Analysis of trends in fuel rod depressurization and determination of 'gas leak' and 'pellet-water interaction' type failures using radiation monitoring techniques of fuel rod leak tightness

Analysis of fuel rod failures in the Light Water Reactor operation is presented. Analysis includes the mechanism of formation and development of fuel rod cladding failure until through-wall defects appear (welding defects; inner hydriding defects; pellet-cladding interaction; crud deposit - intensified corrosion) as well as factors that determine defects propagation after fuel rod depressurization (metal condition in the vicinity of defect determined by the mechanism of formation and propagation defect; operational transients; degree of core cooldown after depressurization during preventive maintenance). Possibilities of in-service monitoring of fuel rod through-wall crack propagation using normal tools of cladding back-tightness monitoring are addressed and used in the course of analysis. Characteristics and values are presented for radiation parameters for fuel assemblies during propagation of defects with different degrees of rod depressurization, including ''gas leak'', cladding crack and ''open pellet-water interaction'' with potential particulate fission product release from the damaged rods as well as after formation of recurring defects. Based on experimental data on specific activity of different iodine isotopes in the primary coolant, a mathematical model to analyse defect propagation trends has been developed. The model describes the rate of radionuclide release from depressurized rods and the rate of nuclear fuel fission processes in the vicinity of defects. The model and results of analysis are illustrated by experimental and statistical data on depressurization of WWER (PWR) and RBMK (BWR) reactor fuel rods. Possibility to solve the problem of predicting defect propagation is considered. (author). 5 refs, 9 figs, 3 tabs