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Chemical thermodynamics of Cs and Te fission product interactions in irradiated LMFBR mixed-oxide fuel pins
375
Journal of Nuclear Materials 130 (1985) 375-392 North-Holland, Amsterdam
CHEMICAL THERMODYNAMICS FUEL PINS
M. G. ADAMSON,
PRODUCT
INTERACTIONS
IN IRRADIATED
LMFBR MIXED-OXIDE
E. A. AITKEN
Electric
General
OF Cs AND Te FISSION
Company,
Advanced
Nuclear Technology
Operation,
Sunnyvale,
California
94086,
U.S.A. T. 6. LINDEMER Oak Ridge National Laboratory, Chemical Oak Ridge, Tennessee 37831, U.S.A.
Technology
Division,
P. 0. Box X,
A combination
of fuel chemistry modelling and equilibrium thermodynamic calculations has been used to predict the atom ratios of Cs and Te fission products (Cs:Te) that find their way into the fuel-cladding interface region of irradiated stainless steel-clad mixed-oxide fast breeder It has been concluded that the ratio of condensed, chemically-associated Cs reactor fuel pins. and Te in the interface region,&?%}, which in turn determines the Te activity, iscontrolled by an equilibrium reaction between Cs Te and the oxide fuel , and that the value of
dynamic
In recent communications, solidus
phase boundaries
liquidus
information
to the interpretation
synergistic
Te,Cs-or
fission
liquid metal embrittlement Type-316
stainless
indicated.2'3 proposed
oxidative
Previously,
as causative
agents
chemical
both incidence
interaction
it is obvious
interpretation.
that
of Cs-Te and
is a prerequisite
0022-3115/85/$03.30
in
for detail-
As noted
0 Eisevier Science Publishers B.V. (North-Holland Physics Publishing Division)
and temperature
relationconse-
employing
to obtain
estimated
the desired
thermodynamic information.
In this paper we present the results of thermochemical
estimations
and
how they have been used to ration-
alize the attainment
of sufficiently
values at the fuel-cladding
low Cs:Te
interface
of ir-
radiated mixed-oxide
fuel pins for promotion
of FPLME
and FCC.1 (Cs:Te<4:1).
(Cs:Te<2:1)
Although
de-
and character
of the thermochemistry
Cs-Te-0 mixtures ed mechanistic
process
have shown that the Cs-to- Te
FPLME and FCC1 processes, knowledge
are
in the conunoner
parameter
functions
indicate
products
thermo-
potential
one must resort to thermochemical
some relevant
out- of-pile
(Cs:Te) is a critical
termining
was
it had been
Since detailed
investigations ratio
calculations
AISI 316 cladding corrosion
known as fuel-cladding
(FCCI).4’8
quently
of
(FPLME) of AISI
steel cladding
derived
ships are lacking for these systems,
of this
product-induced
that Cs and Te fission
implicated
versus composition
for the Cs-Te system
and the relevance
were reported,'
and
experimentally
data such as tellurium
complex
other thermochemical chemical
gen, fuel, reactive ding components
fission
or oxygen chemically
involving
products
between
potential associated
specifically
oxy-
thus
on the
fuel composition
(hi0
of the
and clad-
have been performed,'-"
far none have focussed relationships
analyses
interactions
(0:M)
) and the ratio of
Cs2and Te (a)
in the
376
M.G. Adamson et al. i Cs and Te fission product i~lteract~urls
fuel-cladding pins.
gap of irradiated
Subsequently,"
used to explain
mixed-oxide
these results will be
the Cs:Te-dependences
and FCC1 observed
in laboratory
2. FUEL CHEMISTRY
CONSIDERATIONS
of FPLME
simulations.
fission
products
inside oxide fuel pins is 13,14 well understood, albeit in a
reasonably qualitative words,
sense (see Figure 1).
it is relatively
likely chemical
In this section we examine of current chemical
fuel chemistry
tions of ~reactive/volatile~
semi-quantitative
the capability
models
states and relative
at the fuel-cladding
products
to predict
then, using a model,
pressed
fuel pin of a
for the problem
between
of
Cs:Te and
This relationship
is ex-
in terms of the thermochemical
reaction
equilibria
interface fission
esti-
associated
(Cs:Te). The final step is development
a relationship
at the fuel-cladding
considered
product
most likely to control
reactivity
towards
the clad-
corresponding
more difficult concentrations
to quantify
it the
and distribu-
tions. Our assumed
"chemical
model"
is of neces-
It is based,
sity semi-quantitative.
most part, on the present consensus chemistry chemical
specialists
as to the most likely
states and controlling
equilibria
inside typical
oxide fuel pins. some general
reaction
operating
LMFBR
Our aim here is to arrive at
conclusions
tion ratio of chemically fission products
for the
of fuel
about the concentraassociated
Cs and Te
in, or near, the fuel-cladd-
ing gap.
ding. 2.1 Fuel Chemistry The chemical
phenomena
is considerably
fuel chemistry
fuel composition,
fuel pin, but due to
of kinetic and transport
that also must be taken into consideration,
products
Cs and Te in the gap of an operating
finding
tions in an irradiated
fission
gap loca-
fission
interface,
statement
fuel-cladding
the
concentra-
mate the atom ratio of chemically
thermochemical
states of particular
at discrete
the variety
In other
easy to predict
In addition
Model
behavior
of reactive/mobile
volatile"
to Cs and Te, the "reactive/
fission
FIGURE 1 Chemical Evolution in a Mixed-Oxide Fuel Pin with Emphasis on Fission Product Behavior
products
that must be con-
377
M. G. Adamson et al. / Cs and Te fission product interactions
sidered
are I, MO, Rb, Br and Se.
Se are members
and, principally
tively
short isotopic
tive yields siderably
erally
classified
as a noble metal-type
certain
characteristics
and relative
"reactive/volatile" typical
fuel immediately
following
reactor
and thermo-
of their respective
in the half-lives
fission
are considered
source of error in the present
6:l.
in fissile
(?lO%) resulting
is relatively
irradiation
constant
Table 1.
sink region
Some fission
inner surface.
gradients
fuel pins, although
the resulting
is not usually
of Cs and Rb is strongly
influenced
Xe and Kr precursors ranging from 4 minutes
ior of gaseous sive isotopic
precursors
.594
.071
fractionation
of Cs fission
6.12
0.73
These
pointed out that as much as one-third i.e., approximately
of 133Cs and 135Cs, transport
Br
Te
Se
MO
.052
.006
.097
.016
.654
0.54
0.06
1
0.16
6.74
Concentration
Concentration
(atom ratio)
to 6
can lead to exten-
Concentrations of Fission Products in Fast Neutrpg Irradiated Uo.8Puo.202 Fuel at 10 at% at burnup I
which
on the behav-
as noted by Langer et a1.16
half the yields
Rb
as marked
The transport
of the total Cs inventory,
cs
product
that exist inside
In fact, this dependence
authors
throughout
layer on the
can also occur down the shallow axi-
products,
fission
includes
fuel (T 5 1lOO'C)
cladding
have half-lives
the first few days
(atom percent) Relative
of the fuel-cladding
product
days.
spec-
condense
as well as the corrosion
by their gaseous
from changes
in some direct
down the steep radial
This non-isothermal
behavior
of once
regions of ir-
and eventually
as radial redistribution.
1 indicate
neutron energy
except during
Absolute
a minor
This ratio, which exhibits
composition,
gradient
or react in the vicinity
irradiated
mixed oxide fuel is
trum and uncertainties yields,
fuel, migrate
axial redistribution
ratio of Cs t Rb to Te t Se
(Cs:Te) in irradiated
small variations
in
Cs:Te
fission products,
radiated
transport
prod-
reduced
in Figure 1, a majority
from the hot central
al temperature
assessment.
The fission yield data in Table
approximately
partners
trans-
decay paths
gap.
some of the cool peripheral
Although
of noble gas precursors
such effects
that the overall
com-
due to differences
the case of the alkali metal ucts),
in the fuel-cladding
released
gap.
as Cs, I
of these chemical
(for example,
and radioactive
As illustrated
due to close sim-
behavior
and Br as I, and Te and Se as Te.
is possible
reactions,
of selective
the "volatile/reactive"
in a
shutdown
pounds, we treat Cs and Rb commonly
some fractionation
combination
temperature
in chemical
stabilities
We now show how the
of the
1.
In subsequent discussion, dynamic
Cs:Te
values
mixed oxide fast reactor
are shown in Table
of both I and Te
can also lead to substantially
category.
products
have had the opportunity
will exceed that of Cs (i.e.,
port mechanisms,
that also
concentrations
fission
irradiated
ilarities
fis-
along with Tc, Rh, Ru and Pd,
place it in the "reactive/volatile" Absolute
fission products
gen-
of ir-
Cs fission
to build up to their steady state concentrations, the total inventories
oxide fuel are con-
MO, although
before the longer-lived
chain decay products
of relatheir effec-
than those of their heavier
Cs, I and Te.
sion product displays
on account half-lives,
in irradiated
smaller
homologues
radiation,
of the lighter mass decay
chains
At the very beginning
in fresh fuel.
Rb, Br and
M. G. Adamson et al. i Cs and Te fission product interactions
378 axially
out of the fuel region as noble gas
rather than alkali metal. The chemical products
in the gap region
or by reactions central
fuel.
ments4*17
are determined
occurring
at this location
in the vapor phase over the Thermodynamic
from out-of-pile
nuclides migrate specie deposits
gradient
experi-
that iodine fission
exclusively
This
inner surface,
inner surface
temperatures
its vapor pressure
is suffici-
ently high that it shows some tendency grate axially
product
as CsI(v).
on the cladding
but even at typical (550 to 75O"C),
data and results
temperature
. indicate
to mi-
toward the blanket/plenum
Te also appears
from hot fuel and migrate
radial/axial
states of Cs and Te fission
either by reactions
escape
to migrate
zones.
down temperature
gradients
temperature
Cs(v) or CsOH(v)."
At locations where
oxide fuel and/or cladding
are not known with certainty
but appear to lie between
1:l and
1000°C and below, co-migrating to become associated concomitant transport
in vapor pressure,
(x=2,3,4),
etc,.provided
ities exceed values.'I
Any remaining
gap as Cs(l), however, high vapor pressure, migrate
stable vapor specie,
however
are only capable fraction
it appears
that the re-
vapor phase or gas-solid of converting
to cooler
transport
vapor specie.
such as the oxygen activity, tion/activity temperature proportion
in condensed
products into 4,18 Factors
the MO concentra-
phases,
are known to influence
both the overall
of converted
MO.
locations.
condenses
and shows no tendency
to take part in chemical
equilibria
fuel or other Cs compounds.
The bulk of the Cs fission
that
either to
in the gap exceeds
In addition
fission
to Cs2Te, which appears
in the cool peripheral
product
intermetallics
constitution
(unless
and the putative detected
intermetallic
fuel oxytellurides
by electron
microprobe
(EMPA) at a variety of fuel
locations
and, based on the observations,
have assumed
state.
Other proposed
such as (Ba,Sr,Cs) (Ba,Sr)Te03
fission
product
(Zr, Mo,RE,U,Pu)03
important
balance
The various
they
in determining
reactions
and transport
proces-
by Cs, Te, MO and I, and their
impacts on both the overall
'chemically-associated'
Cs-to-Te
and
ratios in the
gap (Cs:Te and Cs:Te, respectively) marized
in Table 2.
mates,
the following
assumed:
phases and
of Cs and Te.
involving 4,17 that
as
of Te
chemical
are minor and consequently
ses undergone estimated
we
they fall in the same category
- that is, only a small fraction
Cs2Mo04
9OO'C) or
products
and
such as MOTe or M202Te(M=U,Pu).
Both the small Pd, Te-containing inclusions
fuel or
phases are Pd-rich
of uncertain
oxytellurides
to
fuel and
Te-containing
reaction
are not considered
down axial thermal gradients
the temperature
reduces in the hot-
test gap locations.
the overall
gap
to quickly
This axial
Cs:Te and C-
this
Any Cs2Mo04
in the
due to its relatively
it is expected
of excess Cs effectively
and
finds its way into the fuel-cladding
threshold
Cs condenses
(or Cs) ends up in this particular
a small
of the solid MO fission
their transportable
reactions
the Cs and O2 activ-
the corresponding
are occasionally
(v), forms over the hot fuel
(r1300°C),
oxygen,
their
rates drop considerably.
sponsible
migrate
At
with the fuel and, due to
decreases
A reasonably Cs2Mo04
2:l.
Cs and Te tend
products
or U,Pu), CsxCr04
the gap, other possible
process
corrosion
- _
CS~MO~_~(M=U
concentrate
transport
T
such as Cr203 to form the solid compounds Cs2M04,
associated
vapor
do so as
~110O"C, this Cs can react with the mixed-
accompanied by cesium in oxide fuel 4 environments. The ratios of Cs:Te that are in this high temperature
down the
gradients
In performing
are sumthese esti-
orders of stability
for Cs compounds
(condensed
were
phases)
379
M. G. Adamson et al. / Cs and Te fission product interactions
z Cs2MO4_y > CsxCrO4
pin power rating.
(x = 3 or 4, y = 0 or 0.44)
to a hypothetical
CsI>Cs2Mo04>Cs2Te
for Te compounds Cs2Te
stabilities
or estimated
fuel surface
ical estimates essarily
approximate
interrelated
r M202Te.
activ-
compounds at typical 11,17 The numer-
conditions.
presented
such as fuel composi-
form, burnup, Table 2.
