- Email: [email protected]
Chemical equilibria for accident analysis in pressurised water nuclear reactor systems
CALPHAD Vo1.7, No.2, Printed in the USA.
pp. 165-174,
1983
0364-5916/83 $3.00 + .OO (c) 1983 Pergamon Press Ltd.
ILQUILIRRIA mR ACCIPRESURISEDWATERNUCLEARREAClUR-
CHEMICAL
Chemistry
and Materials
ANALYSIS
P. E. Potter and M. H. Rand Development Divisions, AERE, Harwell,
IN
Oxon, OX11 ORA, U.K.
In this paper some chemical equilibria for the safety analysis of water cooled nuclear reactors have been calculated using the programme SOLGASMIX. Particular attention has been paid to the behaviour of the fission product elements Cs, I, Te, Ba and Sr. The chemical states of these elements have been calculated for a variety of conditions likely to be encountered in the core of a reactor before and during degradation.
1.
Introduction
The purpose of this paper is to describe some of the complex chemical equilibria which can occur in or near the core of a water-cooled nuclear reactor such as a pressurised water reactor (PWR). When the 235U atoms
of the enriched Urania (00,) fuel undergo fission, over thirty new fission product elements are formed. A detailed knowledge of the chemical changes which can occur within the fuel as a result of fission 1s of great importance in the interpretation and prediction of the behaviour of fuel in normal operating and accident conditions. We shall discuss some equilibria of relevance to the operation of PWR fuel rods under normal conditions, and then in conditions appropriate to a loss of coolant accident (LOCA). If there is complete loss of coolant, the fuel rods could eventually melt, leading to the complete collapse of the core. The core consists of fuel rods - pellets of Urania, clad in Zircaloy-4 (ca. 95 wt% Zr, 1.5 wt% Sn, remainder Fe, Cr. Ni). In addition, there are absorber and control rods which are borosilicate, and an Ag-Cd-In alloy, both clad in stainless steel. The structural materials are of steel.
During such a melt-down the exothermic reaction
in the presence
of water and steam, the Zircaloy
will be oxidised
by
Zr + 2H20 - zr02 + 2H2 which in fact produces the larger part of the heat which leads to melting or degradation of the core. Were the molten core material or debris to melt through the stainless steel pressure vessel, it would react with the concrete of the containment and also with any water which may have accumulated in the lower region of the containment building. The detailed understanding of the chemical and physical processes which might occur at all stages of reactor operation are essential to the assessment of the quantities of radioactive fission products which could be released from the core into the reactor containment. During operation, the maximum temperature of the centre of the water reactor fuel is approximately 1200°C. The temperature of the coolant is ca. 3OO“C. Under normal operating conditions , the fission product elements will be essentially distributed throughout the fluorite lattice of the UO2 There will be very little movement of fission product elements Received
April
21, 1983 165
P.E.
166
Potter
and
M.H.
Rand
in the temperature gradient of the fuel rod, except, perhaps, for the most volatile elemsnts, the rare gases krypton and xenon, iodine, caesium and also tellurium. The relationship between the quantity of the rare asses in the gap between the fuel and clad and there is also some evidence that and the temperature of the fuel is quite well known[l], caesium and iodine behave rather similarly to the rare gases in their movement within the fuel[Z]. For fuels operating below ca. 13009C, only a very small proportion of the total amount of the fission product elements produced is found in the small gap between the fuel and the The fission product concentration is essentially due to a temperature independent cladding. process involving the recoil of fission product atoms into the fuel-clad gap. Some estimates of these smounts in this region of a fuel pin have been given [l] and for the operating conditions of the fuel in a modern PWR are less than 1% of the total amount of these volatile elements present in the fuel pin.
In addition to these conditions for normal operation, attention has also been given recently to the quantitative prediction of the release of fission products from fuel under accident conditions. Information is required about the nucleation of the fission product phases as the temperature of the fuel rises, and also about the rate of release of fission products to the fuel-clad gap. As the capacity for heat removal is lost from the reactor core, we then have to consider the release of the fission product elements or compounds into steam on failure of the cladding, and finally the behaviour of the compounds and gaseous species of fission product in mixtures of steam and hydrogen (produced by the Zircaloy-water reaction). The program SGLGASMIX is ideal for the calculation of the many complex equilibria which have to be considered in making assessments of the likely consequences of accidents in which the The examples which we have chosen to illustrate some of the reactor core might melt. problems of chemical equiltiria and the application of the program are:
and
1.
The chemical Zircaloy.
constitution
of the fuel-clad
2.
The chemical temperatures
constitution of soms fission product elements higher than those of normal operation.
3.
The behaviour
of caesium
4.
The behaviour
of barium
and iodine
gap of a rod:
in mixtures
and strontium
UO2 pellets
within
clad in
fuel at
of steam and hydrogen.
in the debris
of a reactor
core.
Previous calculations have been carried out on the chemistry of the fuel-clad gap by Zesmann and Lindemer [3] using SGLGASMI X and on the species of caesium, iodine and tellurium in the gas phase for accident analysis in CANDU reactors by Garisto [4]. m2
e
The thermodynamic data have been taken in the main from the data bank (available on-line in Europe via Euronet-Diane) maintained by the Scientific Group, Thennodata Europe (SGTE) [S]. Additional values were taken from Lindemer et al [6] and from assessments by the authors which will be published in the near future. 2.1
The chemical
SDecies
in the fuel-clad
aaP
A fuel rod consists of pellets of Urania (DO,) in a can fabricated from the alloy ZircaloyWe 4. The pins are filled with helium at a pressure of ca. 24 atm. at room temperature. have taken the amount of the volatile fission products, kr, Xr, Cs, I and Te in the gap, to be 1% of the total fission product element concentration within a fuel roU, in which the burn-up of the fissile atoms is 2.9% of the uranium.
EQUILIBRIA OF WATER IN REACTOR SYSTEMS
The
likely
Chemical
Constitution of
a PWR
in the Fuel
I&ion
of the
167
Fuel-Clad
Gap
Rod
Temperature lOOOK Volume of free space in fuel rod Pressure of helium and rare gases Initial amount of u02 Initial amcunts I 2.35xlO-%nol Se + rare gases
case
1.
27.61cm3 84.09 atm 7.33lwl of fission nroduct elements cs 4.11xl0-4mol Te 6.25~lO-~mol 2.61x10-2 mol
After
equilibration
Amounts of condensed phases (mol) 7.33 2.22x10-16 2.25~lO-~ 6.24~lO-~
IJO2 U CSI Cs2Te
Pressures of Predominant Species (atm) cs Cs2 CSI CS2I2 I
Te "2
case
2.
