Application of safety analysis code SIMMER-IV to blockage accidents in FASTEF subcritical core

• Safety analysis SIMMER-IV code is applied to the transient simulations of FASTEF/MYRRHA.
• Unprotected blockage accidents are analyzed.
• The FASTEF core can survive blockages up to 75% of the nominal flowrate.
• In case of 100% TIB, core melting does no propagate to the whole core.

The FAst Spectrum Transmutation Experimental Facility (FASTEF) core proposed for the MYRRHA reactor in Belgium is cooled by Lead–Bismuth Eutectic and can operate in both critical and subcritical modes (as an Accelerator-Driven System, ADS). FASTEF development is sponsored by EURATOM. In this paper subcritical 94 MWth ADS FASTEF is considered at the end of cycle conditions. To achieve a complete view on the safety behavior, several severe accident scenarios are investigated including Unprotected Blockage Accidents (UBA) without switch-off of the ADS neutron source. The UBA transients have been simulated in 3D with the SIMMER-IV code. To provide a basis for transient analyses, the core has been first analyzed under nominal conditions. Then several hypothetical blockage events in a single fuel subassembly (SA), starting from 50% of the nominal coolant flowrate to the Total Instantaneous Blockage (TIB) have been simulated. Using of SIMMER-IV instead of two-dimensional SIMMER-III allows simulation of the blockage event in the non-central fuel SA, while some approximations, such as ignoring coolant flow in gaps between the SAs and restricting the computation domain spatially are made to avoid excessive increase in the computation time. Thanks to the low power density and small variations in the power, no pin failure occurs if the flowrate is reduced by 70–75% from the nominal level. The 85–100% blockages lead to cladding failures in 6–7 s. After 15–20 s, SA walls also fail resulting in coolant redistribution between the blocked SA and neighboring channels thus increasing the flowrate and recovering the cooling in the affected SA. The SIMMER calculations indicate there is sufficient confidence that effects of the single SA blockage are limited in subcritical FASTEF by single and neighboring SA failures. The results are in line with SIMMER-III calculations obtained in the past for UBAs in other ADS designs.