Generation of pressure pulse and water hammer triggered by high-burnup fuel failure under reactivity-initiated accident conditions

Mihara, Takeshi  ; Udagawa, Yutaka  

To investigate generation of mechanical energies such as pressure pulse and water hammer triggered by a failure event of high-burnup fuels during a reactivity-initiated accident (RIA), online-measured data of relevant test cases in the NSRR RIA-simulated test database have been analyzed. The tests were performed on UO and MOX fuels of 44-81 GWd/t, resulting in fuel failure. The analysis revealed a considerable increase in the ratio of mechanical energy to injected thermal energy with burnup above ~50 GWd/t for pressure pulse and water hammer and their correlation with transient fission gas release amount. These findings suggest the mechanism of burnup effect that larger amount of released fission gas produces higher rod internal pressure and thus more intensive gas outflow at the fuel failure instant, accompanied by fuel fragment release, leading to an increased efficient fuel/coolant-thermal interaction. Simultaneously, increasing the specific surface area associated with grain boundary decohesion could enhance the reaction speed.