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Tritium diffusion in 304- and 316-stainless steels in the temperature range 25 to 222 °C
JOURNBL
OF NUCLEAR
43 (19%) 119-125. 0
MATERIALS
TRITIUM
DIFFUSION
J. H.
Engineering Depwtment,
Tritium
diffusion
steels were carried
range
measurements
gradients.
to inject
tritium
initial tritium
region
occurred,
5
appeared
characteristic
diffusion
“tail”
The
profiles:
thick
Surface
partie
The surface trapping surface
domaine
des
reaction
gradients
couches
solutions
Zwischen
(omz/sec).
trations initiales en tritium et
0,007
identiflees superficielle
ppm
zH
6Li
a)aH
(n,
du tritium
se produit,
caracteristique
injecter
La
Trois
parties
un profil
environ
drei Bereiche
de diffusion
rapide
supported
by
USAEC
der
D=O,OlS
Contract
als Haftstellen
Die mutlich Poren
Tritium
das fur eine vermutlich
auf
M~ss~gen
der
an der Oberfliiche
Anteil
aH
ist, und sin rasches das
beruht.
bestiitigen
wirkenden
Ober-
der Volumendiffusion
ist
konsistent :
(+O,Oll-0,007)
x exp-
normale
qui est supposee
fiir
Diffusionsprofil,
charakteristiseh
Der
Anfangs-
ein ca. 5 pm dicker
Haftstellen
Diffusionskurve,
einer
eine
Konzentrationsprofilen
ermittelt: in dem
von Tritium
Existenz
die
der Reaktion
mit der L&sung fur die klassische Diffusion
ett8
en volume
durch
Ko~~nzendiffusion
fllichenschicht.
untersucht.
mit Hilfe
den
em normales
ou Is piegeage
do la diffusion
de diffusion
ont
une region
Introduction
Work
die
unmittelbare
0,0005 und 0,007 Gew.-ppm
Aus
wurde.
Freisetzung
du
Tritium, a ternary fission product, can diffuse through fuel claddings and into the primary coolant system of nuclear reactors. It is *
eingebaut, zwischen
Volumendiffusion einer
die Tritium-Diffusion
304 und 316 durch
wurde in die Proben
Auslaufen
des concen-
de concentration:
de l’echantillon.
Tritium
wurden
comprises entre 0,0005 ppm
de 5 pm d’epaisseur
et une “queue”
pour
a la presence
et localisi?s dans les
der Tritium-Diffusiongradienten
erzeugt
to
dans le
tritium.
et produire
en poids.
dans le profil
qui semble
1.
Stahl
konzentration
dans les aciers
6th entreprises
6Li (n, o()3H a Bte utilis& dans les Bchantillons
a 6th attribue
25 und 222 “C wurde
Messung
and gave:
attributed
du
a
0,Ol eV/kT) cm2,&sec.
par l’helium
superfieielles
in rostfreiem
was
voids in the specimen
diffusion
La
obeissait
measure-
de 25 a 222 “C par mesure de
v&if%
to
of a surface
component
du tritium
316 ont
de temp~ratu~s
dire&e tritium
de diffusion
en volume
(0,Sl +
superficiel
Oberfliichenbereich,
304 et
ont
superflciel.
de diffusion:
stabilises
auftreten, Des mesures
Des mesures
tritium
de piegeage
a la diffusion
Le piegeage
layers.
inoxydables
du
(+O,Oll-0,007)
de vides
which
and a rapid
& 0.01 eV/kT)
the presence of helium stabilized
relative
couche
x exp-
tritium
attributed
was tentatively
d’une
D=O,O18
intergranulaire.
superficiel
une loi classique
(+0.011-0.007) xexp-(0.61
& la diffusion
degagement
l’existence
were
profile
release
diffusion
1971
a surface
where
diffusion
with classical diffusion
D=O.O18
de
was used
components
the existence
bulk
S. ELLEMAN
attribuable
316
and to produce
was tentatively
verified
layer.
consistent
,am
diffusion.
of tritium
trapping
and
of tritium
of bulk diffusion,
which
boundary
ments
Three
a normal
25 TO 222 “C *
17 September
in the range 0.0005 ppm
the concentration
approximately
grain
304-
into the specimens
from
trapping
in
The 6Li (n, a)aH reaction
concentrations
CO., AMSTERDAM
North Carolina State University, Raleigh, North Carolina 27607, USA
measurement
to 0.007 ppm aH by weight. identified
and T.
out over the temperature
25 to 222 “C by direct
diffusion
RANGE
AUSTIN
Received
stainless
PUBLISHING
IN 3OP AND 316~STAINLESS STEELS IN
THE TEMPERATURE
Nuclear
NORTH-HOLLAND
Haftstellen
an
der
auf der Gegenwart
(0,61f
0,Ol event)
Oberflache von
in der Oberfliiohenschicht
beruhen
cmafs. ver-
heliumstabilisierten der Proben.
therefore desirable to gain a thorough understanding of the migration of tritium in the important fuel cladding materials such as 304- and 316-stainless steels, and Zircaloy-2.