Variations
tions of processes reduction,
the relative
always
contribu-
la and 9 towards
however,
no matter
Cs:Te
how these vari-
the final Cs:Te value
falls close to 2:l.
tains because oxidation
etc., and fuel
Cs,Te Balance and Transport
(%5 at% burn-
in power rating and fuel compo-
sition can influence
ables are permutated,
in Table 2 are nec-
since they depend on many
parameters
tion, physical
MOTe
refer
nominally-
up).19
are based on cal-
Cs or Te chemical
ities over the various outer
rated FFTF fuel pin at mid-life
phases)
z (Pd-Te-Sn...)>
The relative culated
(condensed
The actual estimates reference-design
in effect
This result ob-
the fuel and Cr203
layer together
provide excess get-
Sheet: Impact of Cs,Te Fission Product Reactions Behavior on Cs-to-Te Ratios in the Fuel-Cladding
Gap
Impact on Cs:Te+ Reaction or Transport Process 1.
Volatile
Fraction of Total Cs or Te Inventory Affected
Overall
Ratio
'Chemically-Associated' Ratio, Cs:Te
precursor
transport
out of fuel
column
2.
CsI formation
a
,X 0.33 (Cs)
Reduced
(3.9:l)
Reduced
(3.9:1)
b
< 0.05 (Te)
Incleased
(4.1:1)
Increased
(4.1:1)
and
0.09
(Cs)
Unchanged
Reduced
(3.6:1)
?r 0.24
(Cs)
Unchanged
Unchanged
(3.6:1)
< 0.10 (Cs)
Unchanged
Reduced
(3.O:l)
~0.10
Increased
Increased
(3.3:1)
transport 3.
CsxTe formation transport
4.
Cs2Mo04
and
(x -1.5)
formation
and
transport 5.
Pd-Te-Sn
Intermetallic
(Te)
formation 6. 7.
MOTe/M202Te Cs2M04_y
formation
formation
< 0.05 (Te)
Unchanged
Increased
(3.5:l)
> 0.20 (Cs;O:M-
Unchanged
Reduced
(s2:l)
Unchanged
Potential
dependent) 8.
CsxCr04
9.
Axial transport
formation
(x = 3)
?
(Cs)
reduction "excess"
Cs*
+Molar Cs-to-Te other chemical
tt
of
?
(Cs; Rating
and O:M-dependent)* ratio in the vicinity forms.
of the fuel-cladding
Value of x taken from results of thermomigration
*"Excess"
Cs only available
if fuel 0:M
q.95.
Potential
Potential
reduction
reduction
(2:l)
(2:l)
gap; Cs:Te does not include Cs in
experiments. 4,17
380
M. G. Adamson et ul. J Cs and Te fission product ~~ter~cti~~s
tering capacity low
for Cs.
In the case of very
0:M fuel (<1.95), this gettering
stable than
capacity
is much reduced, yet, in terms of impact on m
it is at least partially
an increase
compensated
in the axial transport
tellurium by
of "excess"
or "free" Cs. of Problem
The foregoing
semi-quantitative
has shown that mfalls
analysis
with relative
to 2:1, which corresponds
ease
to the composition
of Cs2Te, the only stable compound
on the Cs-
rich side of the Cs-Te phase diagram. l$*O problem
is to show whether
Our
Cs2Te, by
reacting with the oxide fuel (M02ky) or other oxides such as Cr203 or Moo2 to form complex Cs-containing
oxides,
can cause C?
to fall
below 2:l. This problem closely tered in zircaloy-clad where
stress corrosion
that encoun-
to be the zircaloy
cracking
is the chemically
LWR-SCC
parallels
UO2 LWR fuel elements
iodine is assumed
sion products
('XC) agent and CsI
stable form of iodine fis-
in the fuel-cladding
problem
is generally
gap.
assumed
The
to de-
volve on the relative
thermochemical stabil9,21,22 although ities of CsI and Cs2U04,
Gotzmann
has recently
reaction
product
suggested
activity
attainable
face.23
Irrespective
partial
the maximum
at the cladding
the derived
sur-
the I or I2
inside LWR fuel elements,
absolute
activity
small at typical
and operating
iodine
of which thermochemical
is used to estimate
pressures
extremely
that the Cs
Cs2Mo04 may be more important
than Cs21J04 in determining
equilibrium
temperatures
values are
fuel compositions (e.g. pI s 10q5Pa).
Inside LMFBR fuel pins, the corresponding iodine activities
are also small unless
oxygen
or O:M, at the fuel outer
potential,
surface exceeds (that is, 0:M
activity
impractically > 2.01).
Cs2Te is believed
the
high values
However,
because
to be considerably
less
at this location
ted to be significantly iodine.
The question
the lOCal
2.2 Statement
present
CsI at typical LMFBR fuel outer surface operating temperatures, 10,11,24 the
chemical
is expec-
higher than that of now reduces
conditions
to whether
inside an t_MFBR
oxide fuel pin can raise the Te activity
to
the level corresponding
of
Cs2Te.
The previous
and Lindemer question
to decomposition
evaluations
of Gotzmann
et all' did not address
specifically;
however,
this
they did indi-
cate that Cs2Te would become unstable to Cr, Fe and Ni telluride stoichiometric
formation
fuel compositions
3. THERMOCHEMICAL 3.1 Method
thermodynamics
to the fuel-cladding
face region on account
of its strongly
thermal and thermodynamically In practice,
certain
tendencies
assumptions
calculations
'microdomains'.
and a cladding
and performfor two near-
If, as illustrated
as a fuel microdomain
microdomain
gas- or reaction
(B) separated
product-filled
the essential
assumption
transport
(A) by a
gap which SUP-
ports the bulk of the temperature
across
a
in Figure 2, the interface
is considered
component
of
in this region by making
ing thermochemical
region
aniso-
however, we can learn
number of simplifying
schematically
inter-
"open" charac-
about the reaction
species
isothermal
at near-
(O:Ms2).
EVALUATION
In principle, equilibrium
something
relative
and Assumptions
cannot be applied
ter.
10
difference,
is that the rates of
in and out of A and B, and
the gap, are not
excessive
- that is,
they do not result in rapidly changing compositions
of the species under
consideration
in A and B.
Because
the burnup
transport
rates in
and concomitant
component
nominally-rated
fuel pins are slow,
latter assumption consequently confidence
is probably
this
valid;
we can place some degree of in activity
values derived by
381
M.G. Adamson et al. / Cs and Te fission product interactions
application
to calculate (=RTlnaTe,
adopted
in this evaluation potential
the tellurium
where
reaction
was
activ-
equilibria
calculation
the conventional
activities,
or concentrations
microdomains
A and 6 over a range of typical
temperatures
(800 to 1400K) at selected
fuel
ponding change
employed
is
of deriving
partial
pressures
of both reactants
and prod-
ucts for the process
in
method
2nd Law procedure
the equilibrium
AGTe
aTe is the tellurium
ity) for controlling
thermochemical
thermochemistry.
of equilibrium
The approach
either from the corres-
in the standard
AGT (= RTlnK, where
Gibbs energy,
K is the equilibrium
stant) or from the equilibrium
con-
condition
that
E(G~)- ~(6~) = 0, where E, and $. are the partial molar free energies tials of products ly.25
Of course,
equivalent energies product
poten-
respective-
these two approaches
are
and require as input standard
of formation,
of formation
Gibbs
AG~O,~, for reactant
To express
species.
energies
or chemical
and reactants,
and
these standard
as functions
of tempera-
ture, we employ the linear approximation
aGfO T
,
= AHf0,2g8 - TLIS~~,~~~, where AHf0,2g8 and AS; 2g8 are the respective
,
and standard
entropy
of formation
and T is the temperature expression accurate
compositions
(0:M = 1.99 to 2.001) or their
corresponding potential,
oxygen potentials.
aE
thermodynamic
here as RT lnP* 02' (MPa). For equil-
conclusion
has been reconfirmed treated
source references.
substitutes
eral instances
lack measured
for the
which
in sev-
including
those due to small stoichiometry
differences believed
by this substitution
in the reaction
to be quite small
equations
- are
(slOkJ/mol).
The
from the 298 from the
method. 26
The
data used in the present
The following incurred
study by
- Controlling
Reaction
Equilibria
data. The errors
for several
are listed in Table 3 along with
3.2 Te Activity
thermodynamic
This
in the present
more exact free energy function thermodynamic
the
to phase
with values calculated
pounds as reasonable
(U, Pu) compounds
This
provided
ffiGf" values calculated
calculations
actual mixed
of interest
equilibria
02' P* = Po2 (MPa)/Pi2 02 ibria involving oxide fuel, we used U com-
where
298.15K
of Gibbs energy data
values corresponding
approximation
is defined
at
in the kelvins.
are not neglected. I1
transitions
comparing
Oxygen
enthalpy
was shown to afford an acceptably
representation
at the temperatures FIGURE 2 Schematic of Fuel-Cladding Interface Illustrating Fuel (A) and Cladding (B) Microdomains
standard
-
used throughout
IAEA-recommended the remainder
is
denote
phases:
state;
( ) gaseous state; [ ] solid solution
(subscript
c
notation
of the paper to
denotes
'solid state;
solvent); <
{
!liquid
ldenotes
solid
M.G. Adamson et al. / Cs and Te fission product interactions
382
Table 3.
Thermodynamic Participating (Enthalpy
Element/Compound
Values at 298.15K for Elements and Compounds in Fuel-Fission Product-Cladding Equilibria
Values in kJ/mol,
'"F.298
{Cs}
Entropy Values
Reference
‘;98
2.092
in J mol -lK-1).
(and Notes) 27
92.07
76.65
175.5
27
d-IO>
0
28.6
27
(02)
0
205.0
27
0
49.71
28
ITe}
17.5
73.91
28 (AH and ASm from samC ref.)
(Cs)
(Tel
211.7
182.6
28
(Te2)
160.4
258.9
28
4s Te>
-284.5
174.4
(Cs:Te)
-240.6
214.7
40 >
2 2
11 11,24 (AH and AS estimatedmby auth!!!r)
-1084.0
77.0
28
-1514.0
248.3
11
-1926.0
219.7
11
-1141.0
-1444.7
3 4
-1542.0
296.2
11
-1587.0
361.9
11
>
81.17
-23.0
80.0
-130.5
200.8
29 29
200.8
-87.9
120.3
-301.3
208.0
25,29 (Estimated by authors from data in Ref. 29)
84.06
-144.8
2
30(270-4)
146.0
-57.3
28
0
29 24,25,29
27.28
25
0
29.87
25
0
23.64
25
123.05
30
-346.6
ICSIl
-321.7
150.7
30 (AH, from Ref. 31)
m-1 I
-151.9
275.19
30
or liquid phase, depending
on the melting
>+
temperature. The controlling the interface
reaction
for Te activity
region was concluded
to be
2-y>
+ yco21
in or, for the corresponding
uranium
system,
(1)
383
M.G. Adamson et al. / Cs and Te fission product interactions
(2)
+
reactions
+ $P(02) The other candidate for microdomain
reactions
A (lOOO-1400K)
With the exception
written,
represent reaction
(3).
were: (3)
+
bivariant
the boundary
Other reaction
this evaluation
equilibria;
(4) is a univariant
rium that defines
considered
of (4), these various
equilibria
as
equilib-
between
(2) and
considered
in
are:
+ 0.78(02)
> t 0.22cTe) = 0.22~ Cs2Te)
<“2”3.56
t 0.22
+ 0.78
(4)
(II)
{Cs2Te3}
(12)
and
t+
=
2(02) (5)
= {Te)
(13)
2ITe) = (Te2)
(14)
(Te2) = Z(Te)
(15)
[TelCs =
(16)
(12) = 2(I)
(17)
or t 2 2 2 4 + (02). To allow comparison reaction
describing
with reaction(Z),
the equilibrium
(6)
the
between
oxide fuel, cesium and iodine
+ (I,) = 2CCsIl+
(7)
5$%3,) For microdomain
was also evaluated.
= + 2Cr>
1000K) the possible Te activity controlling reactions
considered
were:
< Cs2Cr04> +
and direct less steel,
813
reaction
In reactions
l=
4/3
(8) and (9),
reduction
(19)
= [Felss + 0.9
(20)
>= 3[Ni],, t 2
(21)
Cr203
urium potential reaction
2.
the procedure
by which
data are calculated,
Writing
tell-
consider
for the equilibrium
con-
dition
.
(IO)
6 ’
was also
was
(slOkJ/mol
at
G(Te2) = '
or.
small effect on the
Te or 02 potentials
2deCr 240 > = 2 t (02)
TO illustrate
in stain-
in Cr203 activity
found to have a relatively
lOOOK).
+ 5/6(02)
chromium
as [Cr203]FeCr204,
The resulting
calculated
(9)
[Crlss, and tellurium:
[Crlss +
considered
between
(18)
5/4(02)
+
t 2(02)
B (BOO-
and setting
activities,
we obtain
2+x' + 9
' $02)
and at unit
(22)
384
M. G. Adamson et al. / Cs and Te fissiort product interactions
AGTe
=
AG;402+x> + AG;
(23)
50
I
I I MIC~DO~AIN
/--
RTlnaCs Te t 2-x L$ 2 2 o2
AGof&2UO4>-AG;{Te}+ or AGTe
=
A(AG",) + RTlnaCs Te t
(24)
2
(0:M
2-x RTlnP
= 2 FOR M
= ~O.?S~UO.25)
a Mo=
02
2 where
I
A f f UEL 1
I
(REACTION
5)
A(AGO~) represents the arithmetically
combined
bGDf terms.