UO2 CSI cs2U03.56 Cs2Te
AfterequLLihratiowithfurther 7.33 2.25~lO-~ 6.41~.lO-~ 6.24~lO-~
CS Cs2 CSI CS2I2 I Te "2
Case
3.
UO2 CSI cs2U03.56 cs2U04 Cs2Te
CS Cs2 CSI Cs2I2
I Te Te2 Case4.
*2 CSI cs2U04 Cs2Te
7.1x10-1 3.5x10-~ 2.1x10-3 3.9x10-4
aDunt
cs Cs2 CSI CS2I2 I Te Tel
-905921
of 0 2 (5xlo-5mol)
3.9xlo-1 1.0x10-2 2.1xloo-3 3.9x10-4
-581644
3.3x10-16 2.7x10-11 1.3x10-13
Of 0,
1.1x10-1 9.2x10-4 2.1x10-3 3.9x10-4
(5rlo-bol) -556224
1.1x10-15 2.9x10-10 1.6x10-l1
AfterequiUhratioawithafurtheraPaurt 7.33 2.25~lO-~ 1.50x10-4 4.35x10-5
potential 051
1.exlo-16 7,6x10-l2 1.1x10-14
AftfzrequiUhrationwithafurthfmammt 7.33 2.25~lO-~ 5.22~lO-~ 5.93x10-5 6.24~lO-~
Oxygen J.Wl
5.6~lO-~ 2.2x10-* 2.1x10-3 3.9x10-4 2.2x10-13 1.2x10-5 2.Elxlo-2
of 0,
(5iuo-5rol) -467647
168
P.E. Potter and M.H. Rand
The conditions and the result6 of the calculations are shown in Table I. It is assumed firstly in these calculations that the Zircaloy clad takes no part in the reactions in this region of a fuel rod at these temperatures due, for example, to the impermeable coating of ZrO2 on the inner surface. Some considerations of the effects of the presence of zirconium are however considered at the end of this section. The calculations shown in Table I indicate the changes in chemical constitution which would occur when the oxygen potential is increased slightly. It is thought that during the process of fission the oxygen potential of the fuel matrix will increase, since the average valency of the fission products is less than four, some of the lJa+ cations being oxidised to Us+ or U6+. For case 1 of Table I, we see that formally thqre is an extremely small amount of elemental uranium present because some of the oxygen of the UO2 is required for the gas phase after equilibration: in practice, this would manifest itself as hypostoichiometric Urania (UO2_x). The addition of a small quantity of oxygen (case 2) results in the oxidation of UO2_x and the appearance of C~~U03.56. The other condensed phases, CsI and Cs2Te remain unchanged in all the four cases considered here. The addition of a further small quantity of oxygen (case 3) results in the appearance of a further condensed phase, there are now five condensed phases present. The result of a further addition of CspuO4i oxygen (case 4) is to remove C~~IJ03.56 leaving the four phases - U02, Cs2UC4, CsI and Cs2Te. The gas phase species which have been considered in these calculations are Cs, Cs2, CsI, Cs212, Cs20, CsU, I, 12, Te, Te2, TeO, TeC2 and Te202. The only species with significant pressures are Cs, Cs2, CsI and Cs212. Te and Te2 are the major species of this element but their pressures are very low. It will be noticed that as the oxygen potential increases the pressure of atomic iodine increases, but for these conditions is always insignificant. After Zircaloy is allowed to react with the elements of the fuel-clad gap then because of the very low iodine pressures those of the four zirconium iodide gaseous species ZrI, Zr12, Zr13 and Zr14 are very low. We have considered two cases, one in which the activities of oxygen and iodine are taken from the conditions given for case 4 in Table I, and those where the iodine activity is the same but the oxygen activity is lower. In the first case, the oxygen is sufficient for the all the zirconium to be oxidised to Zr02, whilst in the second case the oxygen potential is that of the two phase system Zr + Zr02. The details of these two cases are given in Table II. The low pressures of iodine and of zirconium iodide8 in the fuel clad gap make any explanation on a basis of equilibrium thermodynamics for the corrosion of Zircaloy by iodine within an operating fuel rod extremely unlikely. TABLE II
The Reaction Initial Conditions (atm)
of Zirconium
Condensed
with Iodine and Oxygen
Phase
Pressures
of Zr iodide5
(a+=) ZrI
Zr12
Zr13
ZrI4
1.66~10~~~ 3.72~lO-~~
Zr + ZrO2
3.59x10-36
1.03x10-14
3.36~10-1~
1.15~10-~~
02 I2 02
1.66x10-23 6.O3X1O-49
zro2
5.93x10-59
1.66x10-37
5.49x10-39
l.l37Xlo‘-49
12
2.2
at lOOOK
Chemical
eauilibria
in the fuel rods at temoeratures
hiaher
than those of operation
The maximum temperature of the operation of PWR fuel rods is determined by the interaction If the fuel touches the cladding, then a uranium-rich liquid could of Urania with Zircaloy. form at temperatures in the range 1300-1500OC [7]. If the cladding were to become imperviously oxidised or if no contact between fuel and cladding took place, then the fuel rod would remain intact up to higher temperatures than that for the Urania-Zircaloy reaction.
EQUILIBRIA
OF WATER IN REACTOR
169
SYSTEMS
In this section we shall give an example of the calculation of the constitution of a fuel pin at 15OOK. in which the fission products, Cs, I and Te are present, together with MO. Mo was earlier believed to buffer the oxygen potential of irradiated oxide fuels [7] by the equilibrium reaction
The Wo would be a component of the fission product alloy Mo-Tc-Ru-Kh-Pd [a]. The present calculations would demonstrate that in fact the oxygen potential is controlled by the formation of CaeSiUm uranates up to temperatures of ca. 1500K. We shall see that at 1600K. using the available data for the Gibbs energies of formation of the caesium uranates [4], these compound5 have decomposed and thus can no longer control the rise in oxygen potential during irradiation of the Urania fuel. Table III gives the phases and pressures temperatures, 1500 and 1600K.
of the predominant
vapour
species
at the two
It will be noted that for cases l-2 in Table III the condensed phases are identical to cases l-2 of Table I, with the additional of elemental molybdenum. Further addition of oxygen (case 3) simply results in the formation of more CS~UCI~_~~. The oxygen potential is unchanged for cases 2 and 3. At 1600K. for case 4, the requirement5 for oxygen in the gas phase result in the formation On further addition of oxygen (case 5) uranium is no longer present and of uo2-x. hyperstoichiometric Urania is formed. In these calculations, for convenience, we have expressed the single phase Urania as a mixture of 002.00 and 002.06; this will make negligible difference to the calculated values. It will be noted that at this temperature ~~~003~6~ is no longer present in this system. Although we give only the pressure of the predominant gas phase species, we have also considered the following gases in our calculations: Cs20, CsO, TeO, Te02 and Te202. 2.3 2.3.1
Failed
fuel
The behaviour
of caesium.
