of Nuclear Materials 140 (1986) 19-27 North-Holland, Amsterdam
STATE OF FISSION
S. IMOTO Department Received
of Nuclear Engineering, Faculty of Engineering, Osaka University, Yamadaoka 2-1, Suita, Osaka, Japan
24 April 1986
The chemical state of fission products in irradiated UO, fuel has been estimated for FBR as well as LWR on the basis of equilibrium calculation with the SOLGASMIX-PV code. The system considered for the calculation is composed of a gas phase, a CaF, type oxide phase, three grey phases, a noble metal alloy, a mixed telluride phase and several other phases each consisting of single compound. The distribution of elements into these phases and the amount of chemical species in each phase at different temperatures are obtained as a function of oxygen potential for LWR and FBR. Changes of the chemical potential of the fuel-fission products system during burnup are also evaluated with particular attention to the difference between LWR and FBR. Some informations obtained by the calculation are compared with the results of post irradiation examination of UO, fuels.
1. Introduction The irradiated UO, forms a multicomponent multiphase system containing more than 40 elements, in which the chemical state of each element and the amount for respective state are sensitively changed with temperature, pressure, oxygen potential and elementary constitution in the system. An element does not always take a single chemical state but may be distributed into several phases, and a phase is usually composed of a number of elements. For example, Ba may exist in the irradiated UO, as zirconate, molybdate and uranate and the zirconate is supposed to show a wide solid solubility with molybdata and uranate, each of which can involve Sr, Cs and Rb besides Ba. Thus, the chemical state of a fission product (f.p.) element is not determined by a particular reaction but by many competing reactions one another interconnecting and interferring. In this report we present the results of a trial to determine the chemical state of fission products in irradiated UO, assuming that the system is in thermodynamical equilibrium. Naturally, the distribution of elements into phases is not only ruled by the thermodynamical equilibrium of the system but also by the temperature gradient caused by irradiation in the UO, pellet. The latter gives rise to a substantial migration of some elements, particularly vaporable ones, and induces a variation of oxygen potential through the pellet [l].
0022-3115/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
However, introduction of transport theory into the system would require numerical data for the heat of transport of all the elements and of some compounds involved in the system, which we scarcely know. Here, the equilibrium calculation would serve a basis step of investigations before the analysis of the transport phenomena in irradiated UO, fuel.
2. Input data The f.p. elements taken in the present calculation are shown in table 1 with the abundance ratio per fission calculated by the ORIGEN code. The abundance for LWR was calculated for several fuel exposures up to 66 GWd/t with the average power of 33 MW. That for FBR was-obtained on the basis of the operation mode of the prototype FBR “Monju” which consists of cycles each powered 108.1 MW for 148 d. Since the elemental constitution of f.p. in FBR was found not to be so much changed by burnup, only the abundance for the burnup of 48 GWd/t (3 cycles) was taken into further calculation. Elements of which abundance is less than 0.005 per fission were neglected. As seen in table 1, the most notable difference between LWR and FBR is that the fission yields of noble metal elements are much more abundant for FBR compared with LWR. Although Pu is contained by 22.3% in the initial core of the “ Monju” and transmutation of actinide nuclides occurs in LWR
S. Imofo / Chemical state of fission products in irradiated UO,. 1
in fission product
Burnup (MWd/t) 3630
as well as FBR, the difference in thermodynamical properties between U and Pu was neglected in the present work, because the thermodynamical data for Pu compounds required for the calculation were extremely poor. Since Xe and Kr do not form any stable compound at high temperature, these elements were excluded from the system. As for the rare earth elements as well as yttrium, the free energies of formation for the oxides are lower than that of stoichiometric UO, and the valence of a rare earth element does not change during the irradiation except for Ce. Therefore, these were considered to exist always as sesquioxides. According to the review recently presented by Kleykamp , the principal phases appearing in the irradiated UO, fuel pellet are summarized as follows: (1) gas phase, (2) the fuel-f.p. oxide solid solutions, (31 the grey phases, (4) the white inclusions, (5) Pd and Te containing phases, metal compounds. (6) metallic actinide-noble