Since x is small (sO.OOl),
~G;id_iO~+~>
=
bGof, and equation 24 simplifies to
AGTe
=
A(AGO~) + RTlnaCs2Te + AGO*.
The equilibrium
Te potential
to depend on the temperature, and activity
of Cs2Te.
data, and assuming level for Cs2Te
some reasonable
of the thermochemical
Figure 3 illustrates calculations
900
activity
performed
of T and
data.
for reactions
corresponding
2, 5 and
and oxygen
to exactly
stoich-
and Adamson,3'
and, in the case of reaction
the activities
of both
Te potential
tion 9 < reaction
5,
plots
the trend
in the order reac-
2 < reaction
case, the effect of employing value
5.
In each
a lower aCs Te 2
is the same - that is, it produces
essentially
parallel
lated Te potentials. of hGTe for reaction ~o:Cs2Mo04
activity
(400
( K1
decreases
in the calcu-
An additional 5 results
lowering
if the assumed
ratio is reduced
to 0.1: ,; however,
the resulting
still do not match
those corresponding
from 1:l
calculations
conclusion
to
cladding
interface
5 is the Te
equilibrium region.
to indicate
conditions
to draw from these
is that reaction
activity-controlling
appears
AgTe values
2 until T > 1400K.
The obvious
and were The Ellingham-type
of aGTe versus T are seen to display of increasing
TEMPERATURE
reaction
data used here were The mixed oxide hG 02 taken from the recent measurements of Woodley
to be equal.
(200
FIGURE 3 Equilibrium Tellurium Potentials (AG ) of Candidate Te Activity-Controlling Re a: &ions 2, 5 and 9 Plotted as Functions of Temperature
the range of
iometric mixed oxide fuel (Uo,75Puo.2502).
assumed
4000
the results of such
9 with Cs2Te at unit activity potentials
AGof
(say 0.1 to l), we may calcu-
values which fall within
val4dity
is thus seen
oxygen potential
With appropriate
late AGTe for a typical matrix
A$
(25)
in the fuel-
Thermodynamics
that under typical gap
Cs2Te decomposes
lower oxygen potentials
to elemental
over cCs2Mo04>
Te at
+
than over + 2 3 4 However, with kineticalfy limited _ reaction
equilibria,
are sometimes earlier,
thermodynamic
misleading
this appears
to be the case with
Since an analogous
reaction
5.
reaction
5 has been proposed
iodine activities
predictions
and, as mentioned
sufficient
equilibrium
to
as the source of to induce XC
in
385
M. G. Adamson et al. / Cs and Te fission product interactions
zircaloy-clad worthwhile
LWR fuel elements,
to summarize
23
the evidence
+ equilibria
of the fuel may be strictly
trolling
it is
capsule
(a) no tendency
hypothetical:
cladding
the product
of reaction
to form as
between
Cs20 and (b) no tendency
with
for Cs2Mo04
in liquid Cs containing
oxidation
products,
2 give considerably
tials than reaction
to
very
it is migrating
rather than with traces
of Cr203 on the cooler cladding the reactivity
dominantly cladding
as an oxide
as a mobile
surface,
alternative
(Moo2 or Mo03);
temperature
gradient
experiments,
in
Cs+I and Cs+Te mixtures were
allowed
to thermodiffuse
of stoichiometric stoichiometric temperatures
hyper-
oxide fuel pellets
gave no evidence
CsI or Cs2Te had undergone
cific chemical container
interactions
material
specifically
from
as high as 14OO'C inside
long MO capsules, either
over columns
and slightly
that spe-
with the MO
(one such test was
designed
to detect
I2 re-
leased as the result of such inter17,34 action);
unalloyed
liquid;
upon calculating (c-205 kJ/mol)
regime
(~-550
9; he influences
tion 2 were determined
4GTe
that, strictly,
studied,
fied by the small errors
for temperatures
these calculations,
02
was justi-
((lOkJ/mol).
1.9995, 2.0000,
tions; when Cs and MO 'co-habit',
ed by appropriate
in Figure 4.
900
For
2.0005 and 2.0010 were obtaininterpolation
lation of the data described Oxygen potentials
and extrapo-
in reference
plotted
32.
at 900 and 1400K are given
in Table 4; the estimated
uncertainties
these data range from 15 to 25 kJ/mol.
If reaction 5 is excluded as the aTe-con-
The
in the interval
G values corresponding 4 to mixed oxide stoichiometries such as 1.9990,
it is rare to find cs at these loca-
rather than metal-
+
phase equilibrium
incurred at both the
upper and lower 0:M limits
in (outer) unrestruc-
lieved to be oxidic
25 with it is
the
use of this approximation
of irradiated
lic.
poten-
does not apply over the entire O:M/A% range
tured fuel regions
in the gap where the MO is be-
Although
condensed
seen inside cracks
usually
low
value for reac-
from equation
set equal to unit.
to 1400K, are illustrated
it is
extremely
in the applicable
kJ/mol at lOOOK).
MO is occasionally
pins,
repre-
of fuel 0:M or oxygen
tial on the equilibrium
aCs2Te recognized
The
3 and 4 were both rejected
Te potentials A6
in B, the
fuel - Cs2Te equilibria
as candidates
results, although
due to the
Cs2Te is a solid.
sented by reactions
which
inner surface.
of Cs2Te in micro-
fact that in this region it exists pre-
of Cs or Cs20
could only take place
initially
it
region will react with the (hot) fuel through which
domain A (the fuel) is enhanced
tests also showed that a conphase reaction
- is
higher Te poten-
9 but, intuitively,
similar
if MO existed
0
respectively
In addition,
leading to Cs2Mo04
the equilibria
Not only does
straightforward.
low levels of oxygen at T <1000K;4'17y33
densed
reac-
seems more likely that Cs2Te at the interface
MO
metal and liquid Cs supersaturated
decompose
reaction
tests demonstrated
between
Cs2Te and the fuel, and Cs2Te and
between
for Cs2Mo04
represent
tions 2 and 9 - which
that
in outer regions
relatively isothermal
the choice
equilibrium,
of Also
in Figure 4 are AcTe versus tempera-
386
ht. G. Adamson et al. / Cs and Te fission product interactions
Table 4.
Oxygen
Potential
(AE, ,kJ/mol) - Fuel Composition
(O:M;M=U.75Pu.25)
Da&
from Reference
1.9995
ture relationships telluride
316 stainless
tellurides
metals
-640
-630
-443
-264
-244
-470
-460
-394
-259
-239
considered
metal
Fe, Cr and Ni, The metal
are assumed
to be the
with their respective
at typical cladding
thermochemical
+
of the three common AISI
10, 20 and 21.
phases co-existing
2.0010
900
steel components
i.e. reactions
2.0005
1400
for [metal1316
equilibria
2.0000
32.
temperatures.
The
data used for,
and
were either taken di29 rectly from Mills' assessment or estimated
using the techniques
discussed
and Mills' data for related example,
NiTe2.
tellurides.
data for NiTelml,
To calculate
telluride
equilibria,
For
values were obtained coefficients components
000
+ metal to
of Fe, Cr
stainless
steel.
by calculating
Such
activity
(vi) for each of the three
4000
!200
TEMPERATURE
(K)
1400
FIGURE 4 Equilibrium Tellurium Potential of Reaction 2 Plotted as a Function-of Temperature and Fuel 0:M (also shown are AG -T relationships for three Metal-Metal TellJfide Equilibria).
in Fe-Cr and Fe-Ni binary alloys
from appropriate Gibbs energies estimated
from
potentials
it was necessary
for the activities
and Ni in AISI Type-316
11
Ni2Te3 and
the tellurium
with the three [metal]316
values
[Crlsc6ss + (CrnTex) ---
for
listed in Table 3 were estimated
associated
utilize
in reference
the AHf",2g8 and S;g8 values
corresponding
’
tables of partial
of solution35.
activities
5; wherever
experimental reasonable
illustrate
These
are summarized
comparison,
The results
excess
slightly
Table 5.
Estimated Activity Type-316 Stainless
wt%
to very
fuel (AzTe>' 0, or
Cs2Te will be converted
Values for Fe, Cr and Ni in AISI Steel at lOOOK
Concentrations Component
First, Cs2Te
with respect
hyperstoichiometric
aTes l; in reality,
was noted.
in Figure 4
clear trends.
is seen to be unstable
in Table
data were found for agreement
presented
several
Activity
N(mole fraction)
Coefficient (Vi)
Activity (ai = yiNi)
Fe
65.5
0.651
1.1
0.70
Cr
17.0
0.181
2.3
0.42
Ni
12.0
0.113
0.5
0.056
to
381
M. G. Adamson et al. / Cs and Te fission product interactions
Cs2Te3 or some other higher telluride, potential
then being determined
reaction
equilibrium
+ &*UO47
Te potential,
involving
ICs2Te)
Second,
).
is critically
upon oxygen potential u0.75pu0.2502+y
tellurium
to react with chromium
stainless
steel cladding
Te activity
(activity)
calculated
in Type 316
iodine partial
that the calculated
A(&;)
in go
uncertainty
to *lo0 kJ/mol, which
the estimated
3.3
Relationships
2.
At
on AzTe
arises princip-
associated
value of AGfoCs2Te}
for stoichiometric
02 kJ/mol).
is ver 3 sensi-
and in the cor-
term for rPaction
ally from the uncertainties
is
2 on AGo
Te activity
lOOOK the cumulative
and 2
fuel
of the strong de-
for reaction
with
(?42 kJ/mol)
mixed oxide
Between
(*25
Te Activity,
Cs:Te and Fuel 0:M
ships necessary versus
to construct
Cs:Te at a typical
temperature. exercise between
of developing
was 950K, which the ranges
(6) but is slightly
a plot of AETe gap
chosen for this
lies approximately
for cladding
(A) and fuel
below the normal boiling
point of liquid cesium equilibria
eval-
the relation-
fuel-cladding
The temperature
(95210.
are represented
(26)
20.8TlnXTe
where
XTe is the Te mole fraction.
tion 26 was derived
The pertinent
by reactions
11, 12
s01vus~'~~
of the form AGTe = A t BT t
RTnlnXTe where
n= 2.5; this form of expres-
sion, which corresponds
to a Henrian
model, was used previously Phillips
to describe
of pertinent
pression
describing
Cs:Te<3:2,
portion
Te-rich
the Te-rich - fCs2Te$
As a reasonable
was estimated
measurements
The AG;
somewhat
oxygen
using Knights and Phillips'
on {Cs20) +
+ {Cs021 mixtures. 36
workers
found a0 (=p, /pi
if similar behavior Te-Cs mixtures,
and
At 500°C these
) to lie in the
range of 0.01 to20.022for2mixtures
analogous
line
end of the hypothetical tie and pure{ Tel.
value for ICs2Te31
activity
ex-
liquid solutions,
we simply drew a straight
between
of
Due to a complete
data, an analogous
was not derived.
crudely
solution
by Knights and
an equivalent
solvus. 36
absence
approximation,
and 4GTe
11 at 773, 873 and 973K to
values for reaction an expression
Equa-
by fitting Te solubilities
along the {Cs 1 -
>l:l;
The second part of the thermochemical uation consisted
re-
= -315342 + 131 T +
the ICsI -Cs20>
tive to uncertainties
amounts
For
7 is 12 to 13 orders
consequence
of ~~~~
responding
12).
smaller.
One obvious pendence
to convert
over stoichiometric
from reaction
of magnitude
high for
Cr203 layer; as shown later, the
the corresponding
in liquid cesium
16), the following
was used:
AzTe(J/mol)
in the event there is
to ICs2Te31 (i.e. reaction
pressure
lationship
of tellurium
reaction
over
may also be high enough
comparison,
of formation
dependent
fuel is sufficiently
no protective
of
(Cs:Te>3:1;
range 1.9995 to
Third, the Te potential
stoichiometric
For solutions
+
over the narrow
stoichiometry
and 16, and the Gibbs energies
used are those listed in Table 3.
the equilibrium
and hence the propensity
Cs2Te to decompose,
2.0005.
the Te
by a new
with 0:Cs
is assumed i.e.
for the
= 0.015 It
aTe .005 for Te:Cs = l:l, then using AG+!%s2Te>
and reaction
12, a value for AGO" {Cs2Te3} may
be calculated. No attempt was made to estimate
separate
&If0
and ASSfOvalues for Cs2Te3, due to the lack of data on comparable The resulting
families
S-shaped
imposed over a schematic diagram
in Figure 5.
of compounds.
plot is shown superof the Cs-Te phase
The hG,, versus Cs:Te
M. G. Adamson et al. / Cs and Te fission product interactions
388
plot actually
parallels
shaped curves relating
the well-known 2
02 oxide fuel and, to provide reference several of the fuel compositions to the fuel-Cs2Te indicated. potentials [metal]
equilibrium
Also indicated corresponding +
metal
on the fuel side.
points,
corresponding
(reaction
1) are
are equilibrium
Te
transport cladding
This is necessary
because
of Cs and Te across the gap to the through
the gas-phase
the Cs and Te partial ratio; these,
pressures
will depend on and their
in turn, may be sensitive
tions of oxygen potential
to the three
telluride
316SS and the corresponding
S-
and 0:M for mixed
Two equilibria
func-
and temperature.
are relevant
here, namely
equilibria
Te activity
levels.
l.%UO2>
+ 1.5(02) -c (Cs)
(27)
= (Te) + 1.5 and Z
+ 2.5
+ 2.5(02) + (Cs)
(28)
= (Te2) + 2.5
Both equilibria 0 EOUIVALENT F”ELO:M I REACTION II
-260
either
in Figure 6. -300 -
4’1 4
2:1 32
I:,
2’3
,,,
12
,I
I:4 Cr.TeRATIO 4
were considered
because
(Te) or (Te2) may be dominant, The oxygen potential
of the
boundary
between
tracting
(28) from (27) to eliminate
then setting
the two was obtained
pTe = pT
Equations
.
as shown
by sub(Cs) and
27 and
28 were also used to c % lculate the oxygen potential
at several pT
/pcs and pTe/pCs
” Figure 6. values, which also are e% s own in Inspection
of Figures 4 and 6 reveals
both the Te potential the corresponding are markedly CS
80
66 46 COMPOSITION. AT%
Te
26
3.4 The Gas-Phase
(Te2):(Cs)
and (Te):(Cs)
Thus far, we have developed between
fuel O:M, the condensed
ratio, and Te potential understand
the situation
and (Te):(Cs)
responding
to various
phase Te:Cs
or activity.