iodine and tellurium
in mixtures
of steam and
hvdrocfen
The conditions of temperature and pressure at a given moment in a degraded reactor core of depend on the type of failure, for example, on whether there is a rapid depressurisation the primary coolant circuit, due to a large breach of the circuit (in which case the total pressure in the system would rapidly fall to a value of a few atmospheres) or whether the depreasurisation is slow because of a small leak. we have selected total pressure5 of 3atm and 7Oatm as being typical values for these two conditions of failure. We shall first examine the behaviour of the fission product5 failure of a fuel rod, and secondly we examine the behaviour mixtures.
which would be released on of Cs and I in steam-hydrogen
Let us then first consider a pin containing the same fission product concentration as given in Table 1111 on failure of the cladding we shall assume that all of these fission products react with the gaseous atmosphere of hydrogen and steam. We have carried out the calculation at 15OOK. for the two total pressures, and different hydrogen:water ratios. Elemental iodine, HI and E2Te would not condense 50 readily as CsI and the gaseous Te species (Te, Te2, TeOIi, Te(08)2, Tea, Teo2 and Te202). These volatile species would most probably behave differently within the reactor system during an accident and therefore it is important that their amount be known. Most of the Cs will be in the form of CsOli. The details of the proportions of the gaseous species for these conditions are given in Table IV.
P.E. Potter and M.H. Rand
170
The Chemical
Constitution
TABLE III of some Pission Product
15OOK volume 27.61cm2 Temperature Initial WitiOn (ml): 002 7.33 cs 4.11x10-2 I 2.35~lO-~ 1. AfterCondensed phases tmol) 7.33 m2 u 2.22x10-16 MD 5.00~~0-2 l.Slxlo-2 CS CSI 2.23~lO-~ 6.24~lO-~ cs2Te
Te
3.
7.33 5.00x10-~ 1.9Oxlo-2 2.23xlO+ 6.40x10w6 6.24~lO-~
-emmtofoxygen(5d0-%0102) Pressures identical for case 1
Afterequilibrationwiti
IJO2 Ho cs CSI Cs2003.56 CS2Te
7.33 5.00x10-2 1.8oxlo-3
further
CBI
cs2Te
ofoxygen(5x10+m102) -330154
5.79x10-5 6.24~lO-~
7.33 2.22x10-16 5.00x10-2 1.50x10-2 2.13~~0-3 6.2O~lO-~
AftYRrm~*-
002 m2.05 I& cs
-330154
2.231~0-2
179.5 atm as for cases
cs Cs2 CSI cs212 Te m2 I I2
5.
amount
to those
Pressures identical to those for cases 1 and 2
Temperature 16OOK. Pressure Same initial concentrations 4. Afterequilibation 002 u Mo cs CSI cs2Te
Pressure 157.9atm lie + rare gases 2.9O~lO-~ 6.24~lO-~ Te 5.00x10-2 MO
7.69x1O-4 2.67~lO-~ 1,03xlo-9 7.05x10-9
m2 I
Merequillbratiaatith
I=702 ml CB CSI CB2003.56 CS2!re
in Urania
Pressures of predominant Oxygen potential species tatm) J,mol 021 cs 19.59 Re + rare -544619 6.36 gases 129.4 Cs2 4.32~lO-~ CBI CE212 4.54x10 -2
I2 2.
Elements
7.32 2.00x10-4 5.00x10-~ 1.50x10-3 2.13~lO-~ 6.2O~lO-~
1, 2, 3 at 1500K
26.96 13.59 9.41x10-1 9.22x10-2 9.37x10-3 1.05x10-1 7.96~lO-~ 2.19x10-17 m
Pressures identical those of case 4
-499344
Ofaxfgen (5xlo+mlq) to
-304374
Fuel
EQUILIBRIA
The
OF WATER IN REACI‘OR SYS’rFaMS
TABLE IV Chemical Species of the Fission after Release from a Fuel
171
Products Cs, Pin at 1500K
I and
Te
Pressure
Relative
(atm)
ffa
I CSI
H20 3
amounts
Hr CSI
of
sbecies
2 other
Te
CsOH CSI
species
2.4 33.6
5.36~?.0-~ 8.27x10-6
2.74x10-4 4.96xlO-5
5.20~10-~ 1.24X1O-1
15.2 8.3
1 4
2.26~lO-~ 4.33x10-5
4.74x10-3 1.15x10-3
1.64X1O-2 2.55x10-l
10.0 16.1
70
In all conditions in these calculations the contributions to the gas from elemental I and HI The contribution of A2Te compared with those of the otherspecies can bs are very small. The remaining condensed phases are significant under the nwre reducing conditions. hyperetoichiometric Urania and elemental molybdenum. We shall now examine the behaviour of caesium and iodine in steam-hydrogen mixtures, again at the previously chosen pressures of 3 and 70 atm but at higher temperatures, 1750, 2000 411 and 1:l and 2250K. The hydrogenzsteam ratios (14:l. 2:l for the large break accident, for the small break) and the (Cs,I)/(H2+H20) ratios are taken to epan the corresponding ratios obtained from preliminary the-hydraulic modelling calculations of failed core. We shall again be concerned with the amounts of elemental iodine and HI compared with that of The results of the calculations of the relative amounts of these species are given in CSI. Table
V.
The
The
initial
amounts
Total
Temp.
Pressure
(K)
TABLE V of Cs and I in Wixtures of Steam at Temperatures 1750-225OK
Behaviour
of Cs
Initial H2
and
Amount
H20
0.7
and
Proportions I cs1
(atm) 3
I are
0.04
mol
and
Hydrogen
respectively.
of gaseous species a cs CSI CSI
EL CSI
1750
140 100
10 50
3.641UO-~ 1.34x10-3
8.32~lO-~ 2.59x10-3
8.75 14.45
7.77 2.11
2000
140 100 140 100
10 50 10 50
4.87~lO-~ 1.35x10-2 3.96xlO-2 8.61x10T2
5.45x10-3 1.28~10-~ 2.54~lO-~ 4.68x10-2
5.92 13.02 4.22 12.23
10.76 3.94 13.41 6.55
800
200
9.38xlO-4
9.6OxlO-3
15.82
0.87
500 800 500 800 500
500 200 500 200 500
2.861rlO-~ 8.75~lO-~ 2.57~10-~ 5.11x10-2 0.14
2.32~lO-~ 4.38~lO-~ 0.10 0.15 0.32
16.66 15.65 18.08 16.64 23.18
0.29 1.77 0.65 3.32 1.46
2250 70
1750 2000 2250
The calculations show that only for the higher pressure of 70 atm does the amount of HI become appreciable (ca. 10% of CsI) at 2OOOK and that of elemental iodine (ca. 5% of CsI) 2250K. It will be noted that for the lower pressure of 3 aim, the amount of elemental Cs can be greater than that of CsOH. The foxmation of I(g) at high temperatures arises from the dissociation and hydrolysis reactions: CSI(9) CsI(g) CsI(g)
- Cs(g) + I(g) + H2o(g) - CsOH(g) + H20(g) - CaOH(g)
+ HI(g) + I(g) +
;H,(g)
at
172
P.E. Potter and M.B.
