Behavior of Fuel and Structural Materials in Severely Damaged Reactors

To study the fuel debris treatment at Fukushima Daiichi NPP, information on the behaviour of fuel and structural materials in severely damaged reactors, i.e., oxides and metals of uranium and zirconium is essential. Since sea water was introduced to the reactors, situation of fuel debris became different from that for TMI case. In this paper, phase relations of uranium and zirconium oxides were analyzed by powder XRD method at high temperatures. By the heat-treatment of the mixture of UO2 and ZrO2 (U:Zr=1:1) under 10 torr air, UO2 was oxidized to U3O8 over 800 oC, The UO2 like phase appeared again at 1350 oC which may be caused by the decomposition of U3O8. The oxidation behavior of the UO2-ZrO2 system was also investigated by using solid solution sample with different U/Zr ratios under different steam and oxygen pressures. The oxidation of the UO2-ZrO2 mixture seemed to be suppressed with decreasing U/Zr ratio. The behavior of fuel materials in the presence of seawater was also discussed as well as that for other structural materials.