To fully
in the fuel-cladding
gap, we must also consider (Te2):(Cs)
relationships
the ratios of
in the gas phase cor-
condensed
phase ratios
on the fuel 0:M.
How-
ever, it is also clear that for exactly fuel, the gas-phase
composition
will be Cs-rich at all likely temperatures Under these conditions,
(lOOO-1400K). fore, gas-phase
transport
there-
of Te (and Cs)
across the gap could never provide Te in excess of that needed to form
surface.
more positive
Ratios in the Gap
at the fuel surface and
Te:Cs ratio in the gas-phase
dependent
stoichiometric FIGURE 5 Diagram Illustrating the Relationship between the Equilibrium Tellurium Potential in Reaction 2, the Condensed Phase Cs:Te Ratio and the SolidusLiquidus Phase Boundaries of Cs-Te Binary Mixtures.
that
In contrast,
definition
tial for stoichiometric 0:M slightly face,
greater
at the
a slightly
of the oxygen potenU
0 75"O 25'2' Or an than 2 at the fuel sur-
wiZ2 result in Te transport in excess of
that needed to deposit
on the cladd-
not only increases Increase of the A; ing. 02 the Te and Te2 partial pressures - according to the equilibrium
389
M.G. Adamson et at. / Cs and Te fission product iIlter~t~o~s
are measurements
of Cs:Te in irradiated
fuel
pins, the impact of fuel pin linear power rating on
incidence
The evaluation
relative
to
of FCC1 and FPLME.
has shown that the overall
ratio of (condensed)
chemically-associated
and Te that develops
in the gap of irradiated
pins is low relative
to the fission yield
ratio @2:1
versus %:l).
the Te activity equilibrium
It is proposed
between
activity-controlling
condensed surface.
equilibrium
to oxygen potential
stoichiometry
that
in the gap is determined
reaction
and oxide fuel at its outer
sensitive
Cs
This Te
is very
(AE
(0:M).
by an
Cs2Te
02
) or fuel
As shown in figures 4 and 5, stoichiometric I
I
-500 800
o\I,‘i
I
I
$000
fuel establishes
1200
TEMPERATURE
1400
cladding
(Kl
a Te activity
enough to convert
unoxidized
to its telluride
enough to correspond (i.e. Cs:Te
FIGURE 6 The Oxygen Potential - 0:M - Temperature Dependence of the Equilibrium Gas Phase Te-to-C& Ratio in Reaction 2 (calculated from reactions 27 and 28)
tainties
+ + (02)
(29)
= &2UO4>
metric
f (Te),
high
to Cs2Te decomposition
Taking account
of uncer-
data used and
made in the calculations,
noting that slightly
clearly
in the
and is nearly
in the thermochemical
assumptions
is sufficiently
that is high
chromium
and
hyperstoichiometric
oxidizing
fuel
to decompose
it is also possible fuel may be capable
Cs2Te,
that stoichioof reducing
Cs:Te
below 2. for example partial
- but it also decreases
pressure
- according
Calculations
the Cs
corresponding
to the equi-
0:M values
librium
of the gas-phase to various
Cs:Te ratios
fuel outer surface
(or condensed-phase
Cs:Te ratios)
have shown that for typical gas conditions
= Z(Cs) + do*
>+ (02).
(30)
gas-phase
is always more Cs-rich
evaporating
phase transport
4. DISCUSSION In this section
of the paper we discuss
results of our thermochemical Cs,Te fission cladding
product
evaluation
behavior
the
of
in the fuel-
gap in terms of the factors exerting
primary
influences
c-1
ratio at the fuel outer surface
the cladding
condensed
products
and
Also discussed
Thus, if gas-
is the only path for Cs-Te to reach the cladding
face, we would not expect
the cladding
unless the fuel Of course,
face is touching
the gap allow
the gap by capillary
on
corresponds
if the f&l
bridge
to
condensing go
the cladding,
within
sur-
this mechanism
lead to Cs:Te
to 0:H $2.
on the condensed-phase
inner surface.
fission
phase.
the
than the
outer sur-
or if solids
liquid Cs-Te mixtures action,
to
gas-phase
hf. G. Adamson ei al. / Cs and Tc~~si~n product i~lterQcti~ns
390
transport is unnecessary and Cstfe will likely
close to stoichiometric 14'37 and burnup, by
reach the cladding without change in composi-
generating a surplus of metallic fission prod-
.
.
tion, i.e.
ucts that are noble with respect to oxidation,
densed-phase transport is felt to be a more
raises the global 0:M value.14'15 The net
plausible trans-gap transport mechanism in
result is that after 1-2 at% burnup in typical
fuel pins irradiated to burnups 22 at%.
commercial LMFBR fuel pins the peripheral fuel
The factors
that
exert
the major influence
on AGTe, and hence on Q:s:Te),are the fuel oxygen potential and the fuel 0:M. The oxygen
will have attained an oxygen potential corresponding
to near-stoichiometric
fuel.
At this point the Te activity, or cm],
potential of the peripheral fuel, which is in
is SO close to the threshold for Cs2Te de-
turn influenced by fuel temperature, deter-
composition that a relatively small increase
mines the local Te activity, and the fuel 0:M
in fuel oxygen potential will accomplish
value - together with the local Cs and Te con-
Any abrupt increase in fuel operating temper-
centrations - determines
ature, such as accompanies a normal power
this,
or reduction in
ascension or design basis over-power transi-
interaction depends upon the relative quanti-
ent, might be sufficient to raise AE
ties of Cs2Te, fuel and available oxygen within the fuel. Reactions 31 and 32, in which
necessary
the 02
amount.
In this connection
we note that Cs:Te con-
ratios as low as 1:3 have been
Ay(=y-y') represents the 'availableoxygen',
centration
describe the overall stoichiometry:
measured by British workers at the fuelcladding
+
Y
+ + 2-a
y-&': Cs2M04,(31)
cMO~+~,> + +I
interface
region in certain oxide-
fuelled LMFBR-type test pins.6
These measure-
ments included pins reported as suffering exceptionally severe FCC1 at relatively low
Tel
cladding temperatures (380-540°C)and Material
2
unreacted”
+ Ay {Tel =
(32)
&s2TeI+Ay”l.
Test Reactor pins that exhibited
'normal'
FCCI; in both cases the Cs:Te determinations were made at locations where interactionwas
If vaporization of <"Cs2TeI+by'3 is signifi-
most pronounced. Because the measured Cs:Te
cant, as in the case of a true gap, this pro-
ratio is so low, we suspect that the reported
cess will lead to a net decrease in
low temperature "exceptionallysevere FCCI" is
(<2/1+&y). In this case, the relative rates
at least partially the result of Cs,Te-induced
of
liquid metal embrittlement (FPLME).
product supply to the peripheral fuel will
In considering the relationshipsbetween
determine the net concentrationsof Cs and Te
at this location.
of Cs and Te at the fuel-cladding interface,
During irradiation, LMFBR oxide-fuelled pins undergo two processes that tend to simultane-
several interesting questions relating to Te detectability surfaced. Fee and Johnson"
ously raise the 0:M and ~i?~ of peripheral
have argued that because Te is not always de-
fuel to values close to thoge which would de-
tected by EMPA at locations exhibiting FCC1
stabilize Cs2Te. In initially hypostoichio-
the presence of Te is not a necessary condi-
metric fuels, radial oxygen redistribution rapidly brings the peripheral fuel composition
tion for occurrence of FCCI. This argument ignores two important points: first, the in-
M.G. Adamson et al. / Cs and Te fission product interactions
herent
limitation
of EMPA in detecting
ments at low concentration and second,
sections,3g
in irradiated
the roles of Te-Cs mixtures
fuel that
in promoting
FCC1 and FPLME are essentially
REFERENCES
ele-
the possibility
catalytic.
action,
to promote
the corresponding
typical during
shielded
EMPA system,
preliminary
searches.
is universally
observed
may simply reflect fission yield
inter-
Te level probably
often falls below its detection
threshold
in a
particularly The fact the "Cs ,438 regions
in attacked
the high overa
ratio (essentially
1. Y. G. Adamson and J. E. Leighty, J. Nucl. Yater. 114
both 398
In other words, since only trace levels of Te and Cs may be sufficient
As will be proposed exceeding
paper,12
3. M. G. Adamson, W. H. Reineking, T. Lauritzen and S. Vaidyanathan, Liquid Yetal Embrittlement of AISI 316 Stainless Steel by Te-Cs Mixtures, in: Liquid and Solid Metal-Induced Embrittlement of Metals, ed. M. H. Kamdar, TMS-AIME, New York (1984). 4.
E. A. Aitken, S. K. Evans and B. F. Rubin, Proc. Panel on Behavior and Chemical State of Irradiated Ceramic Fuels, (IAEA, Vienna, 1974) 269; see also, E. A. Aitken, M. G. Adamson, D. Dutina and S. K. Evans, Proc. Symp. on Thermodynamics of Nuclear Materials 1974 Vol I (IAEA, Vienna, 1975) 187.
5.
J. E. Antill, K. A. Peakall and E. F. Smart, J. Nucl. Mater. 56 (1975) 47; also, J. E. Anti11 and x B. Warburton, ibid -71 (1977) 134.
Cs:Te all chemical
threshold chemical ceeding
at the interface criterion
In conclusion, fuel-cladding
promotion
limit. the present
interface
evaluation
Cs-to-Te
region of irradiated
of FCC1 (Cs:Te<4:1)
tain conditions,
has
ratio at the
fuel pins is sufficiently
and,
low for
under cer-
FPLME (Cs:Te<2:1).
case of FPLME, the essential condition
than ex-
and system-dependent
shown that the effective
mixed-oxide
Te activity
is a more plausible
for interaction
some arbitrary
EMPA detectability
in a subsequent
some critical
In the
thermochemical
to be met at the interface
corresponds
stoichiometric
hyperstoichio-
metric
fuel.
It
tion is achieved sions,
is suggested during
LMFBR power
or a design basis over-power
trans-
ient. ACKNOWLEDGEMENTS gratefully
by the U.S. Department Reactor
Research
Sciences,
of Energy,
Technology
under Contracts
and W-7405-eng-26,
acknowledge
support
Divisions
and Materials
DE-AT03-76SF71031
respectively.
of
J. Nucl. Mater. e
Nucl.
(1979) 39.
11. T. B. Lindemer, T. M. Besmann and C. E. Johnson, J. Nucl. Mater. -100 (1981) 178. 12. M. G. Adamson published.
The authors
ibid, p. 108 (a) and p.
9. T. M. Besmann and T. B. Lindemer, Technol. -40 (1978) 297. 10. 0. Gotzmann,
power ascen-
such as a normal commercial
increase
M. G. Adamson,
to
that this condi-
reactor
7. A. Delbrassine and A. J. Flipot, Fuel and Cladding Interaction, Intern. Working Group on Fast Reactors, Tokyo, 1977,‘ IAEA/IWGFR-16 (1977) 9.
170 (b).
to be that the oxygen potential or very slightly
6. K. Q. Bagley, W. Batey, J. R. Findlay and R. Paris, in: Proc. Intern. Conf. on Fast Breeder Reactor Fuel Performance, Monterey, CA., 1979, (ANS, 111., 1979) 233.
8.
appears
(1983) 327.
2. Y. 6. Adamson, E. A. Aitken and S. Vaidyanathan, Nature -295 (1982) 49.
forms of Cs find their way into the gap; see Table 2).
391
and E. A. Aitken,
13. E. A. Aitken, General NED0-12328 (1972).
Electric
to be
Report
14. R. L. Gibby, R. E. Woodley, M. G. Adamson and C. E. Johnson, in: Proc. Intern. Conf. 3n Fast Breeder Reactor Fuel Performance, Yonterey, CA. 1979, (ANS, Ill., 1979) 188.
M.G. Adamson et al. / Cs and Te fission product interactions
392
15. J. H. Davies and F. T. Ewart, J. Nucl. Mater. 41 (1971) 143.
26. T. B. Lindemer, (1976) 75.
16. S. Langer, G. Buzzelli, J. R. Lindgren, P. W. Flynn, R. J. Campana, L. A. Neimark, S. Greenburg and C. E. Johnson, in: Proc. Intern. Conf. on Fast Breeder Reactor Fuel Performance, Monterey, CA., 1979, (ANS, Ill., 1979) 353.
