Analysis of mixed-oxide fuel behavior during RIA tests using FALCON MOD01

This paper summarises the development and verification of a MOX fuel pellet transient gaseous swelling model for use in the FALCON fuel performance code and the evaluation results of MOX fuel rods being tested at the Japan Atomic Energy Agency (JAEA) Nuclear Safety Research Reactor (NSRR). The FALCON code, developed under the sponsorship of EPRI, uses a fully two-dimensional finite element analysis methodology to model the thermal and mechanical behavior of the fuel and cladding under a variety of irradiation conditions. The fuel performance code, integrated with the MOX pellet model, was then applied to the analyses of 8 fuel rods tested at the NSRR facility. The test program is composed of 7 tests conducted at cold conditions and one test performed in the high temperature test capsule. The results show that FALCON demonstrates that pellet thermal expansion combined with gaseous swelling enhanced-PCMI can accurately represent the thermo-mechanical behavior of MOX pellets during rapid energy deposition. The comparison of the calculated results to the available experimental measurements for the MOX tests finds that gaseous swelling enhanced-PCMI is an important contributor to the mechanical behavior of MOX fuel rods under RIA conditions. Moreover, the model development and evaluation process has found that the MOX fuel pellet expansion process leading to cladding deformation can be several times higher than that arising from UO₂ pellets at equivalent peak fuel enthalpy levels