Modelling of defective CANDU fuel phenomena

Abstract

A mechanistic model is presented for the modelling of defective CANDU fuel behaviour. This physically-based treatment includes a multi-physics coupling of fission product release, fuel oxidation and sheath hydriding phenomena with the occurrence of fuel failure. The stoichiometry deviation profile is used to evaluate effects on the fission-product diffusivity and heat conduction in the fuel. A time-dependent fission product release model accounts for the transport of fission products of the short-lived radioiodine and noble gas isotopes in the fuel matrix with their release into the fuel-to-sheath gap and coolant. Transport of steam and hydrogen gases along the gap is further considered as a boundary condition of the fuel oxidation model. This treatment provides an estimate of the partial pressure ratio of hydrogen-to-steam in the gap for evaluation of the critical ratio necessary for the promotion of sheath hydriding. Secondary hydriding and fuel element deterioration is subsequently evaluated in accordance with a Locke-type correlation.