Extension of fuel performance code package FEMAXI; Development and validation of reactivity-initiated accident analysis module RANNS for light water reactor fuels

Tasaki, Yudai ; Udagawa, Yutaka  

Japan Atomic Energy Agency (JAEA) has been developing a fuel performance code, FEMAXI, to evaluate the behavior of LWR fuels under normal operation and transient conditions. In March 2019, FEMAXI-8, the first systematically validated and performance evaluated code, was released. Since then, the code has undergone various improvements. In parallel, since the 2000s, JAEA has been developing the RANNS module as a branch for design basis accident (DBA) analysis, with a particular emphasis on computational stability, so that fuel behavior can be tracked even for very steep transients, in this case mainly reactivity-initiated accidents (RIAs). The specific models include boiling heat transfer, fission gas release by grain boundary failure, and cladding failure determination based on fracture mechanics parameters, which are essential for predicting such transient behavior. In this report, prior to the release of RANNS, we present a description of the models for accident behavior analysis, the relationship with FEMAXI-8 in terms of the design and structure of the program, and the results of a large-scale validation using the extensive database of RIA experiments conducted and accumulated by JAEA, to evaluate the overall RIA analysis performance. The code will be made available to users as a packaged FEMAXI/RANNS, enabling them to analyze fuel behavior under various conditions. The model parameter sets determined through the above validation analyses are also presented in this report, and by referring to them, the analysis can be easily performed with almost no change in usability from the previously released FEMAXI-8.