Rand
It is clear from considerations of the law of mm5 action that, as found from the SOLGASMIX results, the factor5 which increase the I/CeI ratio are: -
increase decrease decrease increase
in in in in
temperature pressure (Cs,I)/(H2O+H2) H2O/li2 ratio
These results, however, show that for realietic conditions in the pressure vessel during a core melt, virtually all of the iodine will remain as CsI(g) or HI(g). As the mass of steam, hydrogen and fission product cools, any free I(g) formed above ZOOOK will of course recombine to form CsI (as in Table III). 2.3.2
!J!h e behaviour
of barium
and strontium
in the debr is of a re actor c ore
The volatile fission products Cs, I and Te considered 50 far are removed from the fuel by Other fission products remain dissolved in the (U02,Zr02) the time the core ha5 melted. oxide or the (Zircaloy-steel) melts. In particular, the barium and strontium (which have appreciable fission yields) are likely to dissolve in the UO2 matrix, either as oxides or a5 zirconates. (Zr is also a fission product (Zr/(Ba+Sr) = 2) 50 that the barium and strontium will be in intimate contact with zirconium during operation and heating up of the core). Although soms vaporisation may occur during the period when the core slumps down to the bottom of the pressure vessel, we have studied the release of Ba and Sr for the later period when the molten core has melted through the pressure vessel and is vaporising water, present in the containment (including that bound in the concrete structure). We have therefore calculated the vaporieation of barium and strontium as oxides and hydroxides in realistic flows of hydrogen/steam mixtures, at 2250K. The results are shown in Table VI. Two cases are presented, one where no zirconate formation is assumed, and one with such formation. In both cases, similar compounds of Sa and Sr are assumed to form ideal solid or liquid solutions. As expected the predominant species are the dihydroxide gases, although in keeping with the much higher stability of BaO(g) compared to the other alkaline earth monoxide gases, this species comprises about 5% of the barium vapour species. Barium species are rather more volatile than strontium species, 80 that in cases 3 and 4, where there is a high H,O/(Ba+Sr) ratio, the condensed phase is considerably depleted in barium. The pressures of fission products are not negligible, even in the more probable cases where zirconates are formed.
173
EQUILIBRIA OF WATER IN REACTOR SYSTEMS
Vaporisation
of Ba and Sr from oxide melts
Temperature 2250K. Pressure 3 atm. Initial compositions Ba 7.35 mol, Sr 8.11 mol. Case
B20 (mol)
B2
2 3 4
100 100 1000 1000
10 100 100 1000
phases:
BaxSrl-p(s)
1
a.
-3 Condensed
Caae
1 2 3. 4 b.
1 2 3 4
2 (Ba+Sr) 0.43 0.42 0.10 0.10
Ba(OH)2 3.3x30-2 1.9x10-2 1.9x10-2 9.2x10-3
BaOH
+ BrQ2(S)
Pressures (atm) BaG Sr(OB)2
2.3x10-3
2.6x10-3 2.7xlO-4 5.OxlO-4
1.9x10-3 1.9x10-3 4.6xlO-4 4.4x10-4
2.0x10-3 1.1xlo-3 6.9xlO-3 3.Ex10-3
SrOB
Sr
3.9x10-5 4.3x10-4 2.9x10+ 5.6xlO-4
2.3xlO-5 5.6xlO_5 4.0x10+ 2.7xlO-5
1.2 0.65 2.6 1.4
2.5xlO-6 2.i3xlo-6 1.2x10-6 2.3x10w6
1.5x10-' 3.5x10-7 1.7x10-9 1.1x10-'
1.2 0.67 2.6 1.4
B2G
With sirconatea
0.48 0.49 0.47 0.47
Condensed
phases:
2.0~10-4 1.1x10-4 4.2xlO-4 2.3x10-4
1.3x10-5 1.5x10-5 6.4x.lO+j 1.2x10-5
(BaxSrl_x)Z~3 1.1x10-5 1.1x10-5 1.1x10-5 1.1x1O-5
+ ZrQ2
l.4x10-5 7.6x10+ 2.9x10-5 l.6X1O-5
3. Conclusions This paper describea some of our changea which could occur during reactor. It is our intention to data of all the condensed phases of an accident.
preliminary attempts to exzLmine the complex chemical an accident due to loss of coolant in a PWB type nuclear develop a collection of critically aaaeaaed thermdynamic and gaseous species likely to bs encountered at all stages
We have shown aspects of the behaviour of Cs, I, Te, Ba and Sr, all of which have potentially hazardous radionuclidea. It ia clear that the conditiona of temperature, pressure and oxygen potential must be well defined so that the predominant chemical speciea can be determined for a particular accident sequence. We are most grateful to Dr. Gunnar Eriksson for his generous SOLGASWIX, and for many stimulating diacuasions.
provision
of the program
174
P.R. Potter and M.H. Rand
REFERENCES :
El1
J.B.Gittus. PWR degraded Chapter VII p.%l
core analysis.
UKABA Report
[21
R.A.Lorenz, J.L.Collins, A.P,Malinauskas, Report NURBG-CR-0722 (1990)
O.L.Kirkland,
C53
T.M.Bemann,
141
P.Garisto.
151
T.1.Barz-y. "Contributions of European Thermochemical Rapport6 Techniques CBBEICOR 142 1992 137
Data Banks".
[61
T.B.Lindemer,
J.QQ (1991) 179
[71
P.Hofmann,
[61
P.E.Potter. (1974) p.115
T.B.Lindemer.
Report ORNL/TM-6130
Report AECL-7782
T.M.Bemann,
C.Politis. Behaviour
ND-R61O(S)
1982.
R.L.Towns.
(1979)
(1992)
C.E.Johnson.
J.Nucl.Mata.
J.Nucl.Mat.9. a(1979) and Chemical
22nd CBPA,
375
State of Irradiated
Ceramic
Puels.
