Thermodynamic Assessment of UO 2 Pellet Oxidation in Mixture Atmospheres under Spent Fuel Pool Accident

For an analysis of the oxidation behavior of UO2 nuclear fuel pellet under a loss of water coolant accident in a spent nuclear fuel pool of an LWR, thermodynamic assessments of UO2 oxidation were carried out under various atmospheric conditions. In a steam atmosphere, it was assessed that UO2 would not be fully oxidized into U3O8 due to the relatively lower oxygen partial pressure, while UO2 will be fully oxidized into U3O8 in an air atmosphere. In an air and steam mixture atmosphere, the UO2 oxidation was dominantly affected by the air volumetric fraction, because of the relatively higher oxygen partial pressure of air. In addition, the effect of H2 volumetric fraction on the oxygen partial pressure under a mixture atmosphere was calculated, and it was revealed that UO2 pellet oxidation could be reduced above the critical value of H2 volumetric fraction.


Introduction
Since the Fukushima Daiichi nuclear disaster on March 2011 [1], an accident in the spent nuclear fuel pool of an LWR has been extensively regarded an important concern [2] [3].In particular, if a loss of water coolant accident in a spent nuclear fuel pool occurs, it can bring about a massive failure owing to a drastic increase in the temperature of the spent nuclear fuel.In addition, the possibility of an external leakage of a significant amount of radioactive materials can increase.Under a loss of water coolant accident, an air and/or steam oxidation reaction of the spent nuclear fuel leads to a failure of the fuel cladding, and air and/or steam oxidation of the spent fuel pellet will follow.An oxidized spent fuel pellet will be expanded and pulverized, and a failure of the fuel cladding can additionally take place.
In terms of the accident analysis in a spent nuclear fuel pool, various accident scenarios can be suggested.Therefore, the oxidation data of the UO 2 and spent fuel pellet required for an accident analysis should be secured.The oxidation data of the UO 2 and spent fuel pellet should be verified by experiments under various atmospheres, because reliable experimental data are clearly helpful to achieve valuable results of an accident analysis.
Under an accident in a spent nuclear fuel pool, the UO 2 spent nuclear fuel pellet can be exposed to various environments.If the water coolant in the spent fuel pool is lost wholly or partially, the spent nuclear fuel will be exposed to the air atmosphere.The temperature of the spent nuclear fuel can increase by the loss of water coolant, and a steam atmosphere will be gradually generated.In addition, the hydrogen atmosphere can be generated by the reaction of the spent nuclear fuel clad-ding (Zr-based alloy) with air, steam or water (Zr oxidation).
In this study, for analysis of the oxidation behavior of UO 2 nuclear fuel pellet under a loss of water coolant accident in a spent nuclear fuel pool of an LWR, the thermodynamic assessments of UO 2 oxidation were carried out under various atmospheric conditions.

Thermodynamic Calculations
The thermodynamic calculations of UO 2 oxidation reaction under various atmospheres were performed using a chemical reaction and equilibrium software (Outokumpu HSC Chemistry for Windows, ver.5.1).The thermo-chemical database in the software was utilized to calculate the intended reactions.
As mentioned above, the UO 2 oxidation reactions under various environments were anticipated, and the oxygen partial pressure is the most important factor in the oxidation reaction.Therefore, considering the various atmosphere conditions (steam, air + steam, air + steam + H 2 mixture), a thermodynamic calculation of the oxygen partial pressure in the mixture atmosphere was carried out.In addition, based on the calculated oxygen partial pressure, the equilibrium oxy-gen-to-uranium ratio as a function of temperature was assessed.

