SIMMER-III Code Simulation of High-Pressure Water-Lead Interaction in Westinghouse’s LEWIN Test Facility

Abstract The Lead-cooled Fast Reactor (LFR) is one of the six advanced reactor system types selected in the Generation-IV (Gen IV) program. Westinghouse (WEC) is developing a highly simplified, passively safe, medium-size, economic Gen-IV LFR as its next-generation utility-scale nuclear power plant. Designed as a pool-type reactor, the Westinghouse LFR has all primary components located inside the reactor vessel (RV). The heat transfer from the primary coolant to the secondary side is accomplished through three primary heat exchangers (PHEs). The PHE failure is one of the safety events considered in designing the Westinghouse LFR. Therefore, the transient behavior of the molten lead pool following PHE failure should be investigated to evaluate its consequences and to adopt effective mitigation measures. Based on the secondary system considered in the WEC LFR conceptual design, a high-pressure water injection into molten lead is considered for the characterization of coolant-coolant interaction (CCI) during a postulated PHE failure accident scenario. In the framework of the collaboration between Westinghouse and ENEA, a separate effect testing (SET) facility named LEWIN (LEad-to-Water INteraction) was designed and built, with the aim of identifying and quantifying the main phenomena that occur during the high-pressure water injection into molten lead. To support the conceptual design and experimental pre-test phase, the SIMMER-III (S-III) computer code was used to model the interaction vessel, the injection line, and the dump line. The choice of this code is justified by its distinctive fluid-dynamics features: it uses two-dimensional meshes to simulate a multi-velocity field with multiphase and multicomponent materials. However, since the original version of the S-III code cannot simulate two different types of coolant at the same time, code modifications have been introduced to enable modeling of the lead-water interaction. This paper aims to assess the capability of SIMMER in simulating the main phenomena expected to take place during the experimental transients in LEWIN, and hence to illustrate the pioneering role the code can play, especially in studying the phase change phenomenology in lead-water thermal-hydraulics systems. The simulation results show the flashing of the water injected into the molten lead, the growth of water vapor bubbles in the lead pool, the lead pool swelling, and the lead sloshing motion. The pressure peak, onset of a choked flow, and pressurization of the cover gas region can be observed as well. This activity has also the objective of supporting a systematic validation and verification (V&V) program for the SIMMER code assessment based on test problems involving lead, lead-bismuth eutectic alloy, and water systems, to advance the code as a next-generation standard tool for LFR safety analysis.