Experimental and CFD analysis of the effects of debris deposition across the fuel assemblies

Abstract

During a loss of coolant accident (LOCA) or a main steam line break accident (MSLB) in a pressurize water reactor (PWR), debris of different sorts like particulate, latent fiber or chemical nature might be generated due to jet impingement or ensuing, hot, pressurized steam leaking from the site of damage. This debris gets transported to the emergency core cooling system (ECCS) or the containment recirculation system (CRS) and might get deposited on the sump screens and causes a head loss. Some of these debris particles escape the sump screens and get access to the primary system through the bypass and thereafter challenge the core flow path and cause the head-loss across the fuel assemblies. The deposition of these debris particles in the fuel assemblies generates the possibility of hindering the long term core cooling (LTCC) of the nuclear power plant. The US Nuclear Regulation Commission (NRC) regulates the possibility of a commercial nuclear power plant to be able to maintain long term core cooling in case of a LOCA based event. In light of the aforementioned reasons the study of the debris particles inside the fuel assembly under different conditions becomes necessary to investigate the deposition of these debris particles in the assembly. This paper consists of two parts. The first part reports the experimental results carried out on an actual 15 × 15 experimental fuel assembly for the study of the effects of debris deposition on the pressure drop observed due to it. The second part reports CFD study of debris particles deposited on the spacer grid in the form of a continuous porous medium under different conditions. The study involves steady state (pseudo-transient) analysis of a scaled model of the fuel assembly with the variation of thickness of the porous debris bed, the inlet flow-rate of the fluid and the void fraction of the porous medium against the pressure drop observed.