Some chemical eouilibria for accident analysis in pressurised water reactor systems

Journal of Nuclear Materials 130(1985) North-Holland, Amsterdam

139

139-153

SDMF CHFMI~AL

EDIIILIRRIA FOR ACCInFMT

ANALYSIS

P.E. POTTER*,

M.H. RAND* and C.R. ALCOCK**

IN PRESSIIRISFD WATER REACTOR SYSTEMS

*Chemistrv and *Materials Development Divisions, Atomic Enerqy Research Establishment, Harwell, Didcot, Oxon, OX11 ORA, UK. **Department of Materials Science and Metallurqy, University of Toronto, Canada M5S-lA4. SOfW chemical eauilihria for the safetv analysis of water reactor systems are discussed. Part icular attention has been paid to the behaviour of the fission-product elements Cs, I and Te. An assessment of the thermodynamic properties of tellurium compounds has been made, and these values have been used in the calculations of the speciation of this element in appropriate conditions of temperatures concentration and mixtures of hvdroqen and steam. In Much use has addition some assessments of the chemical forms of Ra, Sr and Ru have been made. been made of the program SDLGASMIX for the calculation of the comolex enuilibria involved.

The likely behaviour

I. INTRDDllCTION In the analysis accidents

of the conseauences

for all nuclear

are concerned

reactor

with the chemical

fate of those radionuclides fission

product

elements

behaviour

hazard

present

some calculations

eouilibria

amounts

shall also consider the elements

reactor

development

accident

of the followinq

.

fuel.

in We

of

of a serious in terms

fuel

rod.

.

The chemical

.

The chemical

.

The behaviour

aspects

of the failure

of

fuel rods.

HZ/H20

fluorite

for water

the fission distributed

lattice

Droducts

product

in the core

of volatile

partfcularlv

environ~nts.

interaction

of Urania as a solid conditions

of fission

in the temperature

gradient elements,

the rare qases fKr and Xel, caesium,

iodine

and oossiblv

of fuel

tellurium.

Fxamination

rods2 showed that for intact rods irradiated

occurred. detected with

the

for that of the most volatile

the rare qases

fission

and ions are

At the normal operatinq

species

fuel

reactor At this

throuqhout

at a ratinq of 730 W cm -l, little behaviour

durinq and after a loss of coolant,

products,

temperature,

except

state of an operatinq

of the Urania

there will be little ~vement

tonics:

The chemical

proqram

STATF nF AN ~PFRATI~I~ FUEL ROD

can be as low as 1200°C.

essentially

solution.

is first reviewed,

equilibria

usinq the comouter

is clad in Zircalov

operation

Ra, Sr and Ru.

The qeneral

of chemical

The centre temperature which

elements

the likely behaviour

have been performed

2. THF CHFMICAL

are rather

from overheated

staqes of operation

conditions.

S~L~ASMIX~

and have been shown to be released

significant

above is then

to

of the chemical

for the fission-product

Cs, Te and I; these elements volatile

a

In this paoer we shall

the environment.

mentioned

at the relevant

The calculations

of the fuel,

if they were released

elements

considered

and accident

or

and other components

of a reactor core which could present radioloqical

product

of

systems, we

of each of the fission

release of

(e 1% of the total inventory)

A trace of Cs was sometimes on the claddinq

wall and there was

alwavs a laver of oxide in the inner surface of the claddinq.

140

P.E. Potter et al. / Some chemical equilibria for accident analysis

There are also observations3 constitution

of fuel (initially

on the

constitution

UO2 oo3) which

recentlv

had been irradiated (- 4% burn-uD)-at -1 of 430 W cm resulting in a centre temDerature

of 1800°C.

outer

3. CHEMICAL

Cs was ohserved

ROM.

In the

fuel rods is crackinq

each other;

this was also the case in some studies4 release

of caesium

to the fuel-clad

The oxygen potential siqnificant

influence

constitution

relationshin Dotential

or

between

lost bv reaction

the thermodynamic is influenced

oxvqen above3

Dotential.

oxvqen

bv the

Of new DhaSeS

In the study mentioned

Dellets

with

indicated

stoichiometric Zircaloy

that oxyqen

of the

amount had been qettered

bv the

cladding.

Evidence

for oxyqen

presented’

has

been

redistribution

in LWR

fuel rods; in fuel irradiated to very low -1 burn-ups, rated at 525 W cm , oxygen migrated

1820°C) whereas

rods with higher burn-up

(ca. 2%1, the oxygen

reacted with the claddinq potential Of

These

respectivelvl.

and the oxvqen

oxvqen are in aqreement on the behaviour stoichiometrv 6-8 . gradient the conditions the chanqe

with the observations

of temDerature

in oxvqen potential

by the claddinq

Some calculations

and burn-uD when with burn-uo

of the fuel rods.

on the chemical

temDeratures

is

represent

74

a-Zr

by a U-Zr sinqle

the number of reaction

and reaction

analvsis

kinetics

layers

is the same for all times.

A very

of the Uo2-Zr couple

of the II-7r-flDhaSe diaqram,

layers,

It would be useful to determine

at room

in two different

separated

and their sequence

in terms

and the transDort

of the system has recentlv

been

The analvsis was carried

and accounted

alloy sandwiched

a temperature

the

region and 24 is Zircaloy-4.