AT-(40-l)-3508. 119
120
J.
H.
AUSTIN
AND
Most studies l-5) of the transport of hydrogen isotopes through stainless steels have been performed by permeation ~,eohniques. This type of experiment
assumes that the rate limiting
process for migration
is volume
results can be influenced boundary
effects.
This
diffusion
and
by surface and grain method
may
not
be
apropos the mi~ation of fission-recoil injected triGurn through a fuel rod cladding. In order t.o better understand the basic diffusion behavior of tritlum m clads and to ascertain the validity of the inherent assumptions in the past studies, it, is desirable to study t~hediffusion of tritium in these materials by a more exact8 technique. This paper presents results on the migration of tritium in 304- and 316-stainless st,eels where initial tritium distributions were well defined and the diffusion anneal profiles were measured directly by a sectioning technique. 2.
Experimental
The cylindrical 304- and 316-stainless steel specimens were strain annealed in a continuously evacuated tube (pressure m 1 x 10-S torr) at 1000 “C for one hour and then air quenched. The samples were wrapped in tantalum foil in order to insure gettering of the evolved gases. All samples were annealed with similar schedules to produce similar grain structures. The average grain diameter was 50,um. Tritium was injected
T.
8.
ELLEMAN
polish indicated that negligible (-c: 0.I $&) tritium was swept from the solution and that tritium wit8hin a section rapidly equiIibrate~1 witoh t,he hydrogen ions present in the solution. ,4fter each individual section removal, an aliquot was withdrawn solut,ion for radioassay activity
was directly
from the electropolish of 51Cr as chromium
proportionai
t#o t,he layer
thickness removed. The remaining liquid was distilled to remove tritiated wat,er from the high levels of background radioactivity present in the solution. A known amount of distillate was added t:o a liquid scintillator and then counted in a Nuclear Chicago liquid scint,illatio~l unit set’ to discriminate against all pulses falling outside of the tritium beta spectrum. Experiment(s were also performed in which t,he t,ritium release rates from t,he surface were measured for correlation with the bulk diffusiol~ coefficients. Tritiated specimens were heated in a flowing stream of P-10 counting gas (90% argon, 10% methane) with the released tritium measured in a gas flow proportional counter. Standard diffusion solutions for the release of a recoil injected gas at the surface of a heated cylinder predict a release fraction which is proportional to (Dt) 1 for release fractions below 0.3 6). The experimental results were therefore summarized as plots of the tritium release fraction versus (heating time)” to
into the surface of the specimens by the transmutation of a surface blanket of enriched 6LiF
determine
in the North Carolina State University research reactor by the reaction SLi (n, M)3H. Maximum tritium concentrations at the specimen surface following irradiation ranged from 0.0005 ppm to 0.007 ppm 3H by weight. The samples were then diffusion annealed at constant temperature in either a furnace or an oil bath. The tritium concelltratio~~ profiles were determined by the successive removal of sections ranging from 0.1 pm to 20 ,um in thickness by the stainless steel in a electropolishing 65% HaPOe, 20% HzS04, and 15% Hz0 solution. Bright, smooth surfaces were obtained and no etching at grain boundaries was observed. Radioassay of the evolved gases during electro-
3.
the apparent
diffusion
coefficients.
Results
In order to establish the validity of the experimental technique and to determine if tritium diffusion during recoil injection was significant, tritium concentration profiles were measured on two specimens which had no diffusion anneal, The initial concentration profile witah depth produced by transmutation of a surface blanket layer should be a linearly decreasing tritium concentration reaching zero at the recoil range. Except for a low tritium surface concentration, which is discussed later, the results were in agreement with prediction. The measured recoil distribution in fig. 1 shows a linearly decreasing tritium concentration
LALCULArED
Fig.
1.
I t I
Experimental
3I
recoil distribution
I I
RE301c
RANGE
191 Sc,,i
of tritium in 304-st&nless steel.
m
122
J.
H.
AUSTIN
AND
T.
S.
ELLEMAN
beyond the first 8 pm with the best fit straight line reaching zero at a measured recoil range of 20.8 ,um. This is in satisfactory the calculated A diffusion
anneal
a profile
produced
agreement with
recoil range value of 19 i: 3 pm. of a tritiated
specimen
with three distinct
charac-
teristics : a high surface tritium concentration a diffusion
gradient
which
persisted
;
over the
range 10 to 100 pm; and a long diffusion “tail” which penetrated to a considerable depth in the specimen (fig. 2). Matzke and DiCola 6) have generated diffusion solutions
for
an
initial
transmutation-recoil
concentration distribution of a diffusing gas and a constant diffusion coefficient, D. The observed tritium concentration profiles in region II as represented in fig. 2 were found to be consistent with these solutions. It was also found that the well-known thin-film diffusion solution : qx,
q =
s
2VnDt
~-w/4w
Fig. 3.