Taking these experimental findings into account, we assumed the following compounds as playing a dominant role in the fuel-f.p. system. (I) Gas phase: As have been reported by Potter et al. , monomeric as well as dimeric Cs, CsI and Te were considered to be involved in the gas phase. As the partial pressure of I, was found to be much lower than that of I, I, was neglected. For obtaining the value of the oxygen potential, 0, was taken as a component of the gas phase though the pressure was in some cases substantiallly low. An inert gas was added to fix the total gas pressure at 1 atm. (2) Oxide phases: ZrO,, CeO,, Ce,O,, BaO and SrO were assumed to make a CaF, type solid solution with UO,. Yttrium oxide and lanthanide oxides were also supposed to be soluble in UO,, but except for Ce oxides they were excluded from calculation because of their fixed chemical states. Since the solubility of MOO, in UO, is negligibly small , MOO, was treated as an independent oxide isolated from the UO, lattice. (3) Grey phases: Compounds expected to constitute the
S. Imoto / Chemical state of fission products in irradiated UO,
Table 2 Compounds supposed to be present in the “grey phase” Cation
Orthorh. Unknown Unknown
Unknown Unknown Unknown
Perovsk. Dist. cubic Perovsk.
Perovsk. ‘) Perovsk.
‘) Thermodynamical data are available.
grey phases are shown in table 2. The anionic oxides with tetravalent metallic ions, ZtO-, UO:and MOO:- form perovskite type compounds with Ba and Sr [4,5], though the lattice of SrUO, shows a slight
Table 3 Free energy of formation Compound
76650 - 90.46T
[131 [131 [ill [111
- 1085000 + 178.2T
- 1098000 + 194.OT
- 592000 + 99.33
- 533500 + 94.5T
- 1090000 + 214.6T
- 1822000 + 300.8T
- 1730000 + 334.7T
- 1774000 + 298.1T
- 284500 + 45.777
 (estimated)  (estimated) Estimated by author
- 1984930 + 339.77
- 1494000 + 357.8T
- 1549000 + 362.1T
PdTe CsI( I) Cs(,) Te(l)
- 37650 - 2.267
-321700-7.5T 2090 - 6.94T 175OC-24.2T
[201 [131 [201
distortion from cubic system . Kleykamp et al. found that Ba zirconate shows a complete solid solubility with Ba uranate and dissolves a limited fraction of Ba molybdate at 17OO’C . They also stated that some of Ba can be replaced by Cs and Sr in this complex . However, the formation of zirconate, uranate and molybdate with Cs and Rb has not been reported with an exception of Rb,ZrO, . The compounds, Cs,UO,  and Cs,MoO,, are unlikely to be stable. Furthermore, the thermodynamic data are known only for one compound, BaZrO,, among tetravalent zirconates, uranates and molybdates shown in table 2. One possible way to estimate the stability of these complex oxides is to utilize the estimated values of the free energy of formation of Rb,ZrO, and Cs,ZrO, reported by Kohli . So, we have chosen here the mixed zirconate, (Rb, Cs, Sr, Ba)ZrO,, as one of the candidates for the “grey phase”. The anionic oxides with hexavalent transition metal ions, UO; and MOO:-, on the other hand, show a relatively strong affinity to Rb, Cs, Sr, and Ba and the well assessed thermodynamical data on these compounds are available [lo]. The hexavalent molybdate was assumed to form a solid solution (Rb, Cs, Sr, Ba)MoO, in spite of different lattice structures between alkali molybdates and alkaline earth molybdates. Because Cs,UO, and Rb,UO, were found not to be so stable as to compete with the mixed molybdate, they have been excluded from the system, and (Sr, Ba)UO, was adopted as the third candidate of the grey phase. (4) White Inclusions: The white inclusions in the irradiated-fuel have been identified to be an alloy composed of MO, Tc, Ru, Rh, Pd and Ag. Since the alloy is mostly single phased (closed packed hexagonal) , these elements were treated to form a solid solution if they were left elementary. (5) Tellurides: BaTe, Cs,Te and PdTe were considered. (6) Pd compounds: Pd reduces UO, under sufficiently
S. Imoto / Chemical state of fission products in irradiated UO,
low oxygen potential forming a very stable intermetallic compound UPd,. UPd, as well as PdTe was taken into calculation. The free energies of formation for these compounds are given in table 3. Besides these compounds we considered several other compounds, such as, ZrI,, UI,, RbI, %I,, BaI,, Cs,U,O,,, CeTe, TcO,, but calculation showed that the amounts of these compounds were very small as to be completely neglected in the range of temperature and oxygen potential we studied. The program SOLGASMIX-PV developed in Oak Ridge National Laboratory by Besmann and Lindemer was utilized to obtain the equilibrium state of the multicomponent system composed of these compounds by finding the minimum for total free energy of the system.