IAEA Vienna
pp. 165-174,
1983
0364-5916/83 $3.00 + .OO (c) 1983 Pergamon Press Ltd.
ILQUILIRRIA mR ACCIPRESURISEDWATERNUCLEARREAClUR-
CHEMICAL
Chemistry
and Materials
ANALYSIS
P. E. Potter and M. H. Rand Development Divisions, AERE, Harwell,
IN
Oxon, OX11 ORA, U.K.
In this paper some chemical equilibria for the safety analysis of water cooled nuclear reactors have been calculated using the programme SOLGASMIX. Particular attention has been paid to the behaviour of the fission product elements Cs, I, Te, Ba and Sr. The chemical states of these elements have been calculated for a variety of conditions likely to be encountered in the core of a reactor before and during degradation.
1.
Introduction
The purpose of this paper is to describe some of the complex chemical equilibria which can occur in or near the core of a water-cooled nuclear reactor such as a pressurised water reactor (PWR). When the 235U atoms
of the enriched Urania (00,) fuel undergo fission, over thirty new fission product elements are formed. A detailed knowledge of the chemical changes which can occur within the fuel as a result of fission 1s of great importance in the interpretation and prediction of the behaviour of fuel in normal operating and accident conditions. We shall discuss some equilibria of relevance to the operation of PWR fuel rods under normal conditions, and then in conditions appropriate to a loss of coolant accident (LOCA). If there is complete loss of coolant, the fuel rods could eventually melt, leading to the complete collapse of the core. The core consists of fuel rods - pellets of Urania, clad in Zircaloy-4 (ca. 95 wt% Zr, 1.5 wt% Sn, remainder Fe, Cr. Ni). In addition, there are absorber and control rods which are borosilicate, and an Ag-Cd-In alloy, both clad in stainless steel. The structural materials are of steel.
During such a melt-down the exothermic reaction
in the presence
of water and steam, the Zircaloy
will be oxidised
by
Zr + 2H20 - zr02 + 2H2 which in fact produces the larger part of the heat which leads to melting or degradation of the core. Were the molten core material or debris to melt through the stainless steel pressure vessel, it would react with the concrete of the containment and also with any water which may have accumulated in the lower region of the containment building. The detailed understanding of the chemical and physical processes which might occur at all stages of reactor operation are essential to the assessment of the quantities of radioactive fission products which could be released from the core into the reactor containment. During operation, the maximum temperature of the centre of the water reactor fuel is approximately 1200°C. The temperature of the coolant is ca. 3OO“C. Under normal operating conditions , the fission product elements will be essentially distributed throughout the fluorite lattice of the UO2 There will be very little movement of fission product elements Received
April
21, 1983 165
P.E.
166
Potter
and
M.H.
Rand
in the temperature gradient of the fuel rod, except, perhaps, for the most volatile elemsnts, the rare gases krypton and xenon, iodine, caesium and also tellurium. The relationship between the quantity of the rare asses in the gap between the fuel and clad and there is also some evidence that and the temperature of the fuel is quite well known[l], caesium and iodine behave rather similarly to the rare gases in their movement within the fuel[Z]. For fuels operating below ca. 13009C, only a very small proportion of the total amount of the fission product elements produced is found in the small gap between the fuel and the The fission product concentration is essentially due to a temperature independent cladding. process involving the recoil of fission product atoms into the fuel-clad gap. Some estimates of these smounts in this region of a fuel pin have been given [l] and for the operating conditions of the fuel in a modern PWR are less than 1% of the total amount of these volatile elements present in the fuel pin.
In addition to these conditions for normal operation, attention has also been given recently to the quantitative prediction of the release of fission products from fuel under accident conditions. Information is required about the nucleation of the fission product phases as the temperature of the fuel rises, and also about the rate of release of fission products to the fuel-clad gap. As the capacity for heat removal is lost from the reactor core, we then have to consider the release of the fission product elements or compounds into steam on failure of the cladding, and finally the behaviour of the compounds and gaseous species of fission product in mixtures of steam and hydrogen (produced by the Zircaloy-water reaction). The program SGLGASMIX is ideal for the calculation of the many complex equilibria which have to be considered in making assessments of the likely consequences of accidents in which the The examples which we have chosen to illustrate some of the reactor core might melt. problems of chemical equiltiria and the application of the program are:
and
1.
The chemical Zircaloy.
constitution
of the fuel-clad
2.
The chemical temperatures
constitution of soms fission product elements higher than those of normal operation.
3.
The behaviour
of caesium
4.
The behaviour
of barium
and iodine
gap of a rod:
in mixtures
and strontium
UO2 pellets
within
clad in
fuel at
of steam and hydrogen.
in the debris
of a reactor
core.
Previous calculations have been carried out on the chemistry of the fuel-clad gap by Zesmann and Lindemer [3] using SGLGASMI X and on the species of caesium, iodine and tellurium in the gas phase for accident analysis in CANDU reactors by Garisto [4]. m2
e
The thermodynamic data have been taken in the main from the data bank (available on-line in Europe via Euronet-Diane) maintained by the Scientific Group, Thennodata Europe (SGTE) [S]. Additional values were taken from Lindemer et al [6] and from assessments by the authors which will be published in the near future. 2.1
The chemical
SDecies
in the fuel-clad
aaP
A fuel rod consists of pellets of Urania (DO,) in a can fabricated from the alloy ZircaloyWe 4. The pins are filled with helium at a pressure of ca. 24 atm. at room temperature. have taken the amount of the volatile fission products, kr, Xr, Cs, I and Te in the gap, to be 1% of the total fission product element concentration within a fuel roU, in which the burn-up of the fissile atoms is 2.9% of the uranium.
EQUILIBRIA OF WATER IN REACTOR SYSTEMS
The
likely
Chemical
Constitution of
a PWR
in the Fuel
I&ion
of the
167
Fuel-Clad
Gap
Rod
Temperature lOOOK Volume of free space in fuel rod Pressure of helium and rare gases Initial amount of u02 Initial amcunts I 2.35xlO-%nol Se + rare gases
case
1.
27.61cm3 84.09 atm 7.33lwl of fission nroduct elements cs 4.11xl0-4mol Te 6.25~lO-~mol 2.61x10-2 mol
After
equilibration
Amounts of condensed phases (mol) 7.33 2.22x10-16 2.25~lO-~ 6.24~lO-~
IJO2 U CSI Cs2Te
Pressures of Predominant Species (atm) cs Cs2 CSI CS2I2 I
Te "2
case
2.