27. JANAF Thermochemical Tables, NSDR-NBS-37 (1971) and Supplements (1974, 1975, 1978).
17. M. G. Adamson, J. H. Davies and E. S. Darlin, Internal Fuel Rod Chemistry, Intern. Working Group on Fuel Performance and Technology for Water Reactors, Erlangen, FRG, 1979, IAEA/IWGFPT-3 (1979), 83: see also Trans. Amer. Nucl. Sot. -17 (1973) 195. 18. I. Johnson and C. E. Johnson, Proc. Symp. on Thermodvnamics of Nuclear Materials 1979 Vol. (IAEA, Vienna, 1975) 99.
i
19. M. G. Adamson and S. Vaidyanathan, Trans. Amer. Nucl. Sot. 38 (1981) 289 and 291. 20. K. A. Chuntonov, A. N. Kuznetsov, V. M. Fedorov, and S. P. Yatsenko, Inorganic Materials (Eng. Trans.) 18 (1982) 937. 21. J. H. Davies, F. T. Frydenbo and M. G. Adamson, J. Nucl. Mater. 80 (1979) 366. 22. 0. Cubicciotti, R. L. Jones and B. C. Syrett, in: Proc. Zirconium in the Nuclear Industry, ASTM 5th Intern. Conf., Boston, 1980 (ASTM, Philadelphia, PA., 1982) 146. 23. 0. Gotzmann, 185.
J. Nucl. Mater. -107 (1982)
24. M. G. Adamson, E. A. Aitken, R. W. Caputi, P. E. Potter and M. A. Mignanelli, Proc. Symp. on Thermodynamics of Nuclear Materials 1979 Vol 1 (IAEA, Vienna, 1980) 503. 25. 0, Kubaschewski and C. B. Alcock, Metallurgical Thermochemistry (Pergamon, Oxford, 1979).
28.
J. Amer. Ceram. Sot. 59
U. S. Department of Interior Bureau of
Mines - Bulletin 672, U.S. Govt. Printing Office, Washington, DC (1982). 29. K. C. Mills, Thermodynamic Data for Inorganic Sulfides, Selenides and Tellurides (Butterworths, London, 1974). 30. D. 0. Wagman and co-workers, Selected Values of Chemical Thermodynamic Properties, U.S. Nat'l. Bur. Stand. Tech. Report Series 270. 31. R. C. Feber, Los Alamos Sci. Laboratory, Los Alamos Report LA-NUREG-6635 (1977). 32. R. E. Woodley and M. G. Adamson, Mater. 82 (1979) 65.
J. Nucl.
33. E. A. Aitken and M. G. Adamson, General Electric Report GEAP-12511 (1974). 34. E. A. Aitken, M. G. Adamson and S. K. Evans, General Electric Report GEAP-12489 (1974). 35. R. Hultgren and co-workers, Selected Values of the Thermodynamic Properties the Elements (ASM, OH., 1973). 36. C. F. Knights and B. A. Phillips, Mater. 84 (1979) 196. 37. F. Ewart, K. Lassmann, Manes and A. Saunders, (1984) 44.
J. Nucl.
Hj. Matzke, L. J. Nucl. Mater.
38. 0. C. Fee and C. E. Johnson, Mater. 96 (1981) 80.
of
124
J. Nucl.
39. See, for example, H. S. Rosenbaum. T. A. Lauritzen, W..V; Cummings and R. T. Peterson. General Electric Report GEAP-5344 (1967).
Journal of Nuclear Materials 130 (1985) 375-392 North-Holland, Amsterdam
CHEMICAL THERMODYNAMICS FUEL PINS
M. G. ADAMSON,
PRODUCT
INTERACTIONS
IN IRRADIATED
LMFBR MIXED-OXIDE
E. A. AITKEN
Electric
General
OF Cs AND Te FISSION
Company,
Advanced
Nuclear Technology
Operation,
Sunnyvale,
California
94086,
U.S.A. T. 6. LINDEMER Oak Ridge National Laboratory, Chemical Oak Ridge, Tennessee 37831, U.S.A.
Technology
Division,
P. 0. Box X,
A combination
of fuel chemistry modelling and equilibrium thermodynamic calculations has been used to predict the atom ratios of Cs and Te fission products (Cs:Te) that find their way into the fuel-cladding interface region of irradiated stainless steel-clad mixed-oxide fast breeder It has been concluded that the ratio of condensed, chemically-associated Cs reactor fuel pins. and Te in the interface region,&?%}, which in turn determines the Te activity, iscontrolled by an equilibrium reaction between Cs Te and the oxide fuel , and that the value of
dynamic
In recent communications, solidus
phase boundaries
liquidus
information
to the interpretation
synergistic
Te,Cs-or
fission
liquid metal embrittlement Type-316
stainless
indicated.2'3 proposed
oxidative
Previously,
as causative
agents
chemical
both incidence
interaction
it is obvious
interpretation.
that
of Cs-Te and
is a prerequisite
0022-3115/85/$03.30
in
for detail-
As noted
0 Eisevier Science Publishers B.V. (North-Holland Physics Publishing Division)
and temperature
relationconse-
employing
to obtain
estimated
the desired
thermodynamic information.
In this paper we present the results of thermochemical
estimations
and
how they have been used to ration-
alize the attainment
of sufficiently
values at the fuel-cladding
low Cs:Te
interface
of ir-
radiated mixed-oxide
fuel pins for promotion
of FPLME
and FCC.1 (Cs:Te<4:1).
(Cs:Te<2:1)
Although
de-
and character
of the thermochemistry
Cs-Te-0 mixtures ed mechanistic
process
have shown that the Cs-to- Te
FPLME and FCC1 processes, knowledge
are
in the conunoner
parameter
functions
indicate
products
thermo-
potential
one must resort to thermochemical
some relevant
out- of-pile
(Cs:Te) is a critical
termining
was
it had been
Since detailed
investigations ratio
calculations
AISI 316 cladding corrosion
known as fuel-cladding
(FCCI).4’8
quently
of
(FPLME) of AISI
steel cladding
derived
ships are lacking for these systems,
of this
product-induced
that Cs and Te fission
implicated
versus composition
for the Cs-Te system
and the relevance
were reported,'
and
experimentally
data such as tellurium
complex
other thermochemical chemical
gen, fuel, reactive ding components
fission
or oxygen chemically
involving
products
between
potential associated
specifically
oxy-
thus
on the
fuel composition
(hi0
of the
and clad-
have been performed,'-"
far none have focussed relationships
analyses
interactions
(0:M)
) and the ratio of
Cs2and Te (a)
in the
376
M.G. Adamson et al. i Cs and Te fission product i~lteract~urls
fuel-cladding pins.
gap of irradiated
Subsequently,"
used to explain
mixed-oxide
these results will be
the Cs:Te-dependences
and FCC1 observed
in laboratory
2. FUEL CHEMISTRY
CONSIDERATIONS
of FPLME
simulations.
fission
products
inside oxide fuel pins is 13,14 well understood, albeit in a
reasonably qualitative words,
sense (see Figure 1).
it is relatively
likely chemical
In this section we examine of current chemical
fuel chemistry
tions of ~reactive/volatile~
semi-quantitative
the capability
models
states and relative
at the fuel-cladding
products
to predict
then, using a model,
pressed
fuel pin of a
for the problem
between
of
Cs:Te and
This relationship
is ex-
in terms of the thermochemical
reaction
equilibria
interface fission
esti-
associated
(Cs:Te). The final step is development
a relationship
at the fuel-cladding
considered
product
most likely to control
reactivity
towards
the clad-
corresponding
more difficult concentrations
to quantify
it the
and distribu-
tions. Our assumed
"chemical
model"
is of neces-
It is based,
sity semi-quantitative.
most part, on the present consensus chemistry chemical
specialists
as to the most likely
states and controlling
equilibria
inside typical
oxide fuel pins. some general
reaction
operating
LMFBR
Our aim here is to arrive at
conclusions
tion ratio of chemically fission products
for the
of fuel
about the concentraassociated
Cs and Te
in, or near, the fuel-cladd-
ing gap.
ding. 2.1 Fuel Chemistry The chemical
phenomena
is considerably
fuel chemistry
fuel composition,
fuel pin, but due to
of kinetic and transport
that also must be taken into consideration,
products
Cs and Te in the gap of an operating
finding
tions in an irradiated
fission
gap loca-
fission
interface,
statement
fuel-cladding
the
concentra-
mate the atom ratio of chemically
thermochemical
states of particular
at discrete
the variety
In other
easy to predict
In addition
Model
behavior
of reactive/mobile
volatile"
to Cs and Te, the "reactive/
fission
FIGURE 1 Chemical Evolution in a Mixed-Oxide Fuel Pin with Emphasis on Fission Product Behavior
products
that must be con-
377
M. G. Adamson et al. / Cs and Te fission product interactions
sidered
are I, MO, Rb, Br and Se.
Se are members
and, principally
tively
short isotopic
tive yields siderably
erally
classified
as a noble metal-type
certain
characteristics
and relative
"reactive/volatile" typical
fuel immediately
following
reactor
and thermo-
of their respective
in the half-lives
fission
are considered
source of error in the present
6:l.
in fissile
(?lO%) resulting
is relatively
irradiation
constant
Table 1.
sink region
Some fission
inner surface.
gradients
fuel pins, although
the resulting
is not usually
of Cs and Rb is strongly
influenced
Xe and Kr precursors ranging from 4 minutes
ior of gaseous sive isotopic
precursors
.594
.071
fractionation
of Cs fission
6.12
0.73
These
pointed out that as much as one-third i.e., approximately
of 133Cs and 135Cs, transport
Br
Te
Se
MO
.052
.006
.097
.016
.654
0.54
0.06
1
0.16
6.74
Concentration
Concentration
(atom ratio)
to 6
can lead to exten-
Concentrations of Fission Products in Fast Neutrpg Irradiated Uo.8Puo.202 Fuel at 10 at% at burnup I
which
on the behav-
as noted by Langer et a1.16
half the yields
Rb
as marked
The transport
of the total Cs inventory,
cs
product
that exist inside
In fact, this dependence
authors
throughout
layer on the
can also occur down the shallow axi-
products,
fission
includes
fuel (T 5 1lOO'C)
cladding
have half-lives
the first few days
(atom percent) Relative
of the fuel-cladding
product
days.
spec-
condense
as well as the corrosion
by their gaseous
from changes
in some direct
down the steep radial
This non-isothermal
behavior
of once
regions of ir-
and eventually
as radial redistribution.
1 indicate
neutron energy
except during
Absolute
a minor
This ratio, which exhibits
composition,
gradient
or react in the vicinity
irradiated
mixed oxide fuel is
trum and uncertainties yields,
fuel, migrate
axial redistribution
ratio of Cs t Rb to Te t Se
(Cs:Te) in irradiated
small variations
in
Cs:Te
fission products,
radiated
transport
prod-
reduced
in Figure 1, a majority
from the hot central
al temperature
assessment.
The fission yield data in Table
approximately
partners
trans-
decay paths
gap.
some of the cool peripheral
Although
of noble gas precursors
such effects
that the overall
com-
due to differences
the case of the alkali metal ucts),
in the fuel-cladding
released
gap.
as Cs, I
of these chemical
(for example,
and radioactive
As illustrated
due to close sim-
behavior
and Br as I, and Te and Se as Te.
is possible
reactions,
of selective
the "volatile/reactive"
in a
shutdown
pounds, we treat Cs and Rb commonly
some fractionation
combination
temperature
in chemical
stabilities
We now show how the
of the
1.
In subsequent discussion, dynamic
Cs:Te
values
mixed oxide fast reactor
are shown in Table
of both I and Te
can also lead to substantially
category.
products
have had the opportunity
will exceed that of Cs (i.e.,
port mechanisms,
that also
concentrations
fission
irradiated
ilarities
fis-
along with Tc, Rh, Ru and Pd,
place it in the "reactive/volatile" Absolute
fission products
gen-
of ir-
Cs fission
to build up to their steady state concentrations, the total inventories
oxide fuel are con-
MO, although
before the longer-lived
chain decay products
of relatheir effec-
than those of their heavier
Cs, I and Te.
sion product displays
on account half-lives,
in irradiated
smaller
homologues
radiation,
of the lighter mass decay
chains
At the very beginning
in fresh fuel.
Rb, Br and
M. G. Adamson et al. i Cs and Te fission product interactions
378 axially
out of the fuel region as noble gas
rather than alkali metal. The chemical products
in the gap region
or by reactions central
fuel.
ments4*17
are determined
occurring
at this location
in the vapor phase over the Thermodynamic
from out-of-pile
nuclides migrate specie deposits
gradient
experi-
that iodine fission
exclusively
This
inner surface,
inner surface
temperatures
its vapor pressure
is suffici-
ently high that it shows some tendency grate axially
product
as CsI(v).
on the cladding
but even at typical (550 to 75O"C),
data and results
temperature
. indicate
to mi-
toward the blanket/plenum
Te also appears
from hot fuel and migrate
radial/axial
states of Cs and Te fission
either by reactions
escape
to migrate
zones.
down temperature
gradients
temperature
Cs(v) or CsOH(v)."
At locations where
oxide fuel and/or cladding
are not known with certainty
but appear to lie between
1:l and
1000°C and below, co-migrating to become associated concomitant transport
in vapor pressure,
(x=2,3,4),
etc,.provided
ities exceed values.'I
Any remaining
gap as Cs(l), however, high vapor pressure, migrate
stable vapor specie,
however
are only capable fraction
it appears
that the re-
vapor phase or gas-solid of converting
to cooler
transport
vapor specie.
such as the oxygen activity, tion/activity temperature proportion
in condensed
products into 4,18 Factors
the MO concentra-
phases,
are known to influence
both the overall
of converted
MO.
locations.
condenses
and shows no tendency
to take part in chemical
equilibria
fuel or other Cs compounds.
