Improvement of a fuel performance code, COSMOS for high burnup UO 2 and MOX fuel

Abstract

A fuel performance code, COSMOS, has been developed for the analysis of MOX fuel as well as UO2 fuel up to a high burnup. The models for the fuel thermal conductivity and corrosion prediction for cladding were recently improved. Both models enable the COSMOS code to precisely predict the in-pile behavior for the high burnup fuel. The other main activity for the COSMOS code is the development of the GUI system to easily make and maintain the input and output files. The GUI system consists of the pre- and post- processor and batch interface. The preprocessor for the input files can easily generate the necessary input for COSMOS code. The postscript format for the output files provides a prompt understanding for the calculation results. The batch interface provides the calculation of a multiple database at once. The fuel performance code, COSMOS, has been verified by using a recently available database from the irradiation results in a commercial reactor and research reactors with instrumented MOX and UO2 fuel. With the rigorously characterized fabrication data and irradiation information, the COSMOS code predicts well the thermal behavior and rod internal pressure for a high burnup fuel. The calculation results by the COSMOS code also demonstrated the code's applicability for instrumented irradiation. According to the present verification, the following two aspects will be improved: (1) the recovery effect on the thermal conductivity due to a significant fission gas release and (2) the fission gas release by microcracks to obtain an the accurate fission gas release behavior in the high burnup MOX fuel with a multiple power up-and-down. Therefore, with all the other verifications of the COSMOS code up to now, it can be concluded that the COMSOS fuel performance code is applicable for the design of MOX as well as UO2 fuel up to a high burnup.