Assessment of the breakaway performance of ZIRLO^® and Optimized ZIRLO^TM high performance claddings for realistic small break LOCA transients

In preparation for proposed change in loss of coolant accident (LOCA) NRC criteria Westinghouse has conducted high temperature oxidation testing of fuel rod cladding. ZIRLO^® and Optimized ZIRLO^TM High Performance Claddings were tested along with Zircaloy-4 cladding. High temperature testing was performed around 1,000degC in order to determine the breakaway temperature and time for each cladding alloy. Breakaway oxidation testing has been performed since 2007 and has been the subject of specific correspondence with the NRC. The reason for this correspondence was to try and resolve the difference between the breakaway time of 3000 seconds that was reported by Argonne National Laboratory (ANL) for ZIRLO cladding with the longer time reported by Westinghouse. Westinghouse concluded that the difference is associated with the difference in heating rates used by the two organizations. Two different apparatus have been used by Westinghouse for testing. In the second and most recent apparatus, the specific protocols including the heat rate requirements from the draft regulatory guide, DG-1261 were followed. Specimens with and without machined scratches were tested. The test specimens were cleaned and weighed prior to testing. The specimens were then exposed to high temperature for times ranging from 300 to 5400 seconds. Following cool down the samples were visually inspected, weighed to determine weight gain, and in selected cases destructively measured for hydrogen content. At 1,000degC, both ZIRLO and Optimized ZIRLO cladding weight gains are within 10% of Cathcart-Pawel (C-P) correlation up to oxidation times of 1200 seconds. At longer exposure times the measured weight gain falls well below the C-P prediction. When using the same steam oxidation apparatus at temperatures from 1050-1200degC, the measured weight gains fall within 10% of C-P predictions. Breakaway times were based on visual appearance and where light oxide was observed on measured hydrogen levels. Based on test results both ZIRLO and Optimized ZIRLO exhibit breakaway times of > 5,400 seconds when subjected to temperatures around 1,000degC with heat up rates that are representative of SBLOCA events. When using the most recent test apparatus with rapid heat up of the cladding and with a limiting scratch present to be consistent with the testing methodology at ANL and to provide a conservative breakaway oxidation time, ZIRLO cladding exhibited breakaway times of > 3,500 seconds and Optimized ZIRLO cladding exhibited breakaway times of > 5,400 seconds. These times are significantly longer than the longest credible small break LOCA event. (author)