RELAP5-3D system thermal-hydraulics code adequacy forMYRRHA safety analyses

RELAP5-3D is a widely used system thermal-hydraulic (STH) code for safety analysis oflight water reactors. It has been extended to 26 other working fluids, including lead and lead-bismuth(LBE), enabling its application in the design and safety of advanced reactors such as the MYRRHAreactor, which uses molten LBE as its primary coolant. This justifies its selection as a main STH code forMYRRHA safety analysis. However, to anticipate its acceptance for the facility licensing, a thoroughverification and validation (V&V) of the code is essential. This paper presents the methodology devel-oped for the RELAP5-3D code V&V in the context of MYRRHA safety analyses. The code is assessed,focusing on important phenomena that determine the evolution of the key figure of merit (FoM) fordesign basis accidents, that is, the cladding behavior. A validation matrix has been established whichincludes separate test effects and integral test effects facilities, including scaled facilities. Selectedresults are presented and important phenomena for the FoM which require code enhancement arehighlighted.