Cosmos: a computer code to analyze LWR UO2 and MOX fuel up to high burnup

A computer code called COSMOS has been developed for the analysis of V02 and MOX fuel during steady-state and transient operating conditions. The main purpose of the COSMOS is to calculate temperature distribution in the fuel and cladding, and fission gas release from the fuel. Experimental findings regarding such areas as rim effect and thermal conductivity degradation with burnup are taken into account to analyze high burnup fuel. In addition, a mechanistic fission gas release model developed based on physical processes is incorporated into the code to calculate gas release during steady-state conditions. An empirical model developed based on the amount of fission gas stored at the grain boundary is used for transient operations. Another important feature of the COSMOS is that it can analyze fuel segments refabricated from base-irradiated fuel rods. This feature makes it possible to utilize database obtained from international projects such as HALDEN and RISO, many of which were collected from refabricated fuel segments. Based on models for V02 fuel, MOX features such as change in themo-mechanical properties and the effect of microscopic heterogeneity on fission gas release have been taken into account. The capacity of the COSMOS has been tested with a number of experimental results from some international fuel irradiation programs. This paper provides a general description of the models contained in the COSMOS and some of the comparison results between the calculations and measurements