Multiphysics Modelisation of an Unprotected Loss of Flow Transient in a Sodium Cooled Fast Reactors Using a Neutronic-Thermal-Hydraulic Coupling Scheme

Abstract

Sodium cooled fast neutron reactors (SFR) are one of the selected reactor concepts in the framework of the Generation IV International Forum. In this concept, unprotected loss of cooling flow transients (ULOF), for which the non-triggering of backup systems is postulated, are regarded as potential initiators of core melting accidents. During an ULOF transient, spatial distributions of fuel, structure and sodium temperatures are affected by the core cooling flow decrease, which will modify the spatial and energy distribution of neutron in the core due to the spatial competition of neutron feedback effects. As no backup systems are triggered, sodium may reach its boiling temperature at some point, leading to local sodium density variations and making the transient fluctuate in a two-phase flow physics where thermal-hydraulics and neutronics may interact with each other. The transient phenomenology involves several physic disciplines at different time and spatial scales, such as core neutronics, coolant thermal-hydraulics and fuel thermo-mechanics. This paper presents the results of thermal-hydraulic/neutronic coupled simulations of an ULOF transient on the SFR project ASTRID. These coupled calculations are based on the supervisor platform SALOME to link the neutron code APOLLO3® to the system thermal-hydraulic code CATHARE3. The physical approach used by the coupling to describe the neutron kinetic is a quasi-static adiabatic one, updating the normalized spatial power distribution periodically by performing static neutron calculations, while a point kinetic model associated to a neutron feedback model calculates the power amplitude variations.

Multiphysics CATHARE2 modeling and experimental validation methodology against CABRI transients

“…The advancements in numerical methods and computer technology together with the continuous increase in operational experience have been moved in the last few years towards the investigation of realistic (or Best-Estimate) simulations of coupled complex phenomena under accidental scenarios in nuclear power plants. Hence, several activities (even international program [1]) have been completed or are in progress to demonstrate the capabilities of coupled methods [2], [3] and multiphysic simulation tools in performing, in a Best-Estimate way, such complex transients. CABRI reactor is an experimental pulse pool-type research reactor, funded by IRSN and operated by CEA at the Cadarache nuclear center in France.…”