3. Results 3.1. Oxygen potential In order to see the effect of oxygen potential upon the chemical state of each element in the system, the amount of oxygen was taken as a variable though the amount of oxygen released by one fission event is essentially equal to O/M. Hereafter we call the virtual amount of oxygen supplied to the f.p. system by a single fission event as O/F. The changes of the oxygen poten-
tial with O/F at three different temperatures, 500°C 1000°C and 1500°C, are shown in fig. 1 for the cases of LWR (33 GWd/t) and FBR (48 GWd/t). The oxygen potential vs. O/F curves show a similar feature irrespective of differences in temperature and type of the reactor. At low O/F the oxygen potential increases very steeply with O/F, then shows a plateau somewhat sloped and lastly comes to a steep again. As well known the plateau is attributed to the equilibrium between metallic molybdenum and its oxide sustained by the high yield of MO. It is seen in the figure that the curves for FBR are shifted towards the low O/F side by about 0.3 in O/F compared with those for LWR. This aspect is originated from the fact that the fission yields of element oxidizable at low oxygen potentials, such as Zr, Sr and lanthanides, are smaller for FBR than for LWR. Fig. 2 indicates that the oxygen potential for LWR slightly increases with bumup. This may be due to generation of Pu with fuel exposure, which has an effect similar to the FBR core on the oxygen potential. In the case of FBR, the increase rate was very low. An LWR is fueled with nearly stoichiometric UO, and the value O/U is kept very close to 2.00 during the exposure up to a high burnup. If there is no oxygen consumption, for example, no pickup by zircaloy cladding, the amount of oxygen supplied to the f.p. system by one fission event, O/F, is equal to O/M. This is also supported by the fact that the UO, of the same oxygen potential as determined by O/F = 2.0 shows a very small deviation from the stoichiometric composition. For example, the oxygen potential at 1000°C for O/F = 2.0 is taken to be - 373.5 kJ/mol from fig. 1, and the
i -z -400 :
Fig. 1. Change
of oxygen 1500°C
potential with O/F at 500, 1000 and for LWR and FBR.
BURN Fig. 2. Change
of oxygen potential
(GWDlt) with burnup
S. Imoto / Chemical state
products in irradiated
FIMA (%I Fig. 4. Change of oxygen potential with bumup in FBR at 1OOO~C. 1.98.
is a thermodynamical equilibrium throughout the total system composed of the f.p. system and the fuel, the oxygen potential of the f.p. system should be equal to that of the fuel. AGo 0
FIMA (% 1 Fig. 3. Change of O/F (a) and O/M (b) with burnup in FBR at 1000°C.