15On"C,

of Urania of varvinq

within

regions

Dublished".

of

of

between

ll-Zr + a-7r(nbl (7 ohasel

oxygen

dissolved

DroDerties

970°C and 1605°C

redistributions

1 a-zr(nla+ II-7r (? Dhasel

with

eleqant

of the fuel was lower at the centre

the rod (temoeratures

gettered

in the lower rated

is observed

and a-Zr(Olb

phase

has

The senuence

where a-Zr(Ola

diphasic

(to

is:

In qeneral,

to the hot centre of the fuel (centre temoerature

lavers at the interface

(2 Dhasel

is c.

does not occur

been DuhliShed".

which

The

study of the

recentlv

un2 + II

in excess

reaction

A detailed

reaction

temDerature

radius of the fuel

aDDlied pressure

between IIn and Zircalov

reactants

, the assessment of the variation of

oxvqen potential

in the II-7r-n ternarv

induce good contact)

reactions

the thermodynamic

of the fuel rods can also be of Un2 with Zircalov.

but without

below N vm”c.

fuel rod; whether

nucleation

occurs could determine

lowest solidus 13OC"C,

on the role

This is

in more detail later in the Daper.

on the chemical

The

for the failure of

due to stress corrosion,

of iodine in this Dhenomenon. discussed

which

nF FllFL

and there has been rmch discussion

The inteqritv

of an oDeratinq

SiqnifiCant

not.

mechanism

could have a

to the fuel clad qaD.

and burn-uo

temperature

on the

gap.

of the fission Droducts

have been released

ASPFCTS OF THF FAILURE

in the

A DOssible

in outer regions of the fuel.

reqion Cs and Zr accomDanied

gap have

a ratinq

Dores and cracks and Cs, MO and Xe were observed

of the fuel-clad

been Dublishedq.

out for

for the uranium-rich

between

the two a-Zr(n)

the SecJuence of nhases,

and the

of laver growth.

4. THF CHEMICAL

RFHAVInllR IN THF CnRF nllRINr;

AblllAFTER A LOSS OF COOLANT This toDic has been considered

in some

P.E. Potter et al. /Some

detail bv Ainscouqh possible

chemical

the

the temperature

increases,

in the core, the

fission nroduct

atoms and ions diffuse

the temperature

qradient

et a112, who discuss reactions

likelv composition

of the molten

ohases formed

and the fate of the fission products, and miscellaneous most

important

absorber

The senuence

materials.

events

141

chemical equilibria for accident analysis

leadinq

of

the qrain boundaries,

and qet released

in the U02

It is assumed that once the various

atoms and ions reach the qrain boundaries Temoerature

Phenomenon

800°C

Fuel rod starts to swell and burst

The more volatile

Zr + H211 reaction leads to rapid rise in temperature of fuel rods.

staqe of the core excursion,

900°C

enuilibrium

First visible linuid formation (either Inconel-Zircalov eutectic or Un2-Zircaloy reaction at exoosed internal clad surfaces).

1300-1500°C

thermodvnamics

fission

products

Stainless steel claddinq of control rod melts; Aq-In-Cd allov ejected. clad starts to melt.

2400-265O'C

Zirconia melt.

and lln2-Zrn2 mixtures

containinq oxygen

II (from Un2),

and metallic

structural

fission

All of the unoxidised

be molten

bv 19OO"C,

entirelv

molten

Zircalov

Zr-U-(Cr,Fe,Ni)-(fission-product)

decreasinq

will

when a phase

elements

of metallic

oxide) molten

oxide

phase will be

as the steam oxidation

of the volatile

of hydroqen

speciation

We have discussed which

the reaction

some release of the volatile to the fuel-clad

FISSIokl PRflpllCTS

can

of 1300-15OO"C, fission

The conditions

and oressure

the development

of

at a riven time

of a severe accident

examole,

on whether

depressurisation

for

there is a rapid

of the orimarv coolant

due to a larqe break in the circuit

svstem would

raoidlv

in the

fall to a value of a few

the depressurisation

due to a small leak.

is slow

We have selected

the

for a small leak, namelv 70 bar and

we have examined

the soeciation mixtures

of Cs, I and

of different

comoositions.

under

of Urania and Zircalov

take place and at temperatures

The

will denend uaon the tvoe of failure,

conditions

oroceeds.

the conditions

and steam has been the

of Cs, I and Te has been

Te in steam-hvdrooen 5. THF REHAVIOIIR OF VOLATILF

fission

assessments.

calculated(15*16*17'.

circuit

is of the

from failed fuel in the

suh.iect of several

durinq

of Te release

claddinq14.

bar) or whether

product

been

upon the extent of oxidation

(in which case the total pressure

metal

on a

Uo2-Zr02-(fission The proportion

(MO, Pd,

but the core will not be

until 2400-255O'C,

will be floatinq

metals,

products

Ru, Rhl.

phase

It has recently

that the extent

temoerature The first liquid phase will be a Zr-rich

but most of the

will remain in the debris of

deoendent

atmosohere

7ircalov

Cs, Rb, I, at an earlv

suqaested

aroduct

1850-1950°C

fission products,

the core materials.

Zircalov

then

can be anolied.

Rr, Te, Se and Sb will be released

The behaviour

1400-150n"c

from

for

which are less

new phases will nucleate

matrix.

is:

down

but in addition,

those fission products volatile,

to core melt

not onlv do the

products 13 qao can be expected . As

fi. IMVIVInllAL FISSInFI 6.1

In the fuel-clad caesium

PROI-MTS

Caesfum

depends

ciao the chemical

on the thermodvnamic

state of oxvgen

142

P.E. Potter et al. i Some c~g~~ca~ equilibria for accident analysis

potential

of the oxide fuel.

the chemical carried

out bv Besmann

indicated combined

Calculations

and Lindemer'

with the fuel as the uranates

and Cs2U207,

the actual constitution

excess of caesium

these ternarv co~ounds.

has been presented

present

to form

with most

bv Cubicciotti

It was also suggested'

examination

and that Cs2Te

no measured

the~dynamic

estimates

of fuels.

There are

data for Cs2Te

have been made using data 20 .

for other alkali metal chalcogenides Potter

and Rand'5 have recentlv

the chemical the gaseous

constitution species

PWR fuel rod, Cs2UO3

in the fuel-clad

uranates

discussed

above.

Amounts of condensed phases (mol)

UO2-x

7.33

csx Cs2Te

2.25~10~ 6.25~10-~

been included and Westrum

which

by Fee

for the

.