UO2 CSI cs2U03.56 Cs2Te
AfterequLLihratiowithfurther 7.33 2.25~lO-~ 6.41~.lO-~ 6.24~lO-~
CS Cs2 CSI CS2I2 I Te "2
Case
3.
UO2 CSI cs2U03.56 cs2U04 Cs2Te
CS Cs2 CSI Cs2I2
I Te Te2 Case4.
*2 CSI cs2U04 Cs2Te
7.1x10-1 3.5x10-~ 2.1x10-3 3.9x10-4
aDunt
cs Cs2 CSI CS2I2 I Te Tel
-905921
of 0 2 (5xlo-5mol)
3.9xlo-1 1.0x10-2 2.1xloo-3 3.9x10-4
-581644
3.3x10-16 2.7x10-11 1.3x10-13
Of 0,
1.1x10-1 9.2x10-4 2.1x10-3 3.9x10-4
(5rlo-bol) -556224
1.1x10-15 2.9x10-10 1.6x10-l1
AfterequiUhratioawithafurtheraPaurt 7.33 2.25~lO-~ 1.50x10-4 4.35x10-5
potential 051
1.exlo-16 7,6x10-l2 1.1x10-14
AftfzrequiUhrationwithafurthfmammt 7.33 2.25~lO-~ 5.22~lO-~ 5.93x10-5 6.24~lO-~
Oxygen J.Wl
5.6~lO-~ 2.2x10-* 2.1x10-3 3.9x10-4 2.2x10-13 1.2x10-5 2.Elxlo-2
of 0,
(5iuo-5rol) -467647
168
P.E. Potter and M.H. Rand
The conditions and the result6 of the calculations are shown in Table I. It is assumed firstly in these calculations that the Zircaloy clad takes no part in the reactions in this region of a fuel rod at these temperatures due, for example, to the impermeable coating of ZrO2 on the inner surface. Some considerations of the effects of the presence of zirconium are however considered at the end of this section. The calculations shown in Table I indicate the changes in chemical constitution which would occur when the oxygen potential is increased slightly. It is thought that during the process of fission the oxygen potential of the fuel matrix will increase, since the average valency of the fission products is less than four, some of the lJa+ cations being oxidised to Us+ or U6+. For case 1 of Table I, we see that formally thqre is an extremely small amount of elemental uranium present because some of the oxygen of the UO2 is required for the gas phase after equilibration: in practice, this would manifest itself as hypostoichiometric Urania (UO2_x). The addition of a small quantity of oxygen (case 2) results in the oxidation of UO2_x and the appearance of C~~U03.56. The other condensed phases, CsI and Cs2Te remain unchanged in all the four cases considered here. The addition of a further small quantity of oxygen (case 3) results in the appearance of a further condensed phase, there are now five condensed phases present. The result of a further addition of CspuO4i oxygen (case 4) is to remove C~~IJ03.56 leaving the four phases - U02, Cs2UC4, CsI and Cs2Te. The gas phase species which have been considered in these calculations are Cs, Cs2, CsI, Cs212, Cs20, CsU, I, 12, Te, Te2, TeO, TeC2 and Te202. The only species with significant pressures are Cs, Cs2, CsI and Cs212. Te and Te2 are the major species of this element but their pressures are very low. It will be noticed that as the oxygen potential increases the pressure of atomic iodine increases, but for these conditions is always insignificant. After Zircaloy is allowed to react with the elements of the fuel-clad gap then because of the very low iodine pressures those of the four zirconium iodide gaseous species ZrI, Zr12, Zr13 and Zr14 are very low. We have considered two cases, one in which the activities of oxygen and iodine are taken from the conditions given for case 4 in Table I, and those where the iodine activity is the same but the oxygen activity is lower. In the first case, the oxygen is sufficient for the all the zirconium to be oxidised to Zr02, whilst in the second case the oxygen potential is that of the two phase system Zr + Zr02. The details of these two cases are given in Table II. The low pressures of iodine and of zirconium iodide8 in the fuel clad gap make any explanation on a basis of equilibrium thermodynamics for the corrosion of Zircaloy by iodine within an operating fuel rod extremely unlikely. TABLE II
The Reaction Initial Conditions (atm)
of Zirconium
Condensed
with Iodine and Oxygen
Phase
Pressures
of Zr iodide5
(a+=) ZrI
Zr12
Zr13
ZrI4
1.66~10~~~ 3.72~lO-~~
Zr + ZrO2
3.59x10-36
1.03x10-14
3.36~10-1~
1.15~10-~~
02 I2 02
1.66x10-23 6.O3X1O-49
zro2
5.93x10-59
1.66x10-37
5.49x10-39
l.l37Xlo‘-49
12
2.2
at lOOOK
Chemical
eauilibria
in the fuel rods at temoeratures
hiaher
than those of operation
The maximum temperature of the operation of PWR fuel rods is determined by the interaction If the fuel touches the cladding, then a uranium-rich liquid could of Urania with Zircaloy. form at temperatures in the range 1300-1500OC [7]. If the cladding were to become imperviously oxidised or if no contact between fuel and cladding took place, then the fuel rod would remain intact up to higher temperatures than that for the Urania-Zircaloy reaction.
EQUILIBRIA
OF WATER IN REACTOR
169
SYSTEMS
In this section we shall give an example of the calculation of the constitution of a fuel pin at 15OOK. in which the fission products, Cs, I and Te are present, together with MO. Mo was earlier believed to buffer the oxygen potential of irradiated oxide fuels [7] by the equilibrium reaction
The Wo would be a component of the fission product alloy Mo-Tc-Ru-Kh-Pd [a]. The present calculations would demonstrate that in fact the oxygen potential is controlled by the formation of CaeSiUm uranates up to temperatures of ca. 1500K. We shall see that at 1600K. using the available data for the Gibbs energies of formation of the caesium uranates [4], these compound5 have decomposed and thus can no longer control the rise in oxygen potential during irradiation of the Urania fuel. Table III gives the phases and pressures temperatures, 1500 and 1600K.
of the predominant
vapour
species
at the two
It will be noted that for cases l-2 in Table III the condensed phases are identical to cases l-2 of Table I, with the additional of elemental molybdenum. Further addition of oxygen (case 3) simply results in the formation of more CS~UCI~_~~. The oxygen potential is unchanged for cases 2 and 3. At 1600K. for case 4, the requirement5 for oxygen in the gas phase result in the formation On further addition of oxygen (case 5) uranium is no longer present and of uo2-x. hyperstoichiometric Urania is formed. In these calculations, for convenience, we have expressed the single phase Urania as a mixture of 002.00 and 002.06; this will make negligible difference to the calculated values. It will be noted that at this temperature ~~~003~6~ is no longer present in this system. Although we give only the pressure of the predominant gas phase species, we have also considered the following gases in our calculations: Cs20, CsO, TeO, Te02 and Te202. 2.3 2.3.1
Failed
fuel
The behaviour
of caesium.