We used the curve for FBR (T = 1000°C) in fig. 1 as the AG,(x,) and an approximation equation AG,(x,)
+ 20.08 ln(y/2(2
O/U of UO, corresponding to this oxygen potential value is calculated to be 2.00003 according to the equation by Blackbum . Thus, there is no oxygen transport from the f.p. system to the UO, and vice versa, and the oxygen potential is completely determined by the f.p. system under the condition O/F = 2.0, if there is a thermodynamical equilibrium. On the other hand, the FBR core is usually loaded with a fuel of hypostoichiometric composition, and oxygen released by fission is not only supplied to f.p. but also taken by the fuel. If the amount of the burn up (FIMA) of a fuel is designated b, the irradiated fuel is considered to be composed of the f.p. system with a fraction b and the mixed oxide MO, with a fraction 1 - b. Since the total amount of oxygen is not changed by the irradiation, the relation bx,$_(l-6)x,=x,
- xu) - 1) (3)
as expressing the oxygen potential of a hypostoichiometric MO, with x, = O/M, where y means the value of Pu/(U + Pu), which is 0.223 in the present case. This equation well reproduces the observed values by Woodley . Fig. 3, giving the change of O/M with the burnup b obtained by solving the eqs. (1) and (2) simultaneously under the condition x0 = 1.93, indicates that the value grows almost linearly with burnup until about 8% where O/M attains the value of 2.0. Since the oxygen potential of hypostoichiometric MO, changes almost vertically with O/M near the stoichiometric composition, O/F of the F.P. system in equilibrium with the oxide is also steeply increased. The variation of the oxygen potential given in fig. 4 shows that the oxygen potential is kept very low until the burnup reaches about 8%. These results show that the oxygen released by fission is mostly taken back to the fuel at the early stage of burnup in FBR.
holds, where xr means the O/F, x, the O/M, and x,, the value of O/M at the fuel loading. The product bx, means the amount of oxygen bound to the fission products and (1 - b)x, that contained in MO,. If there
The chemical state of the f.p. at 1000°C with the fraction of each state is shown in figs. S-10 for Zr, MO, Cs, Rb, Sr and Ba, respectively. The upper block in
Fig. 5. Chemical
0 - 500
Fig. 6. Chemical
form of Zr in FBR (a) and LWR (b).
each figure shows the result for FBR and the lower block that for LWR (33 GWd/t). The vertical line in the upper block corresponds to O/F = 1.3, which is estimated to be the maximum value of O/F actually appearing in the f.p. system of FBR, and that in the lower block to O/F = 2.0. As shown in fig. 5, Zr takes mostly the chemical form of zirconate for FBR and dioxide for LWR. Molybdenum is in a state of an alloy with Tc, Ru, Rh, Pd and Ag for FBR, but molybdate is also possible for LWR (fig. 6). For both types of reactors, CsI is always stable in the range of oxygen potential studied. The amount of CsI is almost equal to the amount of iodine existing in the fuel, indicating that iodine is nearly completely converted into CsI. At low oxygen potentials Cs exists as zirconate and telluride, CszTe, with a small amount of Cs(g). At higher oxygen potentials, which the LWR core may experience, however, the molybdate predominates in the chemical states of Cs (fig. 7). Rubidium shows similar feature to the case of Cs except for telluride (fig. 8). Alkaline earth metals, Sr (fig. 9) and Ba (fig. lo), show a similar trend: at low oxygen potential they form dioxides and zirconates, while at high oxygen potential convert into molybdates and uranates. It is seen that Sr is more soluble in the UO, lattice as oxide than Ba.
form of MO in FBR (a) and LWR (b).
Fig. 7. Chemical
1 k J/mall
form of Cs in FBR (a) and LWR (b).
S. Imoto / Chemical state of fission products in irradiated UO,
0.2 0 -500
- L50 (
z 0.8 z
: 0.6 I: 0.L u. lJ_ 0.2
iRANATE -3 OXYGEN
Fig. 8. Chemical
form of Rb in FBR (a) and LWR (b).
-4 50 OXYGEN
- 350 (k
J / mol 1
Fig. 10. Chemical form of Ba in FBR (a) and LWR (b).
Interesting behavior is observed in the chemical form of Te and Pd. At low oxygen potentials, or low O/F, Te forms Cs,Te and a slight amount of BaTe, while at higher oxygen potentials Te exists in the elementary gaseous state at temperature higher than 800°C and as
0.8 z 2 0.6 I: 0.L LL k 0.2 ZIRCONATE
0 - 650
z : c ; a k
0.6 0.L ZIRCONATE
t 0 - L50
- 500 OXYGEN
form of Sr in FBR (a) and LWR (b).