where : C(x, t) = concentration
of diffusing
(atoms. cm-3), S= quantity of solute onto
the substrate,
initially
plated
(atoms. cm-z)
2 = penetration (cm), D = diffusion coefficient t = diffusion
solute
(cm2 . set-l),
time (set),
would fit these experimental results when applied only to points between the peak in region II and the diffusion tail. Fig. 3 is a plot of In C(z, t) versus x2 for three concentration profiles and shows the agreement with eq. (1). The initial concentration profile produced by transmutation was quite similar to the diffusion profile produced by a Dt = 0.38x 10-Bcm2 in eq. (1). Therefore, the calculated Dt’s were corrected for this initial distribution value. The diffusion coefficients obtained by applying the thin film solution were in agreement with the values obtained with the more exact MatzkeDiCola solution. The diffusion coefficients measured over the
Classical diffusion component 304~stainless steel.
of tritium in
temperature range 25 “C to 222 “C are presented as an Arrhenius plot in fig. 4. Types 304- and 316-stainless steel yielded essentially identical results. The standard deviations calculated for individual points in fig. 4 were found to be less than 1 y0 hence upper and lower limits have not been indicated in the figure. coefficients are given by:
The
diffusion
D = Da e_AHIkT, Do = 0.018?~:~~~ cmz. set-1,
dH=O.61 i:O.OleV, over the temperature range of measurement. The tritium released from the surface was considerably below that predicted from the tritium bulk diffusion coefficients. For fractional releases below 0.25, the observed release curves were consistent with single-valued apparent diffusion coefficients two to three orders of magnitude lower than the bulk diffusion coefficient at the same temperature. Fig. 4 also compares these apparent values of D with the
TRITIUM
DIFFUSION
IN
304-
AND
316-STAINLESS
valued activation range
of
damage
123
STEELS
energy over the temperature
measurement.
Since
the
to the stainless steel lattice
significant in these experiments,
radiation was not
the calculated
diffusion coefficients should represent the normal lattice diffusion of tritium. Chaney and Powell 7) have recently reported tritium diffusion measurements steel in which
the
tritium
in 304-stainless was injected
by
heating in gaseous tritium and layers were machined from the specimen for the tritium profile analysis. The two sets of experiments are in excellent agreement and plots of 1nD versus l/T essentially superimpose. somewhat poorer, however,
Agreement is with diffusion
measurements carried out by permeation techniques. Fig. 5 compares the present work with earlier reported diffusion by hydrogen permeation.
Fig. 4.
Arrhenius
tritium
diffusion
plot
of
diffusion
in 304- and 316-stainless
coefficients
calculated
coefficients
coefficients obtained Differences in D as
of
steels.
from the con-
centration profile. The activation energies are similar but the diffusion coefficients differ significantly. Electropolishing the surface layers from a specimen prior to heating but after tritium recoil produced significantly higher release rates and, consequently, higher apparent diffusion coefficients. Two specimens which appeared to have particularly thick oxide oxide coatings (as evidenced by light diffraction) gave the lowest apparent diffusion coefficients. 4.
Discussion
The measured tritium concentration profiles from 4 ym to roughly 70 pm were consistent with classical diffusion solutions and the calculated diffusion coefficients gave a single-
.6
Fig.
5.
2.0
Summary
of hydrogen and tritium diffusion
in steels (numbers refer to references in bibliography) (hydrogen data correlated to tritium data by Graham’s law).
124
J.
large as lo4 are observed
H.
AUSTIN
AND
in some temperature
tritium
indicates
high surface that
concentration
some retarded
process occurs near the specimen may
be
tentatively
attributed
of
diffusion
surface which to
S.
ELLEMAN
helium. This result appears more consistent wit,h trapping in helium “bubbles” than in the oxide
regions. The observed
T.
trapping.
film. The previously
noted
release rates upon removal specimen
surface
can
also
increase
in tritium
of a few ,um of t,he be interpreted
in
diffusion anneals to Dt’s in the order of 5 x 10-G
terms of removal of part of the helium trapping layer. Surface trapping effects have been observed in preliminary experiment,s with
cm2 were frequently
transmutation-doped
Zircaloy-2,
supports t’he possible trapping. The flat diffusion
importance
Measured
surface
initial tritium
tritium
concentrations
several
concentration,
after
times the highest which implies that
a significant fraction of the diffusing tritium was trapped near the surface. This conclusion is consistent with the ments which gave
surface
release
measure-
low apparent diffusion coefficients for diffusion through the surface layers. Tritium atoms trapped in the surface layers were not permanently retained in traps as prolonged heating reduced the surface concentration of tritium. The most obvious explanation for the surface effect is slow tritium diffusion in a surface oxide film. This interpretation is supported by the observation that specimens with observable surface films gave the lowest measured tritium release rates. However, diffusion measurements carried out by permeation techniques would be expected to give lower values of D than the present method if the specimen had low permeability surface films. Fig. 5 shows that the permeation results are about equally divided above and below the values in the present
work.