The range of homogeneity

is simulated

(0.01 < x < 0.1) with appropriate

-910

After addition

Case 2. 1102

7.33

Cs? Cs2UO3.56

2.25x10_5 ;.;;;;;_5

Cs2Te

by addinq discrete

of formation

of

phases Gibbs

to the phase

assemblage. For the calculation

of the chemical

of a fuel rod we have assumed

that 1% of the total inventorv volatile

7.1x10-1 3.5x101$ Csi 2.1x10 (CsI)2 3.9x10-4

tial kJ mo102'X

-5

.

of SXIO-~ mol 02

ts

3.8x10-1

Cs Cs?

l.OxlO~~ 2.1x10

-577

ICsI12 3.9x10-4 I

Te Te2

3.3x10-16 9.1x10-12 4.6x10-l5

have been taken from Cordfunke

U02yx

constitution

;,,,,n

Te2

has also

UO2+x

eneTqies

Pressures of predominant species (bar)

Te

of Cs Uil 21 2 3.56 have been given bv these authors and bv Adamson Data for Cs2U4012

Te 6.25 x 10e5

CS CS

Gibbs enerqv of formation

et a122.

elements/mol

Case I

qap of a

to the two

Values

and the results

I

as a possible

phase of the system in addition

is

of oxygen

are shown in Table 1.

of fission oroduct

of

In this work the phase

56, which has been described

increases

Temoeratu~ lOOOK. Vogue of free space in fuel rod 27.61 cm3 Pressure of He and rare qases 78.32 bar Initial amount of II02 7.33 mol.

calculated

and the pressure

and Johnson*' ,was included

The conditions

Cs :.ii ; M:; I 2.35 x 10m5 . He + rare Oases

Cs2Te, which malts congruentlv at 810 + 1O+C18,19 has not been observed in postirradiation

as burn-up

oxygen

Table 1 chemical constitutrdirin the region of the tuel-cian qan o+ a PWH tuet ron

Amounts

of this compound

could also be formed in the fuel-clad qap.

althouqh

to the system.

iodine to form CsI -

for the formation

Senecki'.

occurrinq

bv adding small increments

The

ensure

The increasinq

simulated

of the calculations

depen-

Some of the caesium

(about 10%) will also be combined of the available

Cs2uO4

is 2.9%.

potential

The larqe

over iodine would

that there is enough caesium

Te) will be found in the gap and the burn-up of uranium

who

that some of the Cs would be

ding on the oxyqen potential.

evidence

of

state of caesium were first

fission products

of the

(Kr, Xe, Cs, I and

Case 3

After addition

7.33 2.25x10-5 C~2U1?3,56 3.02~10-~ 7.65x10-5 ;:2;;4 6.25~10-~ 2

un cs f

of 1~10~~ mol flp(totall

cs Cs CST fCsTl2 I Te Te2

1.4x10-l 1.4x10-3 2.1x10-3 3.9x10-4

-556

143

P. E. Potter et al. / Some chemical equilibria for accident analysis

Table 1 (continued)

mixtures

Pressures of oredominant sDecies (bar)

Amounts of condensed phases (mol)

nxvqen potential kJ molOpI

of hvdroqen

calculated

of the various

IIn*

7.33

n2(total) -450

2.4~10-~

CS

-5 7.25x10 1.50x10-4 4.38~10-~

CSI

of 1.5x10-4mol

4.1x10-9 cs cs T 2.1x10-3 (CsI12 3.9x1o-4 I Te

CsOH

is

7.33

3.0x10-* 6.5x10-17 2.1x1r3 3.9x10-4

T.S

cs T (CsI12

2.25x10:45 Cs 2 IJtl 4 1.94x10

I Te

It has been assumed

the reactions

phases

extremely

coatinq

Hz+H*O = 9Onwl cs =4.11x10-2 mol 11 = 1.19x1o-~md

70 bar

i

-301

CsOH

0-

P -2 t

to the Dresence

As the oxvqen

with caesium

of an

Dotential

the assemblaqe Iodine

as CsI.

of

of

is always

Except

at

low oxygen potentials

oxygen potential

Cs is combined

siqnificant

Log(H1/&0)

in fact to U02_xI Cs is always

as one or more uranates,

at the highest 5,

Pressure

of Zr02 on the inner wall

is increased,

(corresponding combined

Total

takes no Dart in

in the gap changes.

associated

TemPemturc ncilw

in this region of a fuel at

of the cladding. the svstem

The pressure

at low H2/H20

in these calculations

cladding

lOOOK due, for example, impermeable

constant.

negliqible

and thet of

4.1x10-9 4.1x10-5 4.3x10-2

Te2

that the Zircalov

essentiallv

are

temperature,

ratios.

of ?~lfJ-~ mol n2(total) cs

CSI

of CsI is constant

of Cs becomes

0

““2+x

species

At constant

5.1x10-13 2.2x10-5 2.8~10-~

Te2 Case 5 After addition

are shown in Fiq. 1.

Cs-containing

CsoH, CsI and Cs. the pressure

of the pressures

sDecies with the composition

of the qaseous mixtures The Dredominant

Case 4 After addition

and steam has been

and the variation

and except

qiven in Case

with Te as Cs2Te.

qaseous

species

cs, cs2, CsI and Cs212; Cs20 would be present

The

of caesium

the molecules

are

sDecies

hvdroqen-water

of caesium

and iodine in

mixtures

CsO and

in insignificant

amounts. The soeciation

FIGURE 1 The qaseous

After the formation of caesium

in various

of CsnH and CsI in the

qas phase these species can condense aerosols

which

subsenuentlv

deposit

to form on the

o

144

P.E. Potter et al. /Some

surfaces

of the primary

containment.

circuit

For a detailed

likely behaviour with stainless

or the reactor

assessment

of these mecies

knowledqe

of

and thermodynamics

of

Cs, I and n with the major components stainless

of the

in contact

steel, a thorouqh

the phase relationships

steel is required.

chemical equilibria for accident analysis

of

In this paper we

have carried

out a detailed

assessment

experimental

data on the Cs-Cr-0

lCs,O)

svstem which

we now discuss. system

A review and assessment

included

were used to construct diaaram

(loq

temperature

$

of the

pcs

E

_I

the predominance for the Cs-Cr-0

Cs~cro,

a.__*

-30

is shown in Fia. 7 for a

c54cro4

15x[ol

of ROOK.