The bulk of the Cs fission
that
either to
in the gap exceeds
In addition
fission
to Cs2Te, which appears
in the cool peripheral
product
intermetallics
constitution
(unless
and the putative detected
intermetallic
fuel oxytellurides
by electron
microprobe
(EMPA) at a variety of fuel
locations
and, based on the observations,
have assumed
state.
Other proposed
such as (Ba,Sr,Cs) (Ba,Sr)Te03
fission
product
(Zr, Mo,RE,U,Pu)03
important
balance
The various
they
in determining
reactions
and transport
proces-
by Cs, Te, MO and I, and their
impacts on both the overall
'chemically-associated'
Cs-to-Te
and
ratios in the
gap (Cs:Te and Cs:Te, respectively) marized
in Table 2.
mates,
the following
assumed:
phases and
of Cs and Te.
involving 4,17 that
as
of Te
chemical
are minor and consequently
ses undergone estimated
we
they fall in the same category
- that is, only a small fraction
Cs2Mo04
9OO'C) or
products
and
such as MOTe or M202Te(M=U,Pu).
Both the small Pd, Te-containing inclusions
fuel or
phases are Pd-rich
of uncertain
oxytellurides
to
fuel and
Te-containing
reaction
are not considered
down axial thermal gradients
the temperature
reduces in the hot-
test gap locations.
the overall
gap
to quickly
This axial
Cs:Te and C-
this
Any Cs2Mo04
in the
due to its relatively
it is expected
of excess Cs effectively
and
finds its way into the fuel-cladding
threshold
Cs condenses
(or Cs) ends up in this particular
a small
of the solid MO fission
their transportable
reactions
the Cs and O2 activ-
the corresponding
are occasionally
(v), forms over the hot fuel
(r1300°C),
oxygen,
their
rates drop considerably.
sponsible
migrate
At
with the fuel and, due to
decreases
A reasonably Cs2Mo04
2:l.
Cs and Te tend
products
or U,Pu), CsxCr04
the gap, other possible
process
corrosion
- _
CS~MO~_~(M=U
concentrate
transport
T
such as Cr203 to form the solid compounds Cs2M04,
associated
vapor
do so as
~110O"C, this Cs can react with the mixed-
accompanied by cesium in oxide fuel 4 environments. The ratios of Cs:Te that are in this high temperature
down the
gradients
In performing
are sumthese esti-
orders of stability
for Cs compounds
(condensed
were
phases)
379
M. G. Adamson et al. / Cs and Te fission product interactions
z Cs2MO4_y > CsxCrO4
pin power rating.
(x = 3 or 4, y = 0 or 0.44)
to a hypothetical
CsI>Cs2Mo04>Cs2Te
for Te compounds Cs2Te
stabilities
or estimated
fuel surface
ical estimates essarily
approximate
interrelated
r M202Te.
activ-
compounds at typical 11,17 The numer-
conditions.
presented
such as fuel composi-
form, burnup, Table 2.
Variations
tions of processes reduction,
the relative
always
contribu-
la and 9 towards
however,
no matter
Cs:Te
how these vari-
the final Cs:Te value
falls close to 2:l.
tains because oxidation
etc., and fuel
Cs,Te Balance and Transport
(%5 at% burn-
in power rating and fuel compo-
sition can influence
ables are permutated,
in Table 2 are nec-
since they depend on many
parameters
tion, physical
MOTe
refer
nominally-
up).19
are based on cal-
Cs or Te chemical
ities over the various outer
rated FFTF fuel pin at mid-life
phases)
z (Pd-Te-Sn...)>
The relative culated
(condensed
The actual estimates reference-design
in effect
This result ob-
the fuel and Cr203
layer together
provide excess get-
Sheet: Impact of Cs,Te Fission Product Reactions Behavior on Cs-to-Te Ratios in the Fuel-Cladding
Gap
Impact on Cs:Te+ Reaction or Transport Process 1.
Volatile
Fraction of Total Cs or Te Inventory Affected
Overall
Ratio
'Chemically-Associated' Ratio, Cs:Te
precursor
transport
out of fuel
column
2.
CsI formation
a
,X 0.33 (Cs)
Reduced
(3.9:l)
Reduced
(3.9:1)
b
< 0.05 (Te)
Incleased
(4.1:1)
Increased
(4.1:1)
and
0.09
(Cs)
Unchanged
Reduced
(3.6:1)
?r 0.24
(Cs)
Unchanged
Unchanged
(3.6:1)
< 0.10 (Cs)
Unchanged
Reduced
(3.O:l)
~0.10
Increased
Increased
(3.3:1)
transport 3.
CsxTe formation transport
4.
Cs2Mo04
and
(x -1.5)
formation
and
transport 5.
Pd-Te-Sn
Intermetallic
(Te)
formation 6. 7.
MOTe/M202Te Cs2M04_y
formation
formation
< 0.05 (Te)
Unchanged
Increased
(3.5:l)
> 0.20 (Cs;O:M-
Unchanged
Reduced
(s2:l)
Unchanged
Potential
dependent) 8.
CsxCr04
9.
Axial transport
formation
(x = 3)
?
(Cs)
reduction "excess"
Cs*
+Molar Cs-to-Te other chemical
tt
of
?
(Cs; Rating
and O:M-dependent)* ratio in the vicinity forms.
of the fuel-cladding
Value of x taken from results of thermomigration
*"Excess"
Cs only available
if fuel 0:M
q.95.
Potential
Potential
reduction
reduction
(2:l)
(2:l)
gap; Cs:Te does not include Cs in
experiments. 4,17
380
M. G. Adamson et ul. J Cs and Te fission product ~~ter~cti~~s
tering capacity low
for Cs.
In the case of very
0:M fuel (<1.95), this gettering
stable than
capacity
is much reduced, yet, in terms of impact on m
it is at least partially
an increase
compensated
in the axial transport
tellurium by
of "excess"
or "free" Cs. of Problem
The foregoing
semi-quantitative
has shown that mfalls
analysis
with relative
to 2:1, which corresponds
ease
to the composition
of Cs2Te, the only stable compound
on the Cs-
rich side of the Cs-Te phase diagram. l$*O problem
is to show whether
Our
Cs2Te, by
reacting with the oxide fuel (M02ky) or other oxides such as Cr203 or Moo2 to form complex Cs-containing
oxides,
can cause C?
to fall
below 2:l. This problem closely tered in zircaloy-clad where
stress corrosion
that encoun-
to be the zircaloy
cracking
is the chemically
LWR-SCC
parallels
UO2 LWR fuel elements
iodine is assumed
sion products
('XC) agent and CsI
stable form of iodine fis-
in the fuel-cladding
problem
is generally
gap.
assumed
The
to de-
volve on the relative
thermochemical stabil9,21,22 although ities of CsI and Cs2U04,
Gotzmann
has recently
reaction
product
suggested
activity
attainable
face.23
Irrespective
partial
the maximum
at the cladding
the derived
sur-
the I or I2
inside LWR fuel elements,
absolute
activity
small at typical
and operating
iodine
of which thermochemical
is used to estimate
pressures
extremely
that the Cs
Cs2Mo04 may be more important
than Cs21J04 in determining
equilibrium
temperatures
values are
fuel compositions (e.g. pI s 10q5Pa).
Inside LMFBR fuel pins, the corresponding iodine activities
are also small unless
oxygen
or O:M, at the fuel outer
potential,
surface exceeds (that is, 0:M
activity
impractically > 2.01).
Cs2Te is believed
the
high values
However,
because
to be considerably
less
at this location
ted to be significantly iodine.
The question
the lOCal
2.2 Statement
present
CsI at typical LMFBR fuel outer surface operating temperatures, 10,11,24 the
chemical
is expec-
higher than that of now reduces
conditions
to whether
inside an t_MFBR
oxide fuel pin can raise the Te activity
to
the level corresponding
of
Cs2Te.
The previous
and Lindemer question
to decomposition
evaluations
of Gotzmann
et all' did not address
specifically;
however,
this
they did indi-
cate that Cs2Te would become unstable to Cr, Fe and Ni telluride stoichiometric
formation
fuel compositions
3. THERMOCHEMICAL 3.1 Method
thermodynamics
to the fuel-cladding
face region on account
of its strongly
thermal and thermodynamically In practice,
certain
tendencies
assumptions
calculations
'microdomains'.
and a cladding
and performfor two near-
If, as illustrated
as a fuel microdomain
microdomain
gas- or reaction
(B) separated
product-filled
the essential
assumption
transport
(A) by a
gap which SUP-
ports the bulk of the temperature
across
a
in Figure 2, the interface
is considered
component
of
in this region by making
ing thermochemical
region
aniso-
however, we can learn
number of simplifying
schematically
inter-
"open" charac-
about the reaction
species
isothermal
at near-
(O:Ms2).
EVALUATION
In principle, equilibrium
something
relative
and Assumptions
cannot be applied
ter.
10
difference,
is that the rates of
in and out of A and B, and
the gap, are not
excessive
- that is,
they do not result in rapidly changing compositions
of the species under
consideration
in A and B.
Because
the burnup
transport
rates in
and concomitant
component
nominally-rated
fuel pins are slow,
latter assumption consequently confidence
is probably
this
valid;
we can place some degree of in activity
values derived by
381
M.G. Adamson et al. / Cs and Te fission product interactions
application
to calculate (=RTlnaTe,
adopted
in this evaluation potential
the tellurium
where
reaction
was
activ-
equilibria
calculation
the conventional
activities,
or concentrations
microdomains
A and 6 over a range of typical
temperatures
(800 to 1400K) at selected
fuel
ponding change
employed
is
of deriving
partial
pressures
of both reactants
and prod-
ucts for the process
in
method
2nd Law procedure
the equilibrium
AGTe
aTe is the tellurium
ity) for controlling
thermochemical
thermochemistry.
of equilibrium
The approach
either from the corres-
in the standard
AGT (= RTlnK, where
Gibbs energy,
K is the equilibrium
stant) or from the equilibrium
con-
condition
that
E(G~)- ~(6~) = 0, where E, and $. are the partial molar free energies tials of products ly.25
Of course,
equivalent energies product
poten-
respective-
these two approaches
are
and require as input standard
of formation,
of formation
Gibbs
AG~O,~, for reactant
To express
species.
energies
or chemical
and reactants,
and
these standard
as functions
of tempera-
ture, we employ the linear approximation
aGfO T
,
= AHf0,2g8 - TLIS~~,~~~, where AHf0,2g8 and AS; 2g8 are the respective
,
and standard
entropy
of formation
and T is the temperature expression accurate
compositions
(0:M = 1.99 to 2.001) or their
corresponding potential,
oxygen potentials.
aE
thermodynamic
here as RT lnP* 02' (MPa). For equil-
conclusion
has been reconfirmed treated
source references.
substitutes
eral instances
lack measured
for the
which
in sev-
including
those due to small stoichiometry
differences believed
by this substitution
in the reaction
to be quite small
equations
- are
(slOkJ/mol).
The
from the 298 from the
method. 26
The
data used in the present
The following incurred
study by
- Controlling
Reaction
Equilibria
data. The errors
for several
are listed in Table 3 along with
3.2 Te Activity
thermodynamic
This
in the present
more exact free energy function thermodynamic
the
to phase
with values calculated
pounds as reasonable
(U, Pu) compounds
This
provided
ffiGf" values calculated
calculations
actual mixed
of interest
equilibria
02' P* = Po2 (MPa)/Pi2 02 ibria involving oxide fuel, we used U com-
where
298.15K
of Gibbs energy data
values corresponding
approximation
is defined
at
in the kelvins.
are not neglected. I1
transitions
comparing
Oxygen
enthalpy
was shown to afford an acceptably
representation
at the temperatures FIGURE 2 Schematic of Fuel-Cladding Interface Illustrating Fuel (A) and Cladding (B) Microdomains
standard
-
used throughout
IAEA-recommended the remainder
is
denote
phases:
state;
( ) gaseous state; [ ] solid solution
(subscript
c
notation
of the paper to
denotes
'solid state;
solvent); <
{
!liquid
ldenotes
solid
M.G. Adamson et al. / Cs and Te fission product interactions
382
Table 3.
Thermodynamic Participating (Enthalpy
Element/Compound
Values at 298.15K for Elements and Compounds in Fuel-Fission Product-Cladding Equilibria
Values in kJ/mol,
'"F.298
{Cs}
Entropy Values
Reference
‘;98
2.092
in J mol -lK-1).
(and Notes) 27
92.07
76.65
175.5
27
d-IO>
0
28.6
27
(02)
0
205.0
27
0
49.71
28
ITe}
17.5
73.91
28 (AH and ASm from samC ref.)
