Effect of non-Isothermal Transient Zircaloy Oxidation on Emergency Core Cooling System Criteria

The current Emergency Core Cooling System (ECCS) criteria (LOCA criteria) are set to assure an adequate level of post-LOCA cladding ductility. The criteria limit the Peak Cladding Temperature (PCT) and Equivalent Cladding Reacted (ECR) to 1204C and 17% (calculated by the Baker-Just correlation), respectively. These criteria were developed by quantifying residual ductility of isothermally steam-oxidized and subsequently water quenched Zircaloy cladding specimens in Ring Compression Test. In reality, LOCA accompanies a significant timevarying temperature change of fuel cladding. The current experimental protocols for the aforementioned postLOCA cladding ductility assessment neglect the nonisothermal nature of cladding oxidation. The key knowledge gap of using the isothermal experimental data is twofold; 1. The isothermal ECR prediction is believed to have limited accuracy for nonisothermal oxidation transience, and 2. No assurance is given to the agreement of post-LOCA ductility between isothermal and non-isothermal oxidation. That is, even if ECR prediction is accurate, question remains if postLOCA ductility of non-isothermally oxidized cladding and isothermally oxidized cladding would be acceptably identical. Hence, from the perspective of post-LOCA ductility assurance, the effect of non-isothermal oxidation on both ECR prediction and cladding mechanical behavior needs to be systematically quantified. This study aims at assessing the effect of nonisothermal transience on the predictability of existing ECR correlations, and cladding’s post-LOCA residual ductility. To verify it, cladding steam oxidation experiments were conducted to compare the ECR calculated by an existing isothermal correlation and ECR experimentally obtained in rapidly varying temperature. Ring compression tests are then followed to measure stress-strain curve of both cases from which the residual ductility is assessed.