The Ellinqham

lO’hIC1

diaqram

(see Appendix)

that Cs$rfl4

should be marqinally

than Cs4CrO4

in contact with caesium

and chromium

up to 800K.

difference

2

All of these data

loo p(n211

vs

is well within

shows

more stable

However,

liquid

the

the uncertaintv

D

Cc2 03

5XKJl

1‘I. [Ol -40

0

in

the data for Cs5Cri14, so this phase has been omitted

from the predominance

Other estimated in the diagram potentials

auantities

system,

UP

to CsO2(11.

LcqLpCslbar)

FIGIIRF 2 The

Predominance

In order to estimate

Anproximate

qiven in the Appendix

of these oxides were used

at 800K to obtain

the Gibbs energies

of

at

It was then assumed

the inteqral Gibbs enerqy of formation

to

of the

liquid phase, and a smooth curve was drawn for of composition

values of AEn

system were derived values

so obtained

upper boundarv

that these data gave a good approximation

this as a function

for Cs-Cr-0

the

the three

for the formation

of the oxides.

area diaqram

ROOK.

around 860K and

of the linuid phase,

Gibbs enerqv enuations

formation

-8

are included

(Fig. 3) shows that Cs20

at 768K, Cs2n2 prohablv

oroperties

I

-6

are the oxygen and caesium

Cs-0 phase diagram'6

CsO2 at 723K.

I

-4

in the liquid state for the

caesium-oxygen

melts

I

1

-2

area diaqram.

which

CszCrO&

0” -20

data for this system is

in the Appendix.

system which

_:1 c**o

6.1.1 The Cs-Cr-O

thermochemical

liquid

of the

at ROOK.

diaaram.

solutions These

area

show the oxyqen pressures caesium-oxvqen

studied

authors

were used to obtain the

in the nredominance

Arrows

the homoqeneous

and AEcs across the 2 from this curve and the

by Kniqhts

dilute and Phillips

also made an eouilihrium

of the svstem usinq vanour pressure

and

24

of

.

study

145

P.E. Potter et al. /Some chemical equilibriafor accident analysis galvanic

cell methods.

pressures

They obtained

caesium

reoresent

more correctlv

multiohase

for the ohase fields

The oxvqen potentials and Phillinsz4

Cs2CrO4/Cs3CrO4/Cr203 = -163,000

AL(g)

are also different

(750-95010

from the thermal

in the

by Knights svstems

from the value obtained

data in the Cs4Cr04/Cs3Cr04/ Thfl2-Y2n3 electrolvtes

Cr2n3 enuilibrium.

= -107,000 61: Cs(g)

obtained

for these multinhase

+ 91T + 200 J/mol

Cs3Cr04/Cs4Cr04/Cr203

the ootentials

assemblv.

t 84.7T J/mol

were used to measure

(500-68010

there seems little reason to doubt that the electrolytes

could

function

these temoeratures The authors'

and

satisfactorily

contention

that the assembly

is stable at these

does not apoear to agree with

conclusions

from the thermal

data where

enuilibrium

Cs(l)/Cs4Crf14/Cr

is stable.

is some exoerimental this

SllDD0rt.S

Vh?W25.

in stahilitv assemblaqes

between

information However,

the There

which the difference

the two alternative

is quite small, and kinetic

factors could well determine aopears

at

and oxyqen potentials.

C~(l)/Cs~C~~/Cr20~ temoeratures

these potentials

which

svstem

to be more stable durinq the course of

an exoeriment. Finally ternary

the data can be used to construct

section

FIOOK, in Fig. 4. solutions

FIGIIPF 3 The Cs-0 binary

of the liquid

phase diaqram

and this is shown, aqain for Because

with Cs4Cr04

to fix preciselv

and Cs3Cr04,

q73K, and the predominance

results

pressures

area diaqram

from these

discrepancy

of information.

remembered

between

the two

It should be data are for

individual

oure substances,

and multiphase

eouilibria

which are calculated

from these

data assume no ranqes of homogeneity phases.

The eouilibrium

that the oxvqen

(Fiq.

and Phillips

to the bfnaw

in the data may

and

of

at

area diagram, solution

25 and 30 mole oercent

Accordinq

it is

but comparison

contains

oxyqen.

Cs-0 diaqram

3) Cs202 is still solid at 800K,

there must also he two three-ahase

that the thermal

coexistinq

suqqests between

(Fis. 2) and it can be seen that

there is a marked sources

calculated

at 800K are shown on the oredominance

comdete,

the composition

(Cs, 0) ohase in eouilibrium

the data given by Knights The caesium

the data par the

are not sufficientlv

not possible

a

26 so

fields

cs3crn4 + cs2n2 + csxoI_x(l)

x > 0.5

Cs2Cr04

x < 0.5

+ Cs202 + CS~OI_~(~)

146

P.E. Potter et al. / Some chemical equilibria for accident analysis

has been much discussion

as to the possible

role of iodine as the initiator stress-corrosion claddinq.

crackinq

Some laboratorv

that CsI does not cause stress

corrosion

crackinq2*,

necessarv Zfrcaloy

althouqh

is not completelv

was fndicated3n

for stress corrosion

that such an iodine ootential

stabilfsed

by formation

qetterinq

of the increase

system

at ROOK

would

are not known at all precisely.

but the compositions

It is clear that the existfnq Cs-Cr-0

system are not entirelv

but the assessed

interactions

in mixtures

qaseous molecules with chromium the reactor

and/or aerosol

in stainless

system.

for the future.

svstems

of importance

more uncertain 6.2

oarticles)

steel surfaces

Such calculations

planned

as

of

are

The data for other

(e.g. Cs-Fe-O)

are even

Iodine

In the fuel-clad fuel-rod, as caesfum

gap of an ooerating

iodine will be combined iodide; the pressures

iodine will be extremelv

with caesium of elemental

low (see Table 1)

even at the hfqher oxvqen potentials.

