Reevaluation of safety criteria of Zircaloy oxidation embrittlement for improvement of safety margin under accident conditions

Abstract

The safety margin, the difference between the physical load on the plant and the capacity of the plant to withstand, is considered as one of the indicators of safety analysis. It is necessary to guarantee the safety margin as well as to prepare for Design Extension Condition. We investigated an optimal safety limit which is more realistic than the safety limit of the fuel cladding in the existing design basis accidents. Based on the experimental data of oxidation of the zirconium-based claddings which have been carried out since the 1970s, Lee et. al. proposed new safety limits which exceed the peak cladding temperature of $1204^\circ C$ The high temperature oxidation experiment was performed to evaluate the new safety limits. After the oxidation and quenching experiment, the ring compression tests were conducted to evaluate the integrity of specimens. As a result, the specimen having the oxidation rate of 13% maintained its integrity at the temperature from $1204^\circ C$ to $1300^\circ C$. The oxidation tools of the zirconium-based claddings were used to evaluate the radial distributions of the thickness and oxygen concentration of the beta-zirconium layer, which is believed to be closely related to the ductility of the zirconium. The previous studies claim that the claddings containing β thickness of over 0.3 mm with the oxygen concentration of less than 0.7wt% could maintain its integrity. This criterion was converted to ECR limit of 12% at $1370^\circ C$. This result was regarded as a close result to 13% considering the uncertainties of the tools. We simulated the large break loss of coolant accidents (LBLOCA) with the MARS code and KINS-REM to find out how much power margin the optimal safety limit could guarantee with the best-estimate method. As a result, we found out that the fracture-based optimal safety limit proposed by Lee enabled power to uprate 10% or more under the LBLOCA condition.