An alternative explanation for the anomalous surface behavior is the trapping of tritium in helium stabilized lattice defects. ThesLi (n, a) sH transmutation reaction recoils 4He to a maximum depth of 3 pm in the specimen. The average helium concentration near the surface would be 0.001 ppm He by weight, if diffusion release is negligible. The helium atoms could stabilize small vacancy clusters generated by the recoiling alphas 8) and tritons and these void nuclei could then trap diffusing tritium atoms. The measured concentration profiles revealed that trapping occurred not just at the interface but in the first 5 pm of the specimen surface, which is the region containing the recoiled
which further of helium in
“tail” observed in the tritium gradient experiments is tentatively attributed to grain boundary diffusion. Integration of the tritium in the tail coupled with tritium mass balances indicate that, approximately 10% of the diffusing tritium is contained in the rapid component for a Dt of 6 Y 1OV ems. Experiments are presently underway to measure the grain boundary diffusion coefficientB. The measured initial tritium concentration profile is puzzling in that gas release from the surface occurred whereas the concentration profiles at greater depths agreed with the predicted initial concentration gradient. If general specimen heating had occurred during the irradiation the value of Dt required to produce the observed surface gradient would also have produced significant redistribution of the deeper tritium, which did not occur. Also, the Dt required is considerably
to produce the surface gradient above the Dt which can be
justified from ambient reactor temperatures and heating. The surface release thus gamma appears to result from transient displacement and heating effects produced by the energetic helium and tritium ions. In-pile studies of rare gas release from fissionable solids 9, is) have confirmed the existence of a “knock out” mechanism at low temperatures in which recoil fission fragments which intercept A surface cause some of the rare gas contained in the surface layers to be released. A similar process appears to be occurring in the tritium recoil experiments. The experiments run to date do not definitively distinguish between helium or more
TRITIUM
conventional
surface
DIFFUSION
effects
IN
as the
trapping
process, but they do provide some evidence helium trapping. technology cladding coolant
This has implications
where tritium of
a fuel
is of major
Barnes 11) have processes that
considered
in the PM-l
available
prediction generated
element
to
the
Ray,
diffusion
data
that substantially through ternary
the
AND
316-STAINLESS
assistance
the
profiles the gas
lead
to
the
References
‘)
D.
Randall
all of the tritium fission would be
generated dominant
4) H.
5,
release from the cladding is closer to l%, and this is the design basis for tritium release rates currently assumed in a number of power
7)
voids form and trap
Knolls
Atomic
Report no. KAPL-904
L.
S. Jones R.
Report
Gibson
F.
13 (1963)
and J. A.
Gross
and
and
S.
Schulien,
543
Hj.
Report
Matzke,
Nucl.
Instr.
and
341
K. F. Chaney and G. W.
Powell,
Met. Trans.
1
2356
International
Conference
Voids in Metals, at
Evans,
(1965)
(1966)
57 (1967)
(1970)
Evans,
(1965)
P. M. S. Jones and R. Gibson, UKAEA
Meth.
9
and J. A.
no. AWRE-O-90/65
Eschback,
Albany
on Radiation-Induced
State University
(June
1971)
of New York
(Proceedings
to
be
published)
9)
R. M. Carroll and 0. Sisman, Nucl. Sci. Engr. 21 (1965)
lo)
R.
147
M.
Carroll,
J. Amer. 11
)
J. W.
BMI-1787
12)
Reference Rev.
1,
)
R.
B.
Ceram. Sot.
Ray,
Battelle
Systems
R.
0.
Memorial
Perez
and
48 (1965)
0.
Sisman,
H.
Barnes,
55
Wooton
and R.
Institute
(USA)
Analysis
Report,
Report
no.
Vol.
II,
(1966) Safety 14-iv,
Westinghouse
Nuclear
Energy
(1970)
D. G. Jacobs, Sources of Tritium and Its Behavior upon
the
Salmon,
no. AWRE-O-47/65
S. Jones,
UKAEA
6) G. DiCola
13
Acknowledgements
P. M.
Report
no. AWRE-O-58/66
atoms.
would like to acknowledge
Gibson,
UKAEA
Vacuum
coolant of most power reactors if releases did However, experimental reapproach 100%. sults 1%la) have shown that measured tritium
reactor safety analysis reports. The discrepancy between prediction and observation can be explained by tritium trapping in rare gas stabilized voids. Helium is generated in the cladding through (n,a) reactions and fission product Kr and Xe can diffuse into the cladding from the fuel. The fast neutrons generate large vacancy concentrations and it is possible that
N. (USA)
(1953)
P. M.
from ternary fission would be the source of tritium in the primary
and 0.
Power Laboratory
3,
The authors
in
McDougall concentration
production
and concluded
expected to diffuse to the coolant. The diffusion coefficients measured in the present work are consistent with this interpretation. Tritium
tritium
Frank
and
R.
diffusing
Mr.
of tritium
and Mr. Charles Craft who performed release measurements.
2,
small rare gas stabilized
of
measurement
125
STEELS
primary
Wooten
tritium
reactor
for
in reactor
diffusion through
concern.
