Burnup impact of an ASTRID-like core during a ULOF with SIMMER-III

Innovative Sodium-cooled Fast Reactors (SFRs) are currently investigated in the ESNII+ European project. The goal of the WP6 ^Core safety of this project is to support the development of the ESNII roadmap, the implementation of the ESNII deployment strategy and the licensing of the ESNII systems. This is done by identifying the experimental and theoretical R&D activities which are necessary for improving the present designs, as well as the existing methods, tools and databases for static and transient safety analysis of the ESNII critical reactor cores. One of the main issues of the WP6 ^Core safety of the ESNII+ project, as for ASTRID development, is to assess the behavior of the ESNII+ core (ASTRID-like core) in severe accidents at a representative stage, ie. the end of equivalent cycle (EOC), as the sodium voiding effect is less favorable at this moment. Consequently, the SIMMER-III code system (coupled thermohydraulics, pin mechanics and neutronics) is used as it can represent the accident up to an advanced core degradation. However, it has been developed to perform neutronics calculations at the beginning of life (BOL, without irradiation). This gives good results for homogeneous cores at EOC, but a new methodology needs to be implemented to perform EOC calculations for heterogeneous ones. The aim of this paper is to present the behavior of the ESNII+ heterogeneous core at EOC, so as to highlight the importance of burnup in the accident scenario. Thus, a new methodology developed in the framework of the ESNII+ project to perform neutronics calculations at EOC is presented. Then, Unprotected Loss Of Flow (ULOF) calculation, with a 22s primary flow-rate halving time, is performed at EOC. The sodium boiling and the pin degradation happen earlier at EOC than at BOL, but the core degradation is slow in both calculations. Despite less favorable feedback coefficients at EOC, and thanks to its heterogeneous geometry, the ESNII+ core in ULOF with a 22s halving time, does not lead, with the given hypotheses, to a power excursion. (author)