Verification of practical modeling approaches of an MSLB event simulation: A CFD contribution

• CFD simulations of an MSLB were conducted. • Dynamic mixing maps at the core inlet were created from the CFD simulations and used to conduct transient core calculations. • The evaluation of the practical methodologies shows that the use of homogeneous mixing maps corresponds to the most conservative of the evaluated alternatives. This article describes the development and application of a Computational Fluid Dynamics (CFD) model to simulate the fluid flow, heat transfer, and boron distribution during a Main Steam Line Break (MSLB) event in a three-loop pre-Konvoi Nuclear Power Plant. The CFD simulations were used to produce time-varying flow maps at the core inlet, which were integrated into a transient SIMULATE-3 K core model using a loose-coupling methodology. The simulations predict complex flow patterns within the reactor pressure vessel, characterized by pronounced temperature stratification in the downcomer and a well-defined cold plume descending directly from the affected cold leg. This coolant behavior significantly influences flow and boron distributions, with notable deviations at the core inlet compared to traditional static mixing assumptions. Transient reactivity calculations conducted with SIMULATE-3 K, utilizing mixing maps derived from the CFD simulations and an in-house mixing tool, show a larger margin to recriticality than traditional static homogeneous mixing maps.