First approach to analyse control rod withdrawal transient in SFR with a multi-physics methodology

Control Rod Withdrawal (CRW) accident in Sodium-Cooled Fast reactors (SFR) is considered, from a safety point of view, as a potential trigger of a core melting accident. It occurs when a Control Rod (CR) is accidentally extracted from the core. This causes a power increase locally around the faulty Control Rod and globally in the core, followed by fuel and sodium temperature increases. Temperature variations induce neutron feedback effects, expansion of materials and mechanical stresses on clads. Then, high temperatures and stresses can respectively lead to fuel melting and clad failures. The potential consequences of this accident requires safety studies to be made, at an early design stage, to ensure manageable behaviour of reactor. Such studies rely on fast calculation schemes, which allow performing probabilistic safety assessments. As high fidelity coupling schemes are time and resource consuming, conservative approach based on single-physics (neutronics, thermal-hydraulics and thermomechanics) discretization are often preferred at this stage. It allows treating each physics domain according to the optimal time and space scales but requires introducing important approximations to take into account the multi-physics feedbacks and couplings. A local multi-physics approach is therefore necessary to analyse CRW transient more accurately. In order to improve both the understanding and the modelling of this transient, this paper will describe the multi-physics phenomenology involved in a CRW and presents a simplified multi-physics coupling scheme compatible with the possibility of performing probabilistic safety assessments. A first implemented coupling scheme, MORPHEE based on improved point kinetics, core thermo-hydraulics and in pin fuel thermo-mechanical models is presented and tested on ASTRID core. A comparison to GERMINAL-V2 shows relatively good agreement on thermal resolution. Further development will focus on transient behaviour of thermal properties and mechanical strains to allow comparison with current safety criteria