iodine and tellurium
in mixtures
of steam and
hvdrocfen
The conditions of temperature and pressure at a given moment in a degraded reactor core of depend on the type of failure, for example, on whether there is a rapid depressurisation the primary coolant circuit, due to a large breach of the circuit (in which case the total pressure in the system would rapidly fall to a value of a few atmospheres) or whether the depreasurisation is slow because of a small leak. we have selected total pressure5 of 3atm and 7Oatm as being typical values for these two conditions of failure. We shall first examine the behaviour of the fission product5 failure of a fuel rod, and secondly we examine the behaviour mixtures.
which would be released on of Cs and I in steam-hydrogen
Let us then first consider a pin containing the same fission product concentration as given in Table 1111 on failure of the cladding we shall assume that all of these fission products react with the gaseous atmosphere of hydrogen and steam. We have carried out the calculation at 15OOK. for the two total pressures, and different hydrogen:water ratios. Elemental iodine, HI and E2Te would not condense 50 readily as CsI and the gaseous Te species (Te, Te2, TeOIi, Te(08)2, Tea, Teo2 and Te202). These volatile species would most probably behave differently within the reactor system during an accident and therefore it is important that their amount be known. Most of the Cs will be in the form of CsOli. The details of the proportions of the gaseous species for these conditions are given in Table IV.
P.E. Potter and M.H. Rand
170
The Chemical
Constitution
TABLE III of some Pission Product
15OOK volume 27.61cm2 Temperature Initial WitiOn (ml): 002 7.33 cs 4.11x10-2 I 2.35~lO-~ 1. AfterCondensed phases tmol) 7.33 m2 u 2.22x10-16 MD 5.00~~0-2 l.Slxlo-2 CS CSI 2.23~lO-~ 6.24~lO-~ cs2Te
Te
3.
7.33 5.00x10-~ 1.9Oxlo-2 2.23xlO+ 6.40x10w6 6.24~lO-~
-emmtofoxygen(5d0-%0102) Pressures identical for case 1
Afterequilibrationwiti
IJO2 Ho cs CSI Cs2003.56 CS2Te
7.33 5.00x10-2 1.8oxlo-3
further
CBI
cs2Te
ofoxygen(5x10+m102) -330154
5.79x10-5 6.24~lO-~
7.33 2.22x10-16 5.00x10-2 1.50x10-2 2.13~~0-3 6.2O~lO-~
AftYRrm~*-
002 m2.05 I& cs
-330154
2.231~0-2
179.5 atm as for cases
cs Cs2 CSI cs212 Te m2 I I2
5.
amount
to those
Pressures identical to those for cases 1 and 2
Temperature 16OOK. Pressure Same initial concentrations 4. Afterequilibation 002 u Mo cs CSI cs2Te
Pressure 157.9atm lie + rare gases 2.9O~lO-~ 6.24~lO-~ Te 5.00x10-2 MO
7.69x1O-4 2.67~lO-~ 1,03xlo-9 7.05x10-9
m2 I
Merequillbratiaatith
I=702 ml CB CSI CB2003.56 CS2!re
in Urania
Pressures of predominant Oxygen potential species tatm) J,mol 021 cs 19.59 Re + rare -544619 6.36 gases 129.4 Cs2 4.32~lO-~ CBI CE212 4.54x10 -2
I2 2.
Elements
7.32 2.00x10-4 5.00x10-~ 1.50x10-3 2.13~lO-~ 6.2O~lO-~
1, 2, 3 at 1500K
26.96 13.59 9.41x10-1 9.22x10-2 9.37x10-3 1.05x10-1 7.96~lO-~ 2.19x10-17 m
Pressures identical those of case 4
-499344
Ofaxfgen (5xlo+mlq) to
-304374
Fuel
EQUILIBRIA
The
OF WATER IN REACI‘OR SYS’rFaMS
TABLE IV Chemical Species of the Fission after Release from a Fuel
171
Products Cs, Pin at 1500K
I and
Te
Pressure
Relative
(atm)
ffa
I CSI
H20 3
amounts
Hr CSI
of
sbecies
2 other
Te
CsOH CSI
species
2.4 33.6
5.36~?.0-~ 8.27x10-6
2.74x10-4 4.96xlO-5
5.20~10-~ 1.24X1O-1
15.2 8.3
1 4
2.26~lO-~ 4.33x10-5
4.74x10-3 1.15x10-3
1.64X1O-2 2.55x10-l
10.0 16.1
70
In all conditions in these calculations the contributions to the gas from elemental I and HI The contribution of A2Te compared with those of the otherspecies can bs are very small. The remaining condensed phases are significant under the nwre reducing conditions. hyperetoichiometric Urania and elemental molybdenum. We shall now examine the behaviour of caesium and iodine in steam-hydrogen mixtures, again at the previously chosen pressures of 3 and 70 atm but at higher temperatures, 1750, 2000 411 and 1:l and 2250K. The hydrogenzsteam ratios (14:l. 2:l for the large break accident, for the small break) and the (Cs,I)/(H2+H20) ratios are taken to epan the corresponding ratios obtained from preliminary the-hydraulic modelling calculations of failed core. We shall again be concerned with the amounts of elemental iodine and HI compared with that of The results of the calculations of the relative amounts of these species are given in CSI. Table
V.
The
The
initial
amounts
Total
Temp.
Pressure
(K)
TABLE V of Cs and I in Wixtures of Steam at Temperatures 1750-225OK
Behaviour
of Cs
Initial H2
and
Amount
H20
0.7
and
Proportions I cs1
(atm) 3
I are
0.04
mol
and
Hydrogen
respectively.
of gaseous species a cs CSI CSI
EL CSI
1750
140 100
10 50
3.641UO-~ 1.34x10-3
8.32~lO-~ 2.59x10-3
8.75 14.45
7.77 2.11
2000
140 100 140 100
10 50 10 50
4.87~lO-~ 1.35x10-2 3.96xlO-2 8.61x10T2
5.45x10-3 1.28~10-~ 2.54~lO-~ 4.68x10-2
5.92 13.02 4.22 12.23
10.76 3.94 13.41 6.55
800
200
9.38xlO-4
9.6OxlO-3
15.82
0.87
500 800 500 800 500
500 200 500 200 500
2.861rlO-~ 8.75~lO-~ 2.57~10-~ 5.11x10-2 0.14
2.32~lO-~ 4.38~lO-~ 0.10 0.15 0.32
16.66 15.65 18.08 16.64 23.18
0.29 1.77 0.65 3.32 1.46
2250 70
1750 2000 2250
The calculations show that only for the higher pressure of 70 atm does the amount of HI become appreciable (ca. 10% of CsI) at 2OOOK and that of elemental iodine (ca. 5% of CsI) 2250K. It will be noted that for the lower pressure of 3 aim, the amount of elemental Cs can be greater than that of CsOH. The foxmation of I(g) at high temperatures arises from the dissociation and hydrolysis reactions: CSI(9) CsI(g) CsI(g)
- Cs(g) + I(g) + H2o(g) - CsOH(g) + H20(g) - CaOH(g)
+ HI(g) + I(g) +
;H,(g)
at
172
P.E. Potter and M.B.