Results and Discussion
The oxygen partial pressure in a pure steam atmosphere as a function of temperature is shown in Figure 1.The temperature of the UO 2 nuclear pellet under nuclear reactor operation is typically in the range of 400˚C to 1200˚C.Therefore, it can be thought that the UO 2 pellets are exposed to low oxygen partial pressure (below ~10 −4 ).
The equilibrium oxygen-to-uranium ratio can be calculated using the oxygen partial pressure of the given environment and a phase diagram of the U-O system with superimposed oxygen pressure isobars (Figure 2).The isobars are indicated by index k in p = 10 −k where p is in atm [4].Figure 2 shows that U 3 O 8 and U 3 O 8-z phases are observed above p(O 2 ) = ~10 −3 , that is, a UO 2 pellet can be fully oxidized in this oxygen partial pressure range.
According to the p-C-T relationships for the U-O system, Y.S. Kim suggested equations [4], where p is the  oxygen partial pressure, C is the oxygen-to-uranium ratio, and T is the temperature.The equilibrium oxygen-touranium ratio as a function of temperature was calculated using the equations (Figure 3), and the calculated results have a good agreement with the literature data [5]- [9].In the UO 2 oxidation test under a pure steam atmosphere, the maximum weight gain of the uranium oxide pellet can be anticipated by the calculated results.
Figure 4 shows the oxygen partial pressures under air and steam mixture atmospheres as a function of temperature.It was calculated using various volumetric fractions of mixture atmospheres.Figure 4 shows that the oxygen partial pressure is higher than p(O 2 ) = 10 −3 in the temperature range of concern (400˚C -1100˚C), even though very small amounts of air were added into the environment.It can be thought that the equilibrium oxygen-to-uranium ratio of uranium oxide pellets in an air and steam mixture atmosphere is similar with that in an air atmosphere.
Figure 5 shows the partial pressures of H 2 O, O 2 , and H 2 in a (air + 50 vol% steam) + 10 vol% H 2 mixture atmosphere.Owing to the addition of H 2 , p(H 2 O) in the environment increased and p(O 2 ) decreased.It is thought that the oxidation of the UO 2 pellet can be affected by the p(H 2 ), that is, the pellet oxidation reaction can be reduced owing to the p(O 2 ) decrease.
Figure 6 shows that there is a critical value of p(H 2 ) that can be drastically decreased p(O 2 ) in the environment.In the case of an air + 50 vol% steam mixture, it was revealed that the critical value of p(H 2 ) is 0.17.That is, at above 17 vol% H 2 , the pellet oxidation reaction will be reduced.In addition, the critical value of p(H 2 ) in an air and steam mixture environment is proportional to the amount of the air atmosphere.

Summary
To analyze the oxidation behavior of a UO 2 nuclear fuel pellet under a loss of water coolant accident in a spent nuclear fuel pool of an LWR, the thermodynamic assessments of UO 2 oxidation were carried out under various atmosphere conditions (steam, air + steam, air + steam + H 2 mixture).
Based on a phase diagram analysis, above p(O 2 ) = ~10 −3 , a UO 2 pellet can be fully oxidized into U 3 O 8 and U 3 O 8-z phases.The oxygen partial pressures in the air and steam mixture environment are higher than p(O 2 ) = 10 −3 in the temperature range of concern (400˚C -1100˚C).It can be thought that the equilibrium oxygen-to-uranium ratio of a uranium oxide pellet in an air and steam mixture atmosphere is similar with that in an air atmosphere.
In addition, in an air + steam + H 2 mixture environment, there is a critical value of p(H 2 ) that can be drastically decreased p(O 2 ).That is, the pellet oxidation reaction will be reduced, above the critical value of p(H 2 ).
It is expected that these thermodynamic assessment results are contributed to establish more efficient experimental plan for simulation of the accident scenarios in spent fuel pool.

Figure 1 .
Figure 1.Calculated oxygen partial pressure in pure steam atmosphere as a function of temperature.

Figure 2 .
Figure 2. Phase diagram of the U-O system with oxygen pressure isobars superimposed.The isobars are indicated by index k in p = 10 −k where p is in atm [4].

Figure 3 .
Figure 3. Calculated equilibrium O/U ratio and weight gain in pure steam atmosphere as a function of temperature.

Figure 4 .
Figure 4. Calculated oxygen partial pressure in air and steam mixture atmosphere.