There

ClaD

under these that for the in

operatinq

fuel rods that the hiah stahilftv

CsI would

orevent

sufficiently

the attainment

for the

based on thermodvna-

If iodine is responsible crackinq

of Zircalov

for

then

such as the radiolvsis

must be involved, 31 of CsT , or the non-

attainment

equilibrium

After

mechanisms

of chemical

fuel-clad

in the

qan. release

hvdrooen,

into mixtures

the major

iodine

of steam and

SDeCieS

smaller amount of the dimer.

is

csI(g),

For the

conditions

qiven in Fiq. 1 for 1300K, the

hvdrolvsis

and deCOmoOsitiOn

caesium

of

of

hiqh iodine oressures

of mechanisms

non-equilibrium

with

than those for Cs-Cr-0.

are

low centre

It seems orobable

stress corrosion

of CsOH

claddinq.

(e.a. 12ofJ"C), since MO will not

mic arquments.

of steam and hydroqen

can

is Such

which will be encountered

oneration

data can now be used to

studv the oossfble (present

data for the compatible,

to

to assume that

form in fuels which

to the fuel-clad

conditions section,

of

in oxygen potential

bv the Zircalov

at relatively

conditions.

on the ternarv

It

that there is no

it is inaoorooriate

temoeratures miqrate

of the Tfnuids

renuire

irradiation

oneratinq

for the Cs-Cr-0

of Cs2Mo04.

would

Moreover,

FIGURE 4

.

it is

in a LLIR fuel Din, if caesium

Cs2Moo4

29

crackinq

zirconium;

formation

durinq

accented

is equal to the ootential required

suqaested prevail

this

that the iodine potential

form ZrI with metallic

section

tests have

indicated

observation

An isothermal

of

of the Zircalov

iodide

is neqliaible.

HI(q) and I(q) increase or the temperature

of qaseous The pressure:

as the H20/H2

increases,

ratio

hut thev are

of

147

P.E. Patter et al. / Some chemical equilibria for accident analysis

always neqliqible 6.3

in comparison

with CsI(q).

Tellurium

In an operatinq

fuel rod the predominant

tellurium-containinq Drobably

oxyqen

After

mixtures

release

of hvdroqen

(case 51, where mainly

as Te2(gl

oxyqen

We have taken

6.25 mm01 of Te in mixtures

of 90 moles H2

and H20 at 1300K and a total pressure bar.

It is aoparent

major

qaseous

increase ratio),

at the lower oxvqen

H2Te, Te and Te2, but with

in the oxyqen potential the pressure

pressures

(H20/H2

of H2Te falls whilst

of the oxide and hydroxide

increase.

of 70

from Fig. 5 that the

species

are

all

and steam the speciation

of the system.

potentials

HZ+ $0 = 90 mol Te -6.25x lO-‘mol

from the fuel into

of Te will deDend uDon the Drecise Dotential

Tem~aturc 1300 K Total Pressure 70 bar

will be formed at all the

potentials

the Te will be vaporized, molecules.

0

most

It is seen from Table 1

be Cs2Te.

that this compound hiqhest

species would

The contribution

iodides TeI, Te12

the

species

from the gaseous

and TeI4 is negligible

under all conditions. The thermodynamic employed

data which we have

in these calculations

a recent assessment be published 6.4

by the authors which will

elsewhere.

Rarium

differinq

their compounds the various

are very similar,

stabilities

of some of

formed

(Ra,Srln or (Ba,Sr)Zr03

ratio in

is by no means

elements

6.4.1

Normal

nurinq

normal operation,

fraction

operation

the Urania matrix

the barium and

(the mole

of Ba and Sr are about 0.002).

eouilfbrium

increases

phase assemblaqe

This may be reqarded

will be in intimate

slowlv,

Althouqh

contact

during

Ba12 is almost as stable as CsI,

the much qreater

In the first staqes of a severe accident, as the temperature

is

ratio of

ooerationl.

are likely to be dispersed

throughout

in IlO (zirconium

also a fission oroduct with a molar

Zr/(Ba + Sri around 2, so that the three

constant.

strontium

in hvdroqen-

together,

means that the Ba/Sr

Dhases

species of tellurium

water mixtures

are considered

since their properties

FIGURE 5 The gaseous

and strontium

These elements

althouqh

are taken from

the

of

of BaO (let alone

cornDared with Cs20 (or solutions

Oxyqen

in Csl means that very little Ba12

will be formed during Dostulated

will be formed.

as a solid solution

stability

Ra7M31

6.4.2

accident Behaviour

As noted earlier,

normal operation

of

or any

sequence. in a deqraded

core

it is anticipated

that a

148

P. E. Potter et al. /Some

degrading

core will consist

of a metallic

phase containing

Zr-U-Cr-Fe-Ni

fission products

olus an increasinq

chemical equilibria for accident analysis

activity

and metallic

of the oxide or zirconate

in the

melt.

amount of

an oxide phase of Zr(l2-1102-fission product

Table 2

oxides. The barium and strontium will undoubtedly where

the opportunity

be much enhanced. themselves presence M~H(g) There

not very volatile,

of hydroqen-steam

and M(O~)2(q)

during boil-off,

Temperature Initial

H2 + H+l =

when residual

vessel

is

The data for the condensed

product

core has melted

through

H2/H20

the

water

in the sump area - includinq

difficult

pressure

or (Ba,Sr)ZrO3,

(in each case

for boil-off,

conditions

ratios will and the

If realistic

The pressures

species

are

Barium

than strontium,

the oxide melt can be

these calculations

enable

rate of loss of barium and strontium from the system

oxide or zirconate lower activity, estimate.

