Example of application of the IRSN approach to assess safety criteria for reactivity initiated accidents

IRSN is developing a consistent and original approach to assess safety criteria for Reactivity Initiated Accidents (RIA). This approach is based on 3 axis. First, a thorough understanding of the physical mechanisms involved in each phase (PCMI - Pellet Clad Mechanical Interaction, and post-boiling phases) of the RIA, supported by the interpretation of the experimental database. This experimental data is constituted of global test outcomes, such as CABRI (France) or NSRR (Japan) experiments, and analytical program outcomes, such as PATRICIA tests, intending to understand some particular physical phenomena. Secondly, the development of computing codes, modelling the physical phenomena. The physical phenomena observed during the tests mentioned above were modelled in the SCANAIR code. SCANAIR is a thermal-mechanical code calculating fuel and clad temperatures and strains during RIA. The CLARIS module is used as a post-calculation tool to evaluate the clad failure risk based on critical flaw depth. These computing codes were validated by global and analytical tests results. Thirdly, the development of a methodology. The first step of this methodology is the identification of all the parameters affecting the clad brittleness. Besides, an envelope curve resulting from burst tests giving the hydride rim depth versus oxidation thickness is defined. After that, the critical flaw depth for a given energy pulse is calculated then compared to the hydride rim depth. This methodology results in an energy or enthalpy limit versus burn-up. This approach can be followed for each phase of the RIA. An example of application is presented to evaluate a PCMI limit for a Zircaloy-4 cladding UO₂ rod at Hot Zero Power. This approach will be applied to MOX fuel, other cladding materials and other initial power levels. This methodology will also be applied for the post-boiling phase, for which another failure mode is expected. (authors)