Zircaloy cladding oxidation is an important phenomenon for both design basis accident and severe accident, because it results in cladding embrittlement and rapid fuel temperature escalation. For this reason during the last decade, many experts have been conducting experiments to identify the oxidation phenomena that occur under design basis accident and to develop mathematical analysis models. However, since the study on design extension conditions is relatively insufficient, it is essential to develop and validate a mathematical and physical model simulating the oxidation of the cladding material at high temperatures.
In this study, the QUENCH-05 and -06 experiments were utilized to develop the best-fitted oxidation model and to validate the SPACE code modified with it under the design extension condition. It is found out that the cladding temperature and oxidation thickness predicted by the Cathcart-Pawel oxidation model at low temperature (T < 1853 K) and Urbanic-Heidrick at high temperature (T > 1853 K) were in excellent agreement with the data of the QUENCH experiments.
For LOCA without SI (Safety Injection) accidents, which should be considered in design extension condition, we performed the evaluation of the operator action time to prevent core melting for the APR1400 plant using SPACE. For the LBLOCA and SBLOCA without SI accident, we performed sensitivity analysis for the operator action time in terms of the number of SIT, the recovery number of the SIP, and the break sizes for the SBLOCA. Also, with the extended acceptance criteria, we evaluated the available operator action time margin and the power margin. It is confirmed that the power can be enabled to uprate about 12% through best-estimate calculations.