which

is however

The pressures

much upper limit values. contain

difficult

are therefore

to

vew

Since the gases all

only one Ba or Sr atom, the actual

pressures

6.5

will have a considerably

will be lower bv the the~odvnamic

oxide

in barium.

fRa,Srfn or (Ra,Sr~Z~3.

the

so that

(where the

the condensed

given in Table 2 are those for unit activity In practice,

as the

species are

rates of flow of the H21H2n

throuqh

established,

at high oxyqen

with the oxides

are hiqher)

phase is depleted

mixtures

species are in Figs. 6 and

to be reTaced hv M~Htq)

rather more volatile

pressures

for the interaction

steam from concrete.

of H2/H20

the M(OH)2(g)

for the enuilibria

conditions

The lower X20/H,

be more appropriate

with

of individual

tl,/U20 ratio increases.

with a solid solution

with ZrO (s) also) for tvaicaf

to form ideal solid or

As miqht he expected,

predominant,

the vapour

stabilities

solutions.

potentials,

of the two cases.

calculated

in equilibrium

steam-rich

were assumed

The pressures

7. and

whose

oxides) were taken from

The barium and strontium

et a133.

compounds linuid

to calculate

ratios for either

(see Table 21.

Waqman

that bound in the concrete

the temperature,

of (Ba,Sr)O

for the zirconates,

(from the constituent

shown as a function

We have therefore species

and gaseous 32 are taken from the JANAF tables ,

soecies

accident

vessel and is vaporizinq

It is clearly

< 10

water

structure.

accurately

1200 mol

could occur:

at a later staqe in a possible

particularly

Ba 7.35 mol

0.1 < Hz/M20

except

present

3 bar.

Sr R.ll mol

seouence

heat.

pressure

compositions:

volatile

in an accident

of barium

from oxide melts

2250K; Total nressure

can be formed.

the reactor pressure

if a molten

for vanorization

will

but in the

being boiled off by fission

-

Conditions

and strontium

are

mixtures,

suecies

are two periods

within

products

to form zirconates

These materials

when such interaction -

fission

remain in the oxide phase,

the

saecies

to be estimated.

Ruthenium

As noted earlier, ruthenium associated

it is likely that the

formed bv fission will be with the other

‘noble' metals

Tc, Rh, Ru, Pdf when the core is heatinq to form the familiar

'white inclusions'

(MO, un found

149

P E. Potter et al. / Some chemical equilibria for accident analysis

RuO4(ql,

and the stabilities

hydroxides

have recently 34

bv Krikorian

all these data to calculate

-1

ruthenium-contafninq

t

a metal

sr (OH)*

-2

1

species

alloy containinq

We have used

.

the pressures

of

in contact with

ruthenium.

+--+------+

3

‘p

and oxyhydroxfdes

been estimated

“r

of a number of

_J

z

2

-4

-5

-6

-1.20

-0.60

As for barium and strontium, vaporization

core is heatinq

I

-0 LO

0.00 Log

4

I

0.40

I

I

0.60

occur as the

and meltinq,

IID

or after when

a molten core has melted

throuqh

vessel and is vaoorizino

water

the pressure

from the sump

area.

(HZ/tl~Ol

We have used the same conditions the barium and strontium

FIGURE 6 The qaseous

ruthenium

could in principle

species

thermodvnamic

of Ra and Sr over

metallic

(Ra,Sr)O + Zr02

This

activity

as for

vaporization.

of ruthenium

allov has been taken to be 0.1.

is almost certainly

The mole fraction Pd) metallic However,

an overestimate.

of Ru in the (Mo,Tc,Rh,Ru,

inclusion

is about 0.3.

as noted bv Ainscouqh

et al

12

mole fracton of this fission product

0

the Zr-U-steel

-1

i

so a ruthenium probablv

-6-

w"

closer to 0.01 is when the core is

For vaporization

core-catcher,

from the

the fission-product

phase will be even further

-7-6 -1.20

-0.M

-040

0.00

0.40

0.60

Log(H*IH~O)

FIGURE 7 The gaseous

species

(Ba,Sr)ZrO3

+ Zt-02

of Ba and Sr over

1 120

dissolution other

into melted

structural

materials.

activitv

even smaller.

Aqain,

vapour species

all contain

metallic

diluted by

pressure

thermodvnamic

vessel and

Hence the

of Ru is likelv to be as with

atom, and their pressures proportional

alloy in

nhase has been oxidised,

activitv

more realistic

melting.

the

phase is only - 0.043 after

75% of the metallic

-2

The

in the

(Ba,Sr),

the

only one ruthenium are thus directly

to the assumed

ruthenium

activity. in most irradiated core melting metallic

phases

are not hiqh enough

to form R&2.

form qaseous

During

into the

since the oxyqen

for the steam-hydroqen

does however

fuel.

these will be subsumed

Zr-U-steel

potentials

fast reactor

mixtures Ruthenium

oxides Ruo3(q)

and

Typical

pressures

of Ru-containing

are qiven in Table 3. hvdroqen

conditions

pu qaseous amounts

species

Under the steam/

assumed, are RuNI(

of RuO(OH)(q)

species

the predominant with minor

and Ru(flH)2(q).

As for the vaporization

of Ba and Sr, total

150

P.E. Potter et al. / Some chemical equilibria for accident analysis

release

fractions

from anv assumed

for Ru can be calculated

REFERENCES

flow rates of H2/H2f1

1.

6. Eriksson,

Chem. Scriota -8 (19791 103.

2.

D. Cubicciotti, J.E. Sanecki, Mats. -78 (1978) 96.

3 . .

H. Kleykamp, 109.

4.

