ESFR-SIMPLE: an assessment of the behavior of MOX fuel pins during transients to guide the design

In the context of the HORIZON-EURATOM call on “Safety of advanced and innovative nuclear designs and fuels”, a new project is underway. Named ESFR-SIMPLE, it is a consortium of 16 partners from Europe. This project follows several previous European projects (EFR, CP-ESFR and ESFR-SMART), and aims to challenge the Generation IV ESFR (European Sodium Fast Reactor) design to improve its safety and economics thanks to innovative technologies. With regard to safety enhancement, a key objective of the project is to conduct a parametric study of the behavior of MOX fuel pins during the transients considered in fast-spectrum SMRs (UTOP and ULOF). This aligns with the design of the SMR version of the ESFR, emphasizing the commitment to safety improvement. The primary objective is to assess the impact of each parameter (pin design, irradiation conditions, and characteristics of the transients). This will provide values and facilitate the guidance of design choices and the selection of nominal operating conditions on which the incident behavior depends. The fuel performance codes that are used in this benchmark are GERMINAL, TRANSURANUS, FEMAXI, FRED, OFFBEAT and SIM-SFR. ... mehrFor the purpose of this study, three distinct designs, each corresponding to a unique geometry for the fuel pellet, have been evaluated. The CAPRA, SuperPhénix, and ESFR designs all correspond to annular pellets with thin, medium, and large diameters, respectively. The steady-state phase preceding the different transients has been selected to last 600 EFPD, corresponding to two cycles, given that discrepancies between codes are increasing with the irradiation time. For the UTOP scenarios, two different levels of reactivity insertion were considered. For the ULOF scenarios, two different pump halving times were considered, resulting in different kinetics for the coolant flow decrease and cladding temperature evolution. The results of this study have emphasized the differences in behavior during transients of the three geometry designs. An evaluation of the margin to melt for the fuel and the risk of failure for the cladding has been conducted for each case.