304-
Release
Critical Review
to
the
Environment,
Series (1968)
USAEC
OF NUCLEAR
43 (19%) 119-125. 0
MATERIALS
TRITIUM
DIFFUSION
J. H.
Engineering Depwtment,
Tritium
diffusion
steels were carried
range
measurements
gradients.
to inject
tritium
initial tritium
region
occurred,
5
appeared
characteristic
diffusion
“tail”
The
profiles:
thick
Surface
partie
The surface trapping surface
domaine
des
reaction
gradients
couches
solutions
Zwischen
(omz/sec).
trations initiales en tritium et
0,007
identiflees superficielle
ppm
zH
6Li
a)aH
(n,
du tritium
se produit,
caracteristique
injecter
La
Trois
parties
un profil
environ
drei Bereiche
de diffusion
rapide
supported
by
USAEC
der
D=O,OlS
Contract
als Haftstellen
Die mutlich Poren
Tritium
das fur eine vermutlich
auf
M~ss~gen
der
an der Oberfliiche
Anteil
aH
ist, und sin rasches das
beruht.
bestiitigen
wirkenden
Ober-
der Volumendiffusion
ist
konsistent :
(+O,Oll-0,007)
x exp-
normale
qui est supposee
fiir
Diffusionsprofil,
charakteristiseh
Der
Anfangs-
ein ca. 5 pm dicker
Haftstellen
Diffusionskurve,
einer
eine
Konzentrationsprofilen
ermittelt: in dem
von Tritium
Existenz
die
der Reaktion
mit der L&sung fur die klassische Diffusion
ett8
en volume
durch
Ko~~nzendiffusion
fllichenschicht.
untersucht.
mit Hilfe
den
em normales
ou Is piegeage
do la diffusion
de diffusion
ont
une region
Introduction
Work
die
unmittelbare
0,0005 und 0,007 Gew.-ppm
Aus
wurde.
Freisetzung
du
Tritium, a ternary fission product, can diffuse through fuel claddings and into the primary coolant system of nuclear reactors. It is *
eingebaut, zwischen
Volumendiffusion einer
die Tritium-Diffusion
304 und 316 durch
wurde in die Proben
Auslaufen
des concen-
de concentration:
de l’echantillon.
Tritium
wurden
comprises entre 0,0005 ppm
de 5 pm d’epaisseur
et une “queue”
pour
a la presence
et localisi?s dans les
der Tritium-Diffusiongradienten
erzeugt
to
dans le
tritium.
et produire
en poids.
dans le profil
qui semble
1.
Stahl
konzentration
dans les aciers
6th entreprises
6Li (n, o()3H a Bte utilis& dans les Bchantillons
a 6th attribue
25 und 222 “C wurde
Messung
and gave:
attributed
du
a
0,Ol eV/kT) cm2,&sec.
par l’helium
superfieielles
in rostfreiem
was
voids in the specimen
diffusion
La
obeissait
measure-
de 25 a 222 “C par mesure de
v&if%
to
of a surface
component
du tritium
316 ont
de temp~ratu~s
dire&e tritium
de diffusion
en volume
(0,Sl +
superficiel
Oberfliichenbereich,
304 et
ont
superflciel.
de diffusion:
stabilises
auftreten, Des mesures
Des mesures
tritium
de piegeage
a la diffusion
Le piegeage
layers.
inoxydables
du
(+O,Oll-0,007)
de vides
which
and a rapid
& 0.01 eV/kT)
the presence of helium stabilized
relative
couche
x exp-
tritium
attributed
was tentatively
d’une
D=O,O18
intergranulaire.
superficiel
une loi classique
(+0.011-0.007) xexp-(0.61
& la diffusion
degagement
l’existence
were
profile
release
diffusion
1971
a surface
where
diffusion
with classical diffusion
D=O.O18
de
was used
components
the existence
bulk
S. ELLEMAN
attribuable
316
and to produce
was tentatively
verified
layer.
consistent
,am
diffusion.
of tritium
trapping
and
of tritium
of bulk diffusion,
which
boundary
ments
Three
a normal
25 TO 222 “C *
17 September
in the range 0.0005 ppm
the concentration
approximately
grain
304-
into the specimens
from
trapping
in
The 6Li (n, a)aH reaction
concentrations
CO., AMSTERDAM
North Carolina State University, Raleigh, North Carolina 27607, USA
measurement
to 0.007 ppm aH by weight. identified
and T.
out over the temperature
25 to 222 “C by direct
diffusion
RANGE
AUSTIN
Received
stainless
PUBLISHING
IN 3OP AND 316~STAINLESS STEELS IN
THE TEMPERATURE
Nuclear
NORTH-HOLLAND
Haftstellen
an
der
auf der Gegenwart
(0,61f
0,Ol event)
Oberflache von
in der Oberfliiohenschicht
beruhen
cmafs. ver-
heliumstabilisierten der Proben.
therefore desirable to gain a thorough understanding of the migration of tritium in the important fuel cladding materials such as 304- and 316-stainless steels, and Zircaloy-2.
