Analysis of operational factors that can lead to fuel failure in WWER units
Fuel failure is likely to occur during operation of nuclear power units. Failure may result in escalation of primary coolant activity. Hydrogen uptake by cladding is high in failed fuel rods. Under certain conditions it may lead to secondary degradation and formation of coarse defects in cladding. Fuel washout becomes possible in case of severe cladding degradation with ensuing contamination of primary circuit. Fuel failure necessitates high-cost activities on detection of leaking fuel. Expensive mitigating procedures may be needed as well. The present paper is focused on fuel failure in WWER units in the period of 2003 through 2013. The data of visual inspections and post-irradiation examinations of leaking fuel assemblies in hot cells are reported. Degradation of defective fuel rods is related to fuel operation conditions. The threshold conditions are estimated which lead to severe secondary degradation of leaking fuel and substantial increase in release of fission products into primary coolant. The accumulated data are summarized in relation to possible impact of operational factors on the rate and causes of fuel failures.
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