Nuclear fuel rod fragmentation under accidental conditions

Abstract

This paper deals with fuel rod fragmentation during a core meltdown accident in a Nuclear Power Plant. If water is injected on the degraded core to stop the degradation, embrittled fuel rods may crumble to form a reactor debris bed. The size and the morphology of the debris are two key parameters which determine in particular heat transfer and flow friction in the debris bed and as a consequence its coolability. To address this question, a bibliographic survey is performed with the aim of evaluating the size and the surface area of the fragments resulting from fuel rod fragmentation. On this basis, a model to estimate the mean particle diameter obtained in a reflooded degraded core is proposed. Modelling results show that the particle size distribution is very narrow if we only take into account fuel cracking resulting from normal operating conditions. It leads to minimum mean diameters of 2.5mm (for fuel particles), 1.35mm (for cladding particles) and 2mm (for the mixing of cladding and fuel fragments). These results are obtained with fuel rods of 9.5mm outer diameter and cladding thickness of 570μm. The particle size distribution is larger if fine fragmentation of the highly irradiated fuel rods during temperature rise is accounted for. This is illustrated with the computation by the severe accident code ASTEC, codeveloped by IRSN abd GRS, of the size of the debris expected to form in case of reflooding of a French 900MW reactor core during a core meltdown accident.