(Cs)
(Tel
211.7
182.6
28
(Te2)
160.4
258.9
28
4s Te>
-284.5
174.4
(Cs:Te)
-240.6
214.7
40 >
2
11 11,24 (AH and AS estimatedmby auth!!!r)
-1084.0
77.0
28
-1514.0
248.3
11
-1926.0
219.7
11
-1141.0
-1444.7
-1542.0
296.2
11
-1587.0
361.9
11
>
81.17
-23.0
80.0
-130.5
200.8
29 29
200.8
-87.9
120.3
-301.3
208.0
25,29 (Estimated by authors from data in Ref. 29)
84.06
-144.8
30(270-4)
146.0
-57.3
28
0
29 24,25,29
27.28
25
0
29.87
25
0
23.64
25
123.05
30
-346.6
ICSIl
-321.7
150.7
30 (AH, from Ref. 31)
m-1 I
-151.9
275.19
30
or liquid phase, depending
on the melting
>+
temperature. The controlling the interface
reaction
for Te activity
region was concluded
to be
2-y>
+ yco21
in or, for the corresponding
uranium
system,
(1)
383
M.G. Adamson et al. / Cs and Te fission product interactions
(2)
+
reactions
+ $P(02) The other candidate for microdomain
reactions
A (lOOO-1400K)
With the exception
written,
represent reaction
(3).
were: (3)
+
bivariant
the boundary
Other reaction
this evaluation
equilibria;
(4) is a univariant
rium that defines
considered
of (4), these various
equilibria
as
equilib-
between
(2) and
considered
in
are:
+ 0.78(02)
> t 0.22cTe) = 0.22~ Cs2Te)
<“2”3.56
t 0.22
+ 0.78
(4)
(II)
{Cs2Te3}
(12)
t
=
2(02) (5)
(13)
2ITe) = (Te2)
(14)
(Te2) = Z(Te)
(15)
[TelCs =
(16)
(12) = 2(I)
(17)
or
describing
with reaction(Z),
the equilibrium
(6)
the
between
oxide fuel, cesium and iodine
+ (I,) = 2CCsIl+
(7)
5$%3,) For microdomain
was also evaluated.
1000K) the possible Te activity controlling reactions
considered
were:
< Cs2Cr04> +
and direct less steel,
813
reaction
In reactions
l=
4/3
(8) and (9),
reduction
(19)
= [Felss + 0.9
(20)
>= 3[Ni],, t 2
(21)
Cr203
urium potential reaction
2.
the procedure
by which
data are calculated,
Writing
tell-
consider
for the equilibrium
con-
dition
.
(IO)
6
was also
was
(slOkJ/mol
at
G(Te2) = '
or
small effect on the
Te or 02 potentials
2deCr 240 > = 2
TO illustrate
in stain-
in Cr203 activity
found to have a relatively
lOOOK).
+ 5/6(02)
chromium
as [Cr203]FeCr204,
The resulting
calculated
(9)
[Crlss, and tellurium:
[Crlss +
considered
between
(18)
5/4(02)
+
t 2(02)
B (BOO-
and setting
activities,
we obtain
2+x' + 9
' $02)
and
(22)
384
M. G. Adamson et al. / Cs and Te fissiort product interactions
AGTe
=
AG;402+x> + AG;
(23)
50
I
I I MIC~DO~AIN
/--
RTlnaCs Te t 2-x L$ 2 2 o2
AGof&2UO4>-AG;{Te}+ or AGTe
=
A(AG",) + RTlnaCs Te t
(24)
2
(0:M
2-x RTlnP
= 2 FOR M
= ~O.?S~UO.25)
a Mo=
02
2 where
I
A f f UEL 1
I
(REACTION
5)
A(AGO~) represents the arithmetically
combined
bGDf terms.
Since x is small (sO.OOl),
~G;id_iO~+~>
=
bGof
AGTe
=
A(AGO~) + RTlnaCs2Te + AGO*.
The equilibrium
Te potential
to depend on the temperature, and activity
of Cs2Te.
data, and assuming level for Cs2Te
some reasonable
of the thermochemical
Figure 3 illustrates calculations
900
activity
performed
of T and
data.
for reactions
corresponding
2, 5 and
and oxygen
to exactly
stoich-
and Adamson,3'
and, in the case of reaction
the activities
of both
Te potential
tion 9 < reaction
5,
plots
the trend
in the order reac-
2 < reaction
case, the effect of employing value
5.
In each
a lower aCs Te 2
is the same - that is, it produces
essentially
parallel
lated Te potentials. of hGTe for reaction ~o:Cs2Mo04
activity
(400
( K1
decreases
in the calcu-
An additional 5 results
lowering
if the assumed
ratio is reduced
to 0.1: ,; however,
the resulting
still do not match
those corresponding
from 1:l
calculations
conclusion
to
cladding
interface
5 is the Te
equilibrium region.
to indicate
conditions
to draw from these
is that reaction
activity-controlling
appears
AgTe values
2 until T > 1400K.
The obvious
of aGTe versus T are seen to display of increasing
TEMPERATURE
reaction
data used here were The mixed oxide hG 02 taken from the recent measurements of Woodley
to be equal.
(200
FIGURE 3 Equilibrium Tellurium Potentials (AG ) of Candidate Te Activity-Controlling Re a: &ions 2, 5 and 9 Plotted as Functions of Temperature
the range of
iometric mixed oxide fuel (Uo,75Puo.2502).
assumed
4000
the results of such
9 with Cs2Te at unit activity potentials
AGof
(say 0.1 to l), we may calcu-
values which fall within
val4dity
is thus seen
oxygen potential
With appropriate
late AGTe for a typical matrix
A$
(25)
in the fuel-
Thermodynamics
that under typical gap
Cs2Te decomposes
lower oxygen potentials
to elemental
over cCs2Mo04>
Te at
+
than over
equilibria,
are sometimes earlier,
thermodynamic
misleading
this appears
to be the case with
Since an analogous
reaction
5.
reaction
5 has been proposed
iodine activities
predictions
and, as mentioned
sufficient
equilibrium
to
as the source of to induce XC
in
385
M. G. Adamson et al. / Cs and Te fission product interactions
zircaloy-clad worthwhile
LWR fuel elements,
to summarize
23
the evidence
+
of the fuel may be strictly
trolling
it is
capsule
(a) no tendency
hypothetical:
cladding
the product
of reaction
to form as
between
Cs20 and (b) no tendency
with
for Cs2Mo04
in liquid Cs containing
oxidation
products,
2 give considerably
tials than reaction
to
very
it is migrating
rather than with traces
of Cr203 on the cooler cladding the reactivity
dominantly cladding
as an oxide
as a mobile
surface,
alternative
(Moo2 or Mo03);
temperature
gradient
experiments,
in
Cs+I and Cs+Te mixtures were
allowed
to thermodiffuse
of stoichiometric stoichiometric temperatures
hyper-
oxide fuel pellets
gave no evidence
CsI or Cs2Te had undergone
cific chemical container
interactions
material
specifically
from
as high as 14OO'C inside
long MO capsules, either
over columns
and slightly
that spe-
with the MO
(one such test was
designed
to detect
I2 re-
leased as the result of such inter17,34 action);
unalloyed
liquid;
upon calculating (c-205 kJ/mol)
regime
(~-550
9; he influences
tion 2 were determined
4GTe
that, strictly,
studied,
fied by the small errors
for temperatures
these calculations,
02
was justi-
((lOkJ/mol).
1.9995, 2.0000,
tions; when Cs and MO 'co-habit',
ed by appropriate
in Figure 4.
900
For
2.0005 and 2.0010 were obtaininterpolation
lation of the data described Oxygen potentials
and extrapo-
in reference
plotted
32.
at 900 and 1400K are given
in Table 4; the estimated
uncertainties
these data range from 15 to 25 kJ/mol.
If reaction 5 is excluded as the aTe-con-
The
in the interval
G values corresponding 4 to mixed oxide stoichiometries such as 1.9990,
it is rare to find cs at these loca-
rather than metal-
+
phase equilibrium
incurred at both the
upper and lower 0:M limits
in (outer) unrestruc-
lieved to be oxidic
25 with it is
the
use of this approximation
of irradiated
lic.
poten-
does not apply over the entire O:M/A% range
tured fuel regions
in the gap where the MO is be-
Although
seen inside cracks
usually
low
value for reac-
from equation
set equal to unit.
to 1400K, are illustrated
it is
extremely
in the applicable
kJ/mol at lOOOK).
MO is occasionally
pins,
repre-
of fuel 0:M or oxygen
tial on the equilibrium
aCs2Te recognized
The
3 and 4 were both rejected
Te potentials A6
in B, the
fuel - Cs2Te equilibria
as candidates
results, although
due to the
Cs2Te is a solid.
sented by reactions
which
inner surface.
of Cs2Te in micro-
fact that in this region it exists pre-
of Cs or Cs20
could only take place
initially
it
region will react with the (hot) fuel through which
domain A (the fuel) is enhanced
tests also showed that a conphase reaction
- is
higher Te poten-
9 but, intuitively,
similar
if MO existed
0
respectively
In addition,
leading to Cs2Mo04
the equilibria
Not only does
straightforward.
low levels of oxygen at T <1000K;4'17y33
densed
reac-
seems more likely that Cs2Te at the interface
MO
metal and liquid Cs supersaturated
decompose
reaction
tests demonstrated
between
Cs2Te and the fuel, and Cs2Te and
between
for Cs2Mo04
represent
tions 2 and 9 - which
that
in outer regions
relatively isothermal
the choice
equilibrium,
of Also
in Figure 4 are AcTe versus tempera-
386
ht. G. Adamson et al. / Cs and Te fission product interactions
Table 4.
Oxygen
Potential
(AE, ,kJ/mol) - Fuel Composition
(O:M;M=U.75Pu.25)
Da&
from Reference
1.9995
ture relationships telluride
316 stainless
tellurides
metals
-640
-630
-443
-264
-244
-470
-460
-394
-259
-239
considered
metal
Fe, Cr and Ni, The metal
are assumed
to be the
with their respective
at typical cladding
thermochemical
+
of the three common AISI
10, 20 and 21.
phases co-existing
2.0010
900
steel components
i.e. reactions
2.0005
1400
for [metal1316
equilibria
2.0000
32.
temperatures.
The
data used for
were either taken di29 rectly from Mills' assessment or estimated
using the techniques
discussed
and Mills' data for related example,
NiTe2.
tellurides.
data for NiTelml,
To calculate
telluride
equilibria,
For
values were obtained coefficients components
000
+ metal to
of Fe, Cr
stainless
steel.
by calculating
Such
activity
(vi) for each of the three
4000
!200
TEMPERATURE
(K)
1400
FIGURE 4 Equilibrium Tellurium Potential of Reaction 2 Plotted as a Function-of Temperature and Fuel 0:M (also shown are AG -T relationships for three Metal-Metal TellJfide Equilibria).
in Fe-Cr and Fe-Ni binary alloys
from appropriate Gibbs energies estimated
from
potentials
it was necessary
for the activities
and Ni in AISI Type-316
11
Ni2Te3 and
the tellurium
with the three [metal]316
values
[Crlsc6ss + (CrnTex) ---
for
listed in Table 3 were estimated
associated
utilize
in reference
the AHf",2g8 and S;g8 values
corresponding
’
tables of partial
of solution35.
activities
5; wherever
experimental reasonable
illustrate
These
are summarized
comparison,
The results
excess
slightly
Table 5.
Estimated Activity Type-316 Stainless
wt%
to very
fuel (AzTe>' 0, or
Cs2Te will be converted
Values for Fe, Cr and Ni in AISI Steel at lOOOK
Concentrations Component
First, Cs2Te
with respect
hyperstoichiometric
aTes l; in reality,
was noted.
in Figure 4
clear trends.
is seen to be unstable
in Table
data were found for agreement
presented
several
Activity
N(mole fraction)
Coefficient (Vi)
Activity (ai = yiNi)
Fe
65.5
0.651
1.1
0.70
Cr
17.0
0.181
2.3
0.42
Ni
12.0
0.113
0.5
0.056
to
381
M. G. Adamson et al. / Cs and Te fission product interactions
Cs2Te3 or some other higher telluride, potential
then being determined
reaction
equilibrium
+ &*UO47
Te potential,
involving
ICs2Te)
Second,
).
is critically
upon oxygen potential u0.75pu0.2502+y
tellurium
to react with chromium
stainless
steel cladding
Te activity
(activity)
calculated
in Type 316
iodine partial
that the calculated
A(&;)
in go
uncertainty
to *lo0 kJ/mol, which
the estimated
3.3
Relationships
2.
At
on AzTe
arises princip-
associated
value of AGfoCs2Te}
for stoichiometric
02 kJ/mol).
is ver 3 sensi-
and in the cor-
term for rPaction
ally from the uncertainties
is
2 on AGo
Te activity
lOOOK the cumulative
and 2
fuel
of the strong de-
for reaction
with
(?42 kJ/mol)
mixed oxide
Between
(*25
Te Activity,
Cs:Te and Fuel 0:M
ships necessary versus
to construct
Cs:Te at a typical
temperature. exercise between
of developing
was 950K, which the ranges
(6) but is slightly
a plot of AETe gap
chosen for this
lies approximately
for cladding
(A) and fuel
below the normal boiling
point of liquid cesium equilibria
eval-
the relation-
fuel-cladding
The temperature
(95210.
are represented
(26)
20.8TlnXTe
where
XTe is the Te mole fraction.
tion 26 was derived
The pertinent
by reactions
11, 12
s01vus~'~~
of the form AGTe = A t BT t
RTnlnXTe where
n= 2.5; this form of expres-
sion, which corresponds
to a Henrian
model, was used previously Phillips
to describe
of pertinent
pression
describing
Cs:Te<3:2,
portion
Te-rich
the Te-rich - fCs2Te$
As a reasonable
was estimated
measurements
The AG;
somewhat
oxygen
using Knights and Phillips'
on {Cs20) +
+ {Cs021 mixtures. 36
workers
found a0 (=p, /pi
if similar behavior Te-Cs mixtures,
and
At 500°C these
) to lie in the
range of 0.01 to20.022for2mixtures
analogous
line
end of the hypothetical tie and pure{ Tel.
value for ICs2Te31
activity
ex-
liquid solutions,
we simply drew a straight
between
of
Due to a complete
data, an analogous
was not derived.
crudely
solution
by Knights and
an equivalent
solvus. 36
absence
approximation,
and 4GTe
11 at 773, 873 and 973K to
values for reaction an expression
Equa-
by fitting Te solubilities
along the {Cs 1 -
>l:l;
The second part of the thermochemical uation consisted
re-
= -315342 + 131 T +
the ICsI -Cs20>
tive to uncertainties
amounts
For
7 is 12 to 13 orders
consequence
of ~~~~
responding
12).
smaller.
