Influence of Burnup on Fuel Rod Characteristics, and Implications for Loss-Of-Coolant-Accident Behavior

摘要

NRC has recently developed a hypothesis on the mechanisms and conditions causing the fine, sand-like fuel fragmentation seen in Loss-of-Coolant Accident (LOCA) testing of very high burnup rods. The hypothesis is based on a number of observations available from LOCA tests performed at Halden and Studsvik on high burnup boiling water reactor (BWR), pressurized water reactor (PWR) and VVER fuel rods, namely that:1. Extensive fragmentation was not present before testing 2. The extent of relocation and fragmentation is correlated with local strain and proximity to rupture location 3. A significant change in fragmentation size is observed between high (≈60 GWd/MTU) and very high (≈70 GWd/MTU) burnup 4. Pressure transducers in the plenum measured a slow depressurization after rupture in select tests, indicating restricted gas flow between the plenum and rupture region in these tests. 5. For high burnup (≈60 GWd/MTU) LOCA tests, most fine fragments appear to originate from the periphery of the pellet, but for very high burnup (≈70 GWd/MTU) LOCA tests, fine fragments appear to originate from all radii.