Loss Of Flow Accident (LOFA) with protection in NUR nuclear research reactor; three dimensional analysis of a Fast LOFA and a Slow LOFA

Abstract

The Loss Of Flow Accident (LOFA), with protection and without intervention of natural convection in the cooling process, is studied in the NUR nuclear research reactor for two accident configurations; a Fast Loss Of Flow Accident (FLOFA) and a Slow Loss Of Flow Accident (SLOFA). The accident is due to an inadvertent partial closure of coolant loop isolation valve. All the sequences of each accident are presented, namely the steady-state, the transient (SLOFA or FLOFA), the nuclear reactor emergency shutdown (SCRAM), the cooling of the decay heat and the contribution of 20% of flow to guarantee a good nuclear reactor core cooling. In order to be able to follow the thermalhydraulic accident down to the smallest detail, the equation set-up, based on the three conservation equations of continuity, momentum and energy, in three dimensions of space and one dimension of time, coupled to the turbulence model K−ω SST, is established. The models of the distribution of thermal power and decay heat are introduced into the mathematical model as well as the exponential flow loss equation. The boundary conditions for each sequence of the accident are fixed. The resolution of all the equations was carried out using Computational Fluid Dynamics (CFD) through FLUENT calculation code. It was possible to show the evolution of the temperature of the coolant and the cladding in the respective planes of each physical quantity for each time step of the accident, one second for the FLOFA and 2 s for the SLOFA. The evolution of the reactor power is presented before, during and after the SCRAM for each accident. After the SCRAM, the decay heat is taken into account and the average evolution of the temperature of the coolant and the cladding is presented as a function of time. The cooling of decay heat with no adding flow and with adding 20% flow was also studied as a function of time and the difference between the two approaches is presented. The FLOFA was studied for 2.2 s and the SLOFA for 25 s. The results obtained for the FLOFA and the SLOFA show that these accidents generates high temperatures but always remaining below the critical temperatures whether for the heat transfer fluid or for the cladding for the conditions and the data which have been retained for this work.