Failure-Bearing Capability of Oxidized Zircaloy-4 Cladding under Simulated Loss-of-Coolant Condition

With a view to examining the failure-bearing capability of Zircaloy-4 cladding under postulated Loss-of-Coolant Accident condition in LWRs, integral tests of rod-burst, oxidation and thermal-shock were performed using simulated fuel containing A1203 pellets sheathed in Zircaloy-4 specimen cladding, filled with He gas, and sealed. This simulated fuel rod was oxidized in steam flowing at the isothermal oxidation temperatures between 920 and 1,330°C for duration ranging of 3~180 min after the cladding burst. After isothermal oxidation, the rod was quenched with bottom-flooding water under the condition of constraint or no constraint. The failure boundary oxidation condition of the cladding on quenching under no constraint condition lay in the region of 35~38% ECR for the isothermal oxidation temperatures between 1,050 and 1,330°C. For the temperatures ranging 970~1,050°C, the boundary value of ECR was somewhat lower than that obtained for higher temperatures. The failure boundary oxidation condition of the cladding on quenching under constraint condition lay in the region of 19~24% ECR for the isothermal oxidation temperatures between 930 and 1,310°C. It is sufficiently large compared with the criterion of 15% ECR in Japanese acceptance criteria for ECCS. Hydrogen absorbed by the Zircaloy-4 cladding as well as oxygen played a dominant role in the fracture behavior of the rod during flooding under constraint condition.