Fuel Performance Benchmark for the European Sodium Fast Reactor with Metallic Fuel

Oxide fuel is the reference option for the European Sodium Fast Reactor (ESFR) original design as well as the one with the largest commercial experience world-wide. Conversely, the United States are focusing on metallic fuel for the first generation of commercial fast reactors, profiting from an extensive experience and operational feedback. A metallic-fueled version of the ESFR is being evaluated in the context of the ESFR-SIMPLE project, with the aim of challenging the commercial-size oxide-fueled design in terms of safety features and economic performance. High thermal conductivity, high fuel density, ease of fabrication, inherent safety features, high achievable burnup and fuel/coolant compatibility are among attractive features of metallic fuel. Fuel performance assessment is an essential step towards concluding on the benefits of metallic fuel technology for the ESFR design. The goal of this paper is to report on the ongoing activities tailored to modify and adapt the European codes to simulate this technology. Three different institutions and four different codes (FRED, SIM-SFR, SAS-SFR and OFFBEAT) have been involved in the development and benchmarking of metallic fuel simulation capabilities in Europe, using SAS4A/SASSYS-1 (SAS) as reference. SAS and its fuel performance model MFUEL, developed at Argonne National Laboratory (ANL), represent the state-of-the-art code for metallic fuel simulation under steady-state and transient conditions, extensively validated with EBR-II, FFTF, and PHENIX irradiation data and separate effect eutectic formation, cladding creep rupture tests and integrated TREAT M-Series and furnace tests. The current benchmark presents the results of the six-cycles-long steady-state simulation of two fuel rods under representative irradiation conditions for inner and outer fuel core regions. The comparison of code performance focused on safety-relevant thermo-mechanical quantities; namely fuel and cladding temperatures, fission gas release, plenum pressure, fuel expansion and cladding strain. Additionally, the inner cladding wastage, due to lanthanide migration and precipitation at the boundary between the fuel and the cladding, was considered as one of the relevant phenomena for cladding integrity and has been simulated in some of the codes. The code-to-code comparison shows satisfactory agreement for most of the metrics considered in this study. Future work will focus on resolving the remaining discrepancies in cladding strain axial profile predictions and complementing these efforts with experimental validation.