AT-(40-l)-3508. 119
120
J.
H.
AUSTIN
AND
Most studies l-5) of the transport of hydrogen isotopes through stainless steels have been performed by permeation ~,eohniques. This type of experiment
assumes that the rate limiting
process for migration
is volume
results can be influenced boundary
effects.
This
diffusion
and
by surface and grain method
may
not
be
apropos the mi~ation of fission-recoil injected triGurn through a fuel rod cladding. In order t.o better understand the basic diffusion behavior of tritlum m clads and to ascertain the validity of the inherent assumptions in the past studies, it, is desirable to study t~hediffusion of tritium in these materials by a more exact8 technique. This paper presents results on the migration of tritium in 304- and 316-stainless st,eels where initial tritium distributions were well defined and the diffusion anneal profiles were measured directly by a sectioning technique. 2.
Experimental
The cylindrical 304- and 316-stainless steel specimens were strain annealed in a continuously evacuated tube (pressure m 1 x 10-S torr) at 1000 “C for one hour and then air quenched. The samples were wrapped in tantalum foil in order to insure gettering of the evolved gases. All samples were annealed with similar schedules to produce similar grain structures. The average grain diameter was 50,um. Tritium was injected
T.
8.
ELLEMAN
polish indicated that negligible (-c: 0.I $&) tritium was swept from the solution and that tritium wit8hin a section rapidly equiIibrate~1 witoh t,he hydrogen ions present in the solution. ,4fter each individual section removal, an aliquot was withdrawn solut,ion for radioassay activity
was directly
from the electropolish of 51Cr as chromium
proportionai
t#o t,he layer
thickness removed. The remaining liquid was distilled to remove tritiated wat,er from the high levels of background radioactivity present in the solution. A known amount of distillate was added t:o a liquid scintillator and then counted in a Nuclear Chicago liquid scint,illatio~l unit set’ to discriminate against all pulses falling outside of the tritium beta spectrum. Experiment(s were also performed in which t,he t,ritium release rates from t,he surface were measured for correlation with the bulk diffusiol~ coefficients. Tritiated specimens were heated in a flowing stream of P-10 counting gas (90% argon, 10% methane) with the released tritium measured in a gas flow proportional counter. Standard diffusion solutions for the release of a recoil injected gas at the surface of a heated cylinder predict a release fraction which is proportional to (Dt) 1 for release fractions below 0.3 6). The experimental results were therefore summarized as plots of the tritium release fraction versus (heating time)” to
into the surface of the specimens by the transmutation of a surface blanket of enriched 6LiF
determine
in the North Carolina State University research reactor by the reaction SLi (n, M)3H. Maximum tritium concentrations at the specimen surface following irradiation ranged from 0.0005 ppm to 0.007 ppm 3H by weight. The samples were then diffusion annealed at constant temperature in either a furnace or an oil bath. The tritium concelltratio~~ profiles were determined by the successive removal of sections ranging from 0.1 pm to 20 ,um in thickness by the stainless steel in a electropolishing 65% HaPOe, 20% HzS04, and 15% Hz0 solution. Bright, smooth surfaces were obtained and no etching at grain boundaries was observed. Radioassay of the evolved gases during electro-
3.
the apparent
diffusion
coefficients.
Results
In order to establish the validity of the experimental technique and to determine if tritium diffusion during recoil injection was significant, tritium concentration profiles were measured on two specimens which had no diffusion anneal, The initial concentration profile witah depth produced by transmutation of a surface blanket layer should be a linearly decreasing tritium concentration reaching zero at the recoil range. Except for a low tritium surface concentration, which is discussed later, the results were in agreement with prediction. The measured recoil distribution in fig. 1 shows a linearly decreasing tritium concentration
LALCULArED
Fig.
1.
I t I
Experimental
3I
recoil distribution
I I
RE301c
RANGE
191 Sc,,i
of tritium in 304-st&nless steel.
m
122
J.
H.
AUSTIN
AND
T.
S.
ELLEMAN
beyond the first 8 pm with the best fit straight line reaching zero at a measured recoil range of 20.8 ,um. This is in satisfactory the calculated A diffusion
anneal
a profile
produced
agreement with
recoil range value of 19 i: 3 pm. of a tritiated
specimen
with three distinct
charac-
teristics : a high surface tritium concentration a diffusion
gradient
which
persisted
;
over the
range 10 to 100 pm; and a long diffusion “tail” which penetrated to a considerable depth in the specimen (fig. 2). Matzke and DiCola 6) have generated diffusion solutions
for
an
initial
transmutation-recoil
concentration distribution of a diffusing gas and a constant diffusion coefficient, D. The observed tritium concentration profiles in region II as represented in fig. 2 were found to be consistent with these solutions. It was also found that the well-known thin-film diffusion solution : qx,
q =
s
2VnDt
~-w/4w
Fig. 3.