Rand
It is clear from considerations of the law of mm5 action that, as found from the SOLGASMIX results, the factor5 which increase the I/CeI ratio are: -
increase decrease decrease increase
in in in in
temperature pressure (Cs,I)/(H2O+H2) H2O/li2 ratio
These results, however, show that for realietic conditions in the pressure vessel during a core melt, virtually all of the iodine will remain as CsI(g) or HI(g). As the mass of steam, hydrogen and fission product cools, any free I(g) formed above ZOOOK will of course recombine to form CsI (as in Table III). 2.3.2
!J!h e behaviour
of barium
and strontium
in the debr is of a re actor c ore
The volatile fission products Cs, I and Te considered 50 far are removed from the fuel by Other fission products remain dissolved in the (U02,Zr02) the time the core ha5 melted. oxide or the (Zircaloy-steel) melts. In particular, the barium and strontium (which have appreciable fission yields) are likely to dissolve in the UO2 matrix, either as oxides or a5 zirconates. (Zr is also a fission product (Zr/(Ba+Sr) = 2) 50 that the barium and strontium will be in intimate contact with zirconium during operation and heating up of the core). Although soms vaporisation may occur during the period when the core slumps down to the bottom of the pressure vessel, we have studied the release of Ba and Sr for the later period when the molten core has melted through the pressure vessel and is vaporising water, present in the containment (including that bound in the concrete structure). We have therefore calculated the vaporieation of barium and strontium as oxides and hydroxides in realistic flows of hydrogen/steam mixtures, at 2250K. The results are shown in Table VI. Two cases are presented, one where no zirconate formation is assumed, and one with such formation. In both cases, similar compounds of Sa and Sr are assumed to form ideal solid or liquid solutions. As expected the predominant species are the dihydroxide gases, although in keeping with the much higher stability of BaO(g) compared to the other alkaline earth monoxide gases, this species comprises about 5% of the barium vapour species. Barium species are rather more volatile than strontium species, 80 that in cases 3 and 4, where there is a high H,O/(Ba+Sr) ratio, the condensed phase is considerably depleted in barium. The pressures of fission products are not negligible, even in the more probable cases where zirconates are formed.
173
EQUILIBRIA OF WATER IN REACTOR SYSTEMS
Vaporisation
of Ba and Sr from oxide melts
Temperature 2250K. Pressure 3 atm. Initial compositions Ba 7.35 mol, Sr 8.11 mol. Case
B20 (mol)
B2
2 3 4
100 100 1000 1000
10 100 100 1000
phases:
BaxSrl-p(s)
1
a.
-3 Condensed
Caae
1 2 3. 4 b.
1 2 3 4
2 (Ba+Sr) 0.43 0.42 0.10 0.10
Ba(OH)2 3.3x30-2 1.9x10-2 1.9x10-2 9.2x10-3
BaOH
+ BrQ2(S)
Pressures (atm) BaG Sr(OB)2
2.3x10-3
2.6x10-3 2.7xlO-4 5.OxlO-4
1.9x10-3 1.9x10-3 4.6xlO-4 4.4x10-4
2.0x10-3 1.1xlo-3 6.9xlO-3 3.Ex10-3
SrOB
Sr
3.9x10-5 4.3x10-4 2.9x10+ 5.6xlO-4
2.3xlO-5 5.6xlO_5 4.0x10+ 2.7xlO-5
1.2 0.65 2.6 1.4
2.5xlO-6 2.i3xlo-6 1.2x10-6 2.3x10w6
1.5x10-' 3.5x10-7 1.7x10-9 1.1x10-'
1.2 0.67 2.6 1.4
B2G
With sirconatea
0.48 0.49 0.47 0.47
Condensed
phases:
2.0~10-4 1.1x10-4 4.2xlO-4 2.3x10-4
1.3x10-5 1.5x10-5 6.4x.lO+j 1.2x10-5
(BaxSrl_x)Z~3 1.1x10-5 1.1x10-5 1.1x10-5 1.1x1O-5
+ ZrQ2
l.4x10-5 7.6x10+ 2.9x10-5 l.6X1O-5
3. Conclusions This paper describea some of our changea which could occur during reactor. It is our intention to data of all the condensed phases of an accident.
preliminary attempts to exzLmine the complex chemical an accident due to loss of coolant in a PWB type nuclear develop a collection of critically aaaeaaed thermdynamic and gaseous species likely to bs encountered at all stages
We have shown aspects of the behaviour of Cs, I, Te, Ba and Sr, all of which have potentially hazardous radionuclidea. It ia clear that the conditiona of temperature, pressure and oxygen potential must be well defined so that the predominant chemical speciea can be determined for a particular accident sequence. We are most grateful to Dr. Gunnar Eriksson for his generous SOLGASWIX, and for many stimulating diacuasions.
provision
of the program
174
P.R. Potter and M.H. Rand
REFERENCES :
El1
J.B.Gittus. PWR degraded Chapter VII p.%l
core analysis.
UKABA Report
[21
R.A.Lorenz, J.L.Collins, A.P,Malinauskas, Report NURBG-CR-0722 (1990)
O.L.Kirkland,
C53
T.M.Bemann,
141
P.Garisto.
151
T.1.Barz-y. "Contributions of European Thermochemical Rapport6 Techniques CBBEICOR 142 1992 137
Data Banks".
[61
T.B.Lindemer,
J.QQ (1991) 179
[71
P.Hofmann,
[61
P.E.Potter. (1974) p.115
T.B.Lindemer.
Report ORNL/TM-6130
Report AECL-7782
T.M.Bemann,
C.Politis. Behaviour
ND-R61O(S)
1982.
R.L.Towns.
(1979)
(1992)
C.E.Johnson.
J.Nucl.Mata.
J.Nucl.Mat.9. a(1979) and Chemical
22nd CBPA,
375
State of Irradiated
Ceramic
Puels.
IAEA Vienna