Three dimensional calculations of the primary coolant flow in a 900 MW PWR vessel. Steady state and transient computations

After the Tchernobyl accident a working group was created to analyze the French PWR Safety with a respect to potential risk of reactivity accident. Potentially risky situations are those which can lead to heterogeneous boron concentration or temperature of the primary coolant fluid. This paper reports a Research and Development action based on numerical simulations and experiments on the primary coolant temperature or boron mixing capabilities in a PWR vessel. New numerical results obtained with the thermal hydraulic Finite Element (FE) Code N3S are presented. In these calculations the FE mesh takes into account the geometry of the lower plenum plates and columns. Two configurations have been investigated The first one is a steady state fluid flow mixing case. The reactor is cooled by free convection and the three loops, balanced in mass flow rate, are in operation. The second is a free boron plug transient case. It is related to the mixing of a clear plug injected in the vessel when a primary coolant pump starts-up. Two clear plug volumes have been investigated (3 and 8 m3). Comparisons between these new numerical results and the data previously obtained (see Alvarez et al., 1992, Alvarez, Martin and Schneider, 1994) are presented in this paper