where : C(x, t) = concentration
of diffusing
(atoms. cm-3), S= quantity of solute onto
the substrate,
initially
plated
(atoms. cm-z)
2 = penetration (cm), D = diffusion coefficient t = diffusion
solute
(cm2 . set-l),
time (set),
would fit these experimental results when applied only to points between the peak in region II and the diffusion tail. Fig. 3 is a plot of In C(z, t) versus x2 for three concentration profiles and shows the agreement with eq. (1). The initial concentration profile produced by transmutation was quite similar to the diffusion profile produced by a Dt = 0.38x 10-Bcm2 in eq. (1). Therefore, the calculated Dt’s were corrected for this initial distribution value. The diffusion coefficients obtained by applying the thin film solution were in agreement with the values obtained with the more exact MatzkeDiCola solution. The diffusion coefficients measured over the
Classical diffusion component 304~stainless steel.
of tritium in
temperature range 25 “C to 222 “C are presented as an Arrhenius plot in fig. 4. Types 304- and 316-stainless steel yielded essentially identical results. The standard deviations calculated for individual points in fig. 4 were found to be less than 1 y0 hence upper and lower limits have not been indicated in the figure. coefficients are given by:
The
diffusion
D = Da e_AHIkT, Do = 0.018?~:~~~ cmz. set-1,
dH=O.61 i:O.OleV, over the temperature range of measurement. The tritium released from the surface was considerably below that predicted from the tritium bulk diffusion coefficients. For fractional releases below 0.25, the observed release curves were consistent with single-valued apparent diffusion coefficients two to three orders of magnitude lower than the bulk diffusion coefficient at the same temperature. Fig. 4 also compares these apparent values of D with the
TRITIUM
DIFFUSION
IN
304-
AND
316-STAINLESS
valued activation range
of
damage
123
STEELS
energy over the temperature
measurement.
Since
the
to the stainless steel lattice
significant in these experiments,
radiation was not
the calculated
diffusion coefficients should represent the normal lattice diffusion of tritium. Chaney and Powell 7) have recently reported tritium diffusion measurements steel in which
the
tritium
in 304-stainless was injected
by
heating in gaseous tritium and layers were machined from the specimen for the tritium profile analysis. The two sets of experiments are in excellent agreement and plots of 1nD versus l/T essentially superimpose. somewhat poorer, however,
Agreement is with diffusion
measurements carried out by permeation techniques. Fig. 5 compares the present work with earlier reported diffusion by hydrogen permeation.
Fig. 4.
Arrhenius
tritium
diffusion
plot
of
diffusion
in 304- and 316-stainless
coefficients
calculated
coefficients
coefficients obtained Differences in D as
of
steels.
from the con-
centration profile. The activation energies are similar but the diffusion coefficients differ significantly. Electropolishing the surface layers from a specimen prior to heating but after tritium recoil produced significantly higher release rates and, consequently, higher apparent diffusion coefficients. Two specimens which appeared to have particularly thick oxide oxide coatings (as evidenced by light diffraction) gave the lowest apparent diffusion coefficients. 4.
Discussion
The measured tritium concentration profiles from 4 ym to roughly 70 pm were consistent with classical diffusion solutions and the calculated diffusion coefficients gave a single-
.6
Fig.
5.
2.0
Summary
of hydrogen and tritium diffusion
in steels (numbers refer to references in bibliography) (hydrogen data correlated to tritium data by Graham’s law).
124
J.
large as lo4 are observed
H.
AUSTIN
AND
in some temperature
tritium
indicates
high surface that
concentration
some retarded
process occurs near the specimen may
be
tentatively
attributed
of
diffusion
surface which to
S.
ELLEMAN
helium. This result appears more consistent wit,h trapping in helium “bubbles” than in the oxide
regions. The observed
T.
trapping.
film. The previously
noted
release rates upon removal specimen
surface
can
also
increase
in tritium
of a few ,um of t,he be interpreted
in
diffusion anneals to Dt’s in the order of 5 x 10-G
terms of removal of part of the helium trapping layer. Surface trapping effects have been observed in preliminary experiment,s with
cm2 were frequently
transmutation-doped
Zircaloy-2,
supports t’he possible trapping. The flat diffusion
importance
Measured
surface
initial tritium
tritium
concentrations
several
concentration,
after
times the highest which implies that
a significant fraction of the diffusing tritium was trapped near the surface. This conclusion is consistent with the ments which gave
surface
release
measure-
low apparent diffusion coefficients for diffusion through the surface layers. Tritium atoms trapped in the surface layers were not permanently retained in traps as prolonged heating reduced the surface concentration of tritium. The most obvious explanation for the surface effect is slow tritium diffusion in a surface oxide film. This interpretation is supported by the observation that specimens with observable surface films gave the lowest measured tritium release rates. However, diffusion measurements carried out by permeation techniques would be expected to give lower values of D than the present method if the specimen had low permeability surface films. Fig. 5 shows that the permeation results are about equally divided above and below the values in the present
work.