N. Oi, Internal Fuel Rod Chemistry, IWGFPT/3 (IAEA Vienna 1979).

5.

M.G. Adamson, F.A. Aitken, S.K. Evans, J.H. Davies, Thermodynamics of Nuclear Materials Vol. I (IAEA Vienna 1974) pp. 59-72.

6.

M.H. Rand, L.F.J. Poherts, Vol. I (IAEA Vienna 19661

mixtures. Table 3 Vaoorization

of ruthenium

metallic

melts

snecies

from

la,,, = 0.1)

Temperature

Ru(DH)(gl

5

3.6xlO-(j

1.4~10-~

3.8~10-~’

1~1O-l~

0.2

4.0x10-5

3.8~10-~

4.7x10-8

1.6~10-~

RuO(OH)(gl

Ru(OHl2(g)

Ru03(g)

CONCLUSIONS This paper has reviewed

the chemistry

of

7.

the core of a PWR, both for normal ooeration and durinq core melt following We have attempted

coolant. complex

chemical

occurrinq

fuel rod.

It is our intention

collection

of critically

the in a

to develoo

assessed

a

thermo-

dynamic

data for all the condensed

gaseous

soecies likely to be encountered

phases and at

and accident

situations. We have shown aspects

of the behaviour

Cs, I, Te, Ra, Sr and Ru, all potentially

hazardous

conditions

of temperature,

potential

and steamlhydroqen

defined chemical

or estimated,

of which

radionuclides.

accident

the

P. Hofmann, n.K. Kerwin-Peck, KfK-3552 (19831.

11.

n.~. blander, 271.

12.

J.R. Ainscouqh, F.D. Hindle, P.F. Potter, M.H. Rand, IJKAEA Report ND-R-610(S) (editor J.H. Gittusl, Chaoter V, DD. 199-223.

13.

R.A. Lorenz, J.L. Collins, A.P. Malinauskas, M.F. Osborne, R.L. Towns, Peoort hllIRFG/CR-I3R6 (nRML/FIIIRFG/TM3346) (19RO).

14.

R.A. Lorenz, F.C. Reahm, P.F. Wichner, Proc. International Meeting on LWR Severe Accident Fvaluation, Vol. 1, Amer. Nucl. SOC. (1983) D. 4.4-1.

15.

P.E. Potter, M.H. Rand, CALPHAD -7 (19831 165.

16.

F. Garisto,

17.

R.R. Seqhal, D. Cuhicciotti, Proc. Int. Meetinq on LWR Severe Accident ;va;;a;i;n, Amer. Nucl. Sot. (1983)

oxvqen

flow rate can be

for a

to Dr. Gunnar

for his generous

computer

program SDLGASMIX,

stimulatinq

provision

-38 (1971)

10.

ACKNOWLEDGEMENT We are most qrateful

Mats.

T.M. Resmann, T.R. Lindemar, Technology -40 (1978) 297.

seouence.

Eriksson

blucl.

9.

of

the predominant

species can be determined

particular

J.

Nuclear

ReDort

J. Nucl. Mats. -115 (19831

have If

oressure,

M.G. Adamson, 213.

Thermodvnamics l-31.

DD.

M.G. Adamson, R.F.A. Carnev, J. Nucl. Mats. -54 (1974) 121.

a loss of

to examine

equilibria

all stages of operation

J. Nucl. Mats. -84 (1979

2250K.

"2 F

7.

J. Nut 1.

of the

and for many

discussions.

. 18.

Reoort AECL-77A2

(1982).

.-.

A. Rergmann, (1937) 269.

Z. Anorq. Allq. Chem. -231

151

P.E. Potter et al. / Some chemical equilibriafor accident analysis

19. M.G. Adamson, J. Vucl. Mats. -114 (1983) 327.

37.

20. T.R. Lindemer, T.M. Resmann, C.F. Johnson, J. Nucl. Mats. -100 (1981) 178.

311. P.A.G. O'Hare, J. Roerio and K.J. Jensen,

21.

39.

D.C. Fee, C.E. Johnson, (1981) 107.

J. Nucl. Mats. -99

22. M.G. Adamson, E.A. Aitken, R.W. Caputi, P.E. Potter, M.A. Mignenellf, Thermodynamics of Nuclear Materials (IAFA Vienna 1qADl Vol. I, D. 503.

23.

F.H.P. Cordfunke, Vol. II, p. 125.

F.F. Westrum

Jr.,

D.R. Frederickson, G.K. Johnson and P.A.G. O'Hare ibidE (19801 801.

ibid 2 (1976) 381. W.G. Lyon, D.W. Osborne ibid 8 (1976) 373.

40. K-Y. Kim, G.K. Johnson, C.E. Johnson, H.E. Flotow, E.H. Appelman, P.A.G. n'Hare and R.A. Phillips, ibid -13

I979

ibid,

Hiqh Temperature themistrv of Inorqanic and Ceramic Materials, ed. F.P. Glasser and P.E. Potter, Chem. Sot. London (1977) 0.134.

41. K-Y. Kim, G.K. Johnson, P.A.G. O'Hare and R.A. Phillios, ibid2 (1981) 695. 42.

S.P. Rerardinelli and D.L. Kraus, Tnorqanic Chem. _I3_(19741 189.

APPFNDIX

25. D.G. Fee, K.Y. Kim and C.F. Johnson, J.

Thermochemical

26. C.F. Kniahts and R.A. PhilliDs, J. Nucl.

calorimetric

Nucl. Mats. -84 (1979) 286.

Arqonne

27. D.R. nlander, J. Nucl. Mats. -110 (1982)

343-345.

S.H. Shann, D.R. Olander, -113 (19R3) 234.

J. Cucl. Mats.

R. Cox, V.C. Linq, Report (1984).

AFCL-8269

30. 0. Goetzmann, 185.

J. Nucl. Mats. -107 (1982)

J.H. Davis, Nucl. Sci. Eng. -60 (1976) 314.

