Study on mechanism and threshold conditions for fuel fragmentation during loss-of-coolant accident conditions
Narukawa, Takafumi ; Udagawa, Yutaka
To clarify the mechanism and temperature threshold for fuel fragmentation during loss-of-coolant accidents (LOCAs), out-of-pile heating tests on bare fuel pellet pieces taken from a high-burnup PWR UO fuel rod (segment average burnup: 81 GWd/tU) were performed. The fuel pellet pieces taken from various regions in the radial direction of the fuel pellet were inductively heated with no cladding restraint in vacuum up to 1473 K at a rate of 5 K/s. During the heating tests, the fission gases released from the fuel pellet pieces were continuously analyzed in-situ using a quadrupole mass spectrometer. Following the heating tests, microstructural observation of the fuel pellet fragments was carried out. Based on the relationship between the extent of fuel fragmentation and the terminal temperature, and the time history of fission gas release, temperature thresholds for minor fuel fragmentation and slightly more fuel fragmentation were estimated to be 973 - 1073 K and 1173 - 1273 K, respectively. The extent of fuel fragmentation and the amount of fission gas release became more pronounced with increasing temperature. Further, the microstructural observations after the heating tests revealed that most of the fuel fragments smaller than approximately 500 - 750 m have microstructures consisting of many micropores and subgrains, which are characteristic of the dark zone or high-burnup structure. On the basis of these results, the mechanism of fuel fragmentation during LOCAs was discussed.