An alternative explanation for the anomalous surface behavior is the trapping of tritium in helium stabilized lattice defects. ThesLi (n, a) sH transmutation reaction recoils 4He to a maximum depth of 3 pm in the specimen. The average helium concentration near the surface would be 0.001 ppm He by weight, if diffusion release is negligible. The helium atoms could stabilize small vacancy clusters generated by the recoiling alphas 8) and tritons and these void nuclei could then trap diffusing tritium atoms. The measured concentration profiles revealed that trapping occurred not just at the interface but in the first 5 pm of the specimen surface, which is the region containing the recoiled
which further of helium in
“tail” observed in the tritium gradient experiments is tentatively attributed to grain boundary diffusion. Integration of the tritium in the tail coupled with tritium mass balances indicate that, approximately 10% of the diffusing tritium is contained in the rapid component for a Dt of 6 Y 1OV ems. Experiments are presently underway to measure the grain boundary diffusion coefficientB. The measured initial tritium concentration profile is puzzling in that gas release from the surface occurred whereas the concentration profiles at greater depths agreed with the predicted initial concentration gradient. If general specimen heating had occurred during the irradiation the value of Dt required to produce the observed surface gradient would also have produced significant redistribution of the deeper tritium, which did not occur. Also, the Dt required is considerably
to produce the surface gradient above the Dt which can be
justified from ambient reactor temperatures and heating. The surface release thus gamma appears to result from transient displacement and heating effects produced by the energetic helium and tritium ions. In-pile studies of rare gas release from fissionable solids 9, is) have confirmed the existence of a “knock out” mechanism at low temperatures in which recoil fission fragments which intercept A surface cause some of the rare gas contained in the surface layers to be released. A similar process appears to be occurring in the tritium recoil experiments. The experiments run to date do not definitively distinguish between helium or more
TRITIUM
conventional
surface
DIFFUSION
effects
IN
as the
trapping
process, but they do provide some evidence helium trapping. technology cladding coolant
This has implications
where tritium of
a fuel
is of major
Barnes 11) have processes that
considered
in the PM-l
available
prediction generated
element
to
the
Ray,
diffusion
data
that substantially through ternary
the
AND
316-STAINLESS
assistance
the
profiles the gas
lead
to
the
References
‘)
D.
Randall
all of the tritium fission would be
generated dominant
4) H.
5,
release from the cladding is closer to l%, and this is the design basis for tritium release rates currently assumed in a number of power
7)
voids form and trap
Knolls
Atomic
Report no. KAPL-904
L.
S. Jones R.
Report
Gibson
F.
13 (1963)
and J. A.
Gross
and
and
S.
Schulien,
543
Hj.
Report
Matzke,
Nucl.
Instr.
and
341
K. F. Chaney and G. W.
Powell,
Met. Trans.
1
2356
International
Conference
Voids in Metals, at
Evans,
(1965)
(1966)
57 (1967)
(1970)
Evans,
(1965)
P. M. S. Jones and R. Gibson, UKAEA
Meth.
9
and J. A.
no. AWRE-O-90/65
Eschback,
Albany
on Radiation-Induced
State University
(June
1971)
of New York
(Proceedings
to
be
published)
9)
R. M. Carroll and 0. Sisman, Nucl. Sci. Engr. 21 (1965)
lo)
R.
147
M.
Carroll,
J. Amer. 11
)
J. W.
BMI-1787
12)
Reference Rev.
1,
)
R.
B.
Ceram. Sot.
Ray,
Battelle
Systems
R.
0.
Memorial
Perez
and
48 (1965)
0.
Sisman,
H.
Barnes,
55
Wooton
and R.
Institute
(USA)
Analysis
Report,
Report
no.
Vol.
II,
(1966) Safety 14-iv,
Westinghouse
Nuclear
Energy
(1970)
D. G. Jacobs, Sources of Tritium and Its Behavior upon
the
Salmon,
no. AWRE-O-47/65
S. Jones,
UKAEA
6) G. DiCola
13
Acknowledgements
P. M.
Report
no. AWRE-O-58/66
atoms.
would like to acknowledge
Gibson,
UKAEA
Vacuum
coolant of most power reactors if releases did However, experimental reapproach 100%. sults 1%la) have shown that measured tritium
reactor safety analysis reports. The discrepancy between prediction and observation can be explained by tritium trapping in rare gas stabilized voids. Helium is generated in the cladding through (n,a) reactions and fission product Kr and Xe can diffuse into the cladding from the fuel. The fast neutrons generate large vacancy concentrations and it is possible that
N. (USA)
(1953)
P. M.
from ternary fission would be the source of tritium in the primary
and 0.
Power Laboratory
3,
The authors
in
McDougall concentration
production
and concluded
expected to diffuse to the coolant. The diffusion coefficients measured in the present work are consistent with this interpretation. Tritium
tritium
Frank
and
R.
diffusing
Mr.
of tritium
and Mr. Charles Craft who performed release measurements.
2,
small rare gas stabilized
of
measurement
125
STEELS
primary
Wooten
tritium
reactor
for
in reactor
diffusion through
concern.
304-
Release
Critical Review
to
the
Environment,
Series (1968)
USAEC