32. JANAF Thermochemical

Tables, Project Directors D.R. Stull, H. Prophet, 2nd Fdition, Washington D.C. (1971) and supolements.

Waqman, W.H. Fvans, V.R. Parker, R.H. Schumn, I. Halow, S.M. Railev, K.L. Churnev and R.L. Huttall, J. Phys. Chem. Ref. Data 11 Supplement No. 2 (19821).

33. 0.0.

D.H. Krikorfan, High Temperatures-Hiqh Pressures 14 (1982) 387.

35. P.A.G. D'Hare and J. Poerio, J. Chem. Thermo.

36.

7 (1975) 1195.

W.G. Lvon, n.w. nsborne ibid -7 (1975) 1189.

studies

National

carried

(equilibrium

and H.F. FlOtow,

formation

Svstem

data are derived oriqinating

Laboratorv

and vapour pressure mainlv

from

in the

(thermal data),

and qalvanic

cell studies

out at AFRF Harwell

data).

The thermal

31. IT. Cubicciotti,

34.

Data for the Cs-Cr-D

The thermochemical

Mats. -84 (19791 196.

29.

(1981)

333.

24. C.F. Kniqhts and B.A. Phillips,

28.

and H.E. Flotow,

data provide

and entropies

enthalpies

at 2qP.15K

of

for the

Cs2C,r2f1738s3g, and the heat capacities comDounds

up to

lOOOKfor all of these

except Cs4CrG4.

of this substance experimental

The heat capacitv

was estimated

from the

value at 300K, and by comparison

with the temperature

variations

the other compounds,

removinq

obtained

the effects

for of

second order transitions. Comhinfnq

these data for each COInpOund with

tabulated

data for the elements,

followinq

Gibbs enerqv of formation equations

2CSll)

we obtain the

+ 2Cr + 315n2 + cs2cr2r17 AGo = -2083,000

2Cs(ll + Cr +

+ 587.0 T J mol-'

2n2 + cs2cro4

AG" = -1419,000

+ 357.6 T J mol-'

P.E. Potter et al. I Some chemical equilibria for accident analysis

152

3CsIl) + Cr + 202 + Cs3CrC AG"

4 + 394.2 T J ,1-l

-1542,000

q

were ohtained formation

by combination

In a similar manner, 4CsIT) + Cr + 2n2 + Cs4Cr04 AGo = -1591,000

data32

+ 427.6 T J mol-'

with use of tabulated

for the evaooration

the caesium

oressures

ohase equilibria These enuations represent the Gibbs energies -1 to -+ 2 kJ mol for carefully prepared

CS~C~~/CS~CPO~

samples.

of the thermal

data.

of liquid caesium,

in the two condensed

Cs3Cr04/Cs2Cr04

and

were calculated:

Cs(q) + cs2crf14 + cs3crn4

The possibility

of Cs5Cr04

taking

part in

AG" = -194,200 + 111.43 T J mol-'

the corrosion reactions has been tentatively 25 sugqested and the data for this substance have been estimated

from the corresponding

data for the other monochromates. enerqv equation

Cs(q) + cs3crfJ4

so obtained

AG"

The Gibbs

was

These

thermal

Fllinqham

q

+ cs4cro4

-120,180

+ 108.23 T J II&'

data are oresented

diaqram

in the

(Fiq. 8).

5Cs(l) + Cr + 202 + Cs5Crn4 AG" = -1613,000

+ 457.4 T J mol-' (+ 3000)

llsinq tabulated

data

32

for the chromium/Cr2G3

eauilibrium

2Cr + 3/202 + Cr203 AG” = -1121,000

the oxvqen ootentials phase

+ 253.4 T J mol'l

at which

the three

systems occur may be calculated.

for the co-existence

of Cs2CrG4,

Cs3Cr04

Thus and

Cr203 the eouation

8/5Cs3Cr04

+ 2/5Cr203 AG’

was derived, 12

+ O2 + 12/5Cs2Cr04

= -489,400

-1 t 126.16 T J mol-'

and for eouilibria

/5Cs4Cr04

+ 2/5Cr203

+ O2 + 16/5cs3cr04

AGo = -667,600 and 16 /5Cs(l) + 2/5Cr203

+ 133.84 T J mol-'

+ O2 + 4/5Cs4Cr04

AG" = -824.400

FIGIIRE 8

+ 240.72 T J mol-' Fllinqham

diaqram

for the Cs-Cr-0

system

P. E. Potter et al. /Some

The equilibrium caesium-oxgen

properties

for the

svstem have been determined bv 26 and Rerardinelli and

Knights

and Phillips

Kraus4*

usinq solid electrolyte

cells and direct dissociation measurements Knights

respectivelv.

and Phillips

the solutions

of oxygen

the svstem.

information

The results

of Rerardinelli

Kraus cover the formation

for the oxides

2Cs(l)

+ Jso* + cs2n t 136 T J mol-'

2Cs(i) t n2 + AC" = -306,500

for and

These

with the following

AC' = -348,400

solutions

and Phillips

to

for

in liauid caesium were

bv the authors

UP to 16 mole percent

and it was found that the oxygen

potentials

could be represented

by the

equation

= -583,800+156T+2RT(2140/T-0.33)

AE

cs2n2 + 162 T J mol-'

Cs(l) + n2 + Cs02 AGo = -234,901) t 119 T J mol-I

In

O2 J mol-'

of the oxides ts202

up to 500°C) and Csn2 (UD to 425°C).

eouations

in these data is estimated

of Cs2fl up to

point, and the phase diaqram

data can be summarized

oxygen, for

153

The results of Kniqhts

analvzed

in liquid caesium,

the Gibbs enerqv of formation the meltinq

The uncertaintv -1 be 2 kJ mol .

the oxvqen

galvanic

pressure

The data of

orovide

chemical equilibria for accident analysis

where

x o is the mole fraction

of oxvqen.

X

0