One obvious pendence
to convert
over stoichiometric
from reaction
of magnitude
high for
Cr203 layer; as shown later, the
the corresponding
in liquid cesium
16), the following
was used:
AzTe(J/mol)
in the event there is
pressure
lationship
of tellurium
reaction
over
may also be high enough
comparison,
of formation
dependent
fuel is sufficiently
no protective
of
(Cs:Te>3:1;
range 1.9995 to
Third, the Te potential
stoichiometric
For solutions
+
over the narrow
stoichiometry
and 16, and the Gibbs energies
used are those listed in Table 3.
the equilibrium
and hence the propensity
Cs2Te to decompose,
2.0005.
the Te
by a new
with 0:Cs
is assumed i.e.
for the
= 0.015 It
aTe .005 for Te:Cs = l:l, then using AG+!%s2Te>
and reaction
12, a value for AGO" {Cs2Te3} may
be calculated. No attempt was made to estimate
separate
&If0
and ASSfOvalues for Cs2Te3, due to the lack of data on comparable The resulting
families
S-shaped
imposed over a schematic diagram
in Figure 5.
of compounds.
plot is shown superof the Cs-Te phase
The hG,, versus Cs:Te
M. G. Adamson et al. / Cs and Te fission product interactions
388
plot actually
parallels
shaped curves relating
the well-known 2
02 oxide fuel and, to provide reference several of the fuel compositions to the fuel-Cs2Te indicated. potentials [metal]
equilibrium
Also indicated corresponding +
metal
on the fuel side.
points,
corresponding
(reaction
1) are
are equilibrium
Te
transport cladding
This is necessary
because
of Cs and Te across the gap to the through
the gas-phase
the Cs and Te partial ratio; these,
pressures
will depend on and their
in turn, may be sensitive
tions of oxygen potential
to the three
telluride
316SS and the corresponding
S-
and 0:M for mixed
Two equilibria
func-
and temperature.
are relevant
here, namely
equilibria
Te activity
levels.
l.%UO2>
+ 1.5(02) -c (Cs)
(27)
= (Te) + 1.5
+ 2.5
+ 2.5(02) + (Cs)
(28)
= (Te2) + 2.5
Both equilibria 0 EOUIVALENT F”ELO:M I REACTION II
-260
either
in Figure 6. -300 -
4’1 4
2:1 32
I:,
2’3
,,,
12
,I
I:4 Cr.TeRATIO 4
were considered
because
(Te) or (Te2) may be dominant, The oxygen potential
of the
boundary
between
tracting
(28) from (27) to eliminate
then setting
the two was obtained
pTe = pT
Equations
.
as shown
by sub(Cs) and
27 and
28 were also used to c % lculate the oxygen potential
at several pT
/pcs and pTe/pCs
” Figure 6. values, which also are e% s own in Inspection
of Figures 4 and 6 reveals
both the Te potential the corresponding are markedly CS
80
66 46 COMPOSITION. AT%
Te
26
3.4 The Gas-Phase
(Te2):(Cs)
and (Te):(Cs)
Thus far, we have developed between
fuel O:M, the condensed
ratio, and Te potential understand
the situation
and (Te):(Cs)
responding
to various
phase Te:Cs
or activity.
To fully
in the fuel-cladding
gap, we must also consider (Te2):(Cs)
relationships
the ratios of
in the gas phase cor-
condensed
phase ratios
on the fuel 0:M.
How-
ever, it is also clear that for exactly fuel, the gas-phase
composition
will be Cs-rich at all likely temperatures Under these conditions,
(lOOO-1400K). fore, gas-phase
transport
there-
of Te (and Cs)
across the gap could never provide Te in excess of that needed to form
surface.
more positive
Ratios in the Gap
at the fuel surface and
Te:Cs ratio in the gas-phase
dependent
stoichiometric FIGURE 5 Diagram Illustrating the Relationship between the Equilibrium Tellurium Potential in Reaction 2, the Condensed Phase Cs:Te Ratio and the SolidusLiquidus Phase Boundaries of Cs-Te Binary Mixtures.
that
In contrast,
definition
tial for stoichiometric 0:M slightly face,
greater
at the
a slightly
of the oxygen potenU
0 75"O 25'2' Or an than 2 at the fuel sur-
wiZ2 result in Te transport in excess of
that needed to deposit
on the cladd-
not only increases Increase of the A; ing. 02 the Te and Te2 partial pressures - according to the equilibrium
389
M.G. Adamson et at. / Cs and Te fission product iIlter~t~o~s
are measurements
of Cs:Te in irradiated
fuel
pins, the impact of fuel pin linear power rating on
incidence
The evaluation
relative
to
of FCC1 and FPLME.
has shown that the overall
ratio of (condensed)
chemically-associated
and Te that develops
in the gap of irradiated
pins is low relative
to the fission yield
ratio @2:1
versus %:l).
the Te activity equilibrium
It is proposed
between
activity-controlling
condensed surface.
equilibrium
to oxygen potential
stoichiometry
that
in the gap is determined
reaction
and oxide fuel at its outer
sensitive
Cs
This Te
is very
(AE
(0:M).
by an
Cs2Te
02
) or fuel
As shown in figures 4 and 5, stoichiometric I
I
-500 800
o\I,‘i
I
I
$000
fuel establishes
1200
TEMPERATURE
1400
cladding
(Kl
a Te activity
enough to convert
unoxidized
to its telluride
enough to correspond (i.e. Cs:Te
FIGURE 6 The Oxygen Potential - 0:M - Temperature Dependence of the Equilibrium Gas Phase Te-to-C& Ratio in Reaction 2 (calculated from reactions 27 and 28)
tainties
+
(29)
= &2UO4>
metric
f (Te),
high
to Cs2Te decomposition
Taking account
of uncer-
data used and
made in the calculations,
noting that slightly
clearly
in the
and is nearly
in the thermochemical
assumptions
is sufficiently
that is high
chromium
and
hyperstoichiometric
oxidizing
fuel
to decompose
it is also possible fuel may be capable
Cs2Te,
that stoichioof reducing
Cs:Te
below 2. for example partial
- but it also decreases
pressure
- according
Calculations
the Cs
corresponding
to the equi-
0:M values
librium
of the gas-phase to various
Cs:Te ratios
fuel outer surface
(or condensed-phase
Cs:Te ratios)
have shown that for typical gas conditions
>+ (02).
(30)
gas-phase
is always more Cs-rich
evaporating
phase transport
4. DISCUSSION In this section
of the paper we discuss
results of our thermochemical Cs,Te fission cladding
product
evaluation
behavior
the
of
in the fuel-
gap in terms of the factors exerting
primary
influences
c-1
ratio at the fuel outer surface
the cladding
condensed
products
and
Also discussed
Thus, if gas-
is the only path for Cs-Te to reach the cladding
face, we would not expect
the cladding
unless the fuel Of course,
face is touching
the gap allow
the gap by capillary
on
corresponds
if the f&l
bridge
to
condensing go
the cladding,
within
sur-
this mechanism
lead to Cs:Te
to 0:H $2.
on the condensed-phase
inner surface.
fission
phase.
the
than the
outer sur-
or if solids
liquid Cs-Te mixtures action,
to
gas-phase
hf. G. Adamson ei al. / Cs and Tc~~si~n product i~lterQcti~ns
390
transport is unnecessary and Cstfe will likely
close to stoichiometric 14'37 and burnup, by
reach the cladding without change in composi-
generating a surplus of metallic fission prod-
.
.
tion, i.e.
ucts that are noble with respect to oxidation,
densed-phase transport is felt to be a more
raises the global 0:M value.14'15 The net
plausible trans-gap transport mechanism in
result is that after 1-2 at% burnup in typical
fuel pins irradiated to burnups 22 at%.
commercial LMFBR fuel pins the peripheral fuel
The factors
that
exert
the major influence
on AGTe, and hence on Q:s:Te),are the fuel oxygen potential and the fuel 0:M. The oxygen
will have attained an oxygen potential corresponding
to near-stoichiometric
fuel.
At this point the Te activity, or cm],
potential of the peripheral fuel, which is in
is SO close to the threshold for Cs2Te de-
turn influenced by fuel temperature, deter-
composition that a relatively small increase
mines the local Te activity, and the fuel 0:M
in fuel oxygen potential will accomplish
value - together with the local Cs and Te con-
Any abrupt increase in fuel operating temper-
centrations - determines
ature, such as accompanies a normal power
this,
or reduction in
ascension or design basis over-power transi-
interaction depends upon the relative quanti-
ent, might be sufficient to raise AE
ties of Cs2Te, fuel and available oxygen within the fuel. Reactions 31 and 32, in which
necessary
the 02
amount.
In this connection
we note that Cs:Te con-
ratios as low as 1:3 have been
Ay(=y-y') represents the 'availableoxygen',
centration
describe the overall stoichiometry:
measured by British workers at the fuelcladding
Y
+ + 2-a
y-&': Cs2M04,(31)
cMO~+~,> + +I
interface
region in certain oxide-
fuelled LMFBR-type test pins.6
These measure-
ments included pins reported as suffering exceptionally severe FCC1 at relatively low
Tel
cladding temperatures (380-540°C)and Material
2
unreacted”
+ Ay {Tel =
(32)
&s2TeI+Ay”l.
Test Reactor pins that exhibited
'normal'
FCCI; in both cases the Cs:Te determinations were made at locations where interactionwas
If vaporization of <"Cs2TeI+by'3 is signifi-
most pronounced. Because the measured Cs:Te
cant, as in the case of a true gap, this pro-
ratio is so low, we suspect that the reported
cess will lead to a net decrease in
low temperature "exceptionallysevere FCCI" is
(<2/1+&y). In this case, the relative rates
at least partially the result of Cs,Te-induced
of
liquid metal embrittlement (FPLME).
product supply to the peripheral fuel will
In considering the relationshipsbetween
determine the net concentrationsof Cs and Te
at this location.
of Cs and Te at the fuel-cladding interface,
During irradiation, LMFBR oxide-fuelled pins undergo two processes that tend to simultane-
several interesting questions relating to Te detectability surfaced. Fee and Johnson"
ously raise the 0:M and ~i?~ of peripheral
have argued that because Te is not always de-
fuel to values close to thoge which would de-
tected by EMPA at locations exhibiting FCC1
stabilize Cs2Te. In initially hypostoichio-
the presence of Te is not a necessary condi-
metric fuels, radial oxygen redistribution rapidly brings the peripheral fuel composition
tion for occurrence of FCCI. This argument ignores two important points: first, the in-
M.G. Adamson et al. / Cs and Te fission product interactions
herent
limitation
of EMPA in detecting
ments at low concentration and second,
sections,3g
in irradiated
the roles of Te-Cs mixtures
fuel that
in promoting
FCC1 and FPLME are essentially
REFERENCES
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the possibility
catalytic.
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the corresponding
typical during
shielded
EMPA system,
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searches.
is universally
observed
may simply reflect fission yield
inter-
Te level probably
often falls below its detection
threshold
in a
particularly The fact the "Cs ,438 regions
in attacked
the high overa
ratio (essentially
1. Y. G. Adamson and J. E. Leighty, J. Nucl. Yater. 114
both 398
In other words, since only trace levels of Te and Cs may be sufficient
As will be proposed exceeding
paper,12
3. M. G. Adamson, W. H. Reineking, T. Lauritzen and S. Vaidyanathan, Liquid Yetal Embrittlement of AISI 316 Stainless Steel by Te-Cs Mixtures, in: Liquid and Solid Metal-Induced Embrittlement of Metals, ed. M. H. Kamdar, TMS-AIME, New York (1984). 4.
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5.
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Cs:Te all chemical
threshold chemical ceeding
at the interface criterion
In conclusion, fuel-cladding
promotion
limit. the present
interface
evaluation
Cs-to-Te
region of irradiated
of FCC1 (Cs:Te<4:1)
tain conditions,
has
ratio at the
fuel pins is sufficiently
and,
low for
under cer-
FPLME (Cs:Te<2:1).
case of FPLME, the essential condition
than ex-
and system-dependent
shown that the effective
mixed-oxide
Te activity
is a more plausible
for interaction
some arbitrary
EMPA detectability
in a subsequent
some critical
In the
thermochemical
to be met at the interface
corresponds
stoichiometric
hyperstoichio-
metric
fuel.
It
tion is achieved sions,
is suggested during
LMFBR power
or a design basis over-power
trans-
ient. ACKNOWLEDGEMENTS gratefully
by the U.S. Department Reactor
Research
Sciences,
of Energy,
Technology
under Contracts
and W-7405-eng-26,
acknowledge
support
Divisions
and Materials
DE-AT03-76SF71031
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such as a normal commercial
increase
M. G. Adamson,
to
that this condi-
reactor
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