O/F Fig. 11. Change
Fig. 9. Chemical
of the amounts of Pd/Te O/F in FBR at 500°C.
S. Imoio / Chemical state of fission producis rn irradiated U02
PdTe below 800°C. Fig. 11 shows the amounts of Pd/Te compounds at 500°C plotted against O/F for FBR. With increasing O/F, the amounts of UPd, and Cs,Te decrease and those of PdTe and Pd increase until saturated. The figure seems to indicate that Cs,Te and UPd, convert into PdTe at oxygen potentials higher than about - 510 kJ/mol (O/F = 1.29). The amount of PdTe saturated at high oxygen potentials is equal to that of Cs,Te at low oxygen potentials, both limited by the total amount of Te present in the system. Thus, the reaction which takes place with increase of oxygen potential may be represented by the equation: Cs,Te + UPd,
+ (U, Cs) oxide + PdTe + 2Pd (in alloy). The chemical constitution of the gas phase for LWR at 1000°C is given in fig. 12. In the calculation the total gas pressure was assumed to be fixed to 1 atm which was maintained by inert gas supply. It is seen in the figure that the pressure of CsI and Cs,I z are nearly constant irrespective of values of oxygen potential, and the pressures of Cs and Cs, decrease with increasing oxygen potential. This trend is related to the enhanced stability of cesium zirconate with the increase of the oxygen potential. The increase of pressures for Te and Te, with the oxygen potential is attributed to the loss of
- 600 OXYGEN Fig.
stability of telluride as discussed above. The equilibrium pressure of elementary iodine is negligibly small unless the system is in much higher oxygen potentials, and that of I, is calculated to be furthermore low.
4. Discussion It would be important to compare the results of the present calculation with experimental findings. In the post-irradiation examination of FBR fuel some particles of (U, Pu)Pd, or Pd-Fe alloy containing a small amount of U have been observed at the outer part near the cladding-pellet gap [23,24]. According to the present calculation. UPd, is stable at lower oxygen potentials than that corresponding to O/F = 1.6 at both 500°C and 1OOO’C. Thus, our supposition that the f.p. system is kept below O/F = 1.3 for FBR does not contradict with the observation. Cesium molybdate was once suggested to be present in the irradiated FBR fuel to explain the transport of MO out of the hot region of the fuel , but the existence of Cs,MoO, has not yet been definitely confirmed. Calculation shows that the value of O/F corresponding to the appearance of Cs,MoO, is 1.4 at 500°C and 1000°C in the case of FBR. This is another negative answer to the presence of the compound within the fuel. However, that CszTe has not hitherto been observed suggests whether the f.p. system is just near the state of O/F = 1.30 or there are some more stable compounds of Te than Cs,Te under low oxygen potentials, because Cs,Te should be stable under O/F < 1.30 at 5OO’C and O/F < 1.25 at 1000°C according to the calculation. The most perplexing problem faced with the case of LWR is that the grey phase observed in the irradiated fuel is not molybdate but zirconate, while the calculation suggests the molybdate as the predominant phase for Cs, Rb and Sr (figs. 7-9). One possibility to explain this controversy is that the LWR core is perhaps in the state of lower oxygen potentials than that corresponding to O/F = 2.0. The most possible cause of this shift is the pick up of oxygen by Zircaloy. Naturally, that the thermodynamical data base is very poor for zirconates may also cause such a discrepancy, and these chemical species should deserve an active thermodynamical study for the understanding of the grey phase.
- LOO Note added in proof
1 KJ I mot I
12. Equilibrium pressures of gaseous constituents LWR at various oxygen potentials at 1000°C.
It has recently been reported (A. Ohuchi, paper presented at IAEA Technical Committee on “Fuel Rod
S. Imoto / Chemical state of fission products in irradiated UO,
Internal Chemistry and Fission Products Behaviour”, Karlsruhe, FRG, 12-14 November 1985) that Cs,Te was detected in massive deposits formed on the inside surface of BWR fuel rod cladding by post irradiation examination. This suggests that the fuel-cladding gap of LWR is in an atmosphere of substantially lower oxygen potential than described by O/F = 2.0.
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