Abstract
Best-estimate computational methods are here used to analyse the thermo-mechanical behaviour of high-burnup UO2 fuel rods under postulated reactivity initiated accidents in light water reactors. The considered accident scenarios are the hot zero power rod ejection accident in pressurised water reactors and the cold zero power control rod drop accident in boiling water reactors. For these accidents, fuel enthalpy thresholds associated with clad tube failure and fuel incipient melting are determined with respect to fuel burnup in the range from 30 to 70MWd(kgU) 1. The thresholds for clad tube failure are calculated by means of a strain-based clad failure criterion, which rests upon data from more than 200 out-of-pile mechanical property tests on recrystallized Zircaloy-2 and stress relieved annealed Zircaloy-4 cladding. The enthalpy thresholds for fuel incipient melting are calculated, considering the depression of fuel melting point with increasing burnup as well as burnup-related changes to the pellet radial temperature distribution. The calculated enthalpy thresholds for both clad tube failure and fuel incipient melting drop moderately with increasing burnup, and the thresholds are similar for the pressurised and boiling water reactor accidents under consideration. Extensive sensitivity studies are carried out, in order to quantify uncertainties in the calculated bestestimate thresholds.
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