Regresar

Three dimensional calculations of the primary coolant flow in a 900 MW PWR vessel. Numerical simulation of the accurate RCP start-up transient flow rate

Abstract:

Pointed out by the 1986 French PWR Safety studies and by a 1995 OCDE/NEA Specialist Meeting, a potential risk of a reactivity accident may exist in association with boron dilution events and a poor degree of mixing of the diluted fluid in a PWR vessel (Figure 1). The diluted slug transport and mixing in the vessel when a Reactor Coolant Pump starts-up is the most significant case. This safety problem has been taken into account by EDF since 1991 and a R&D program was set up to investigate the primary fluid flow mixing in PWR's vessels. This program is based on numerical simulations and experiments in a 1/5th mock-up. A special focus is set on the scenario of the diluted plug, previously made-up in the cross over leg and then shot into the reactor vessel when the Reactor Coolant Pump starts up. In this paper we present the numerical simulation with the FEM code N3S of this scenario with the accurate fluid flow motion involved by the RCP start-up. Previous simulations performed without taking into account the transient behaviour of the fluid flow are compared with this new computation. This comparison given in terms of the plug mixing in the downcomer and boron dilution maps at the core inlet. We show that the degree of mixing of a diluted plug when simulating the real pump start up is poorer than considering a steady state fluid flow behaviour.