An Assessment of the Fuel Pulverization Threshold During LOCA-Type Temperature Transients

Abstract

To investigate the potential disintegration to powder of high-burnup fuel pellets during a rapid temperature transient, the Nuclear Fuels Industry Research (NFIR) Program commissioned two independent scoping studies. The first investigated the effect of hydrostatic restraint pressure on fission gas release during a series of fast temperature ramps. In the second study laser heating was used to investigate the temperature at which small samples of fuel fragmented. From the observations made in these studies, local burnup and temperature thresholds of 71 MWd/kg HM and 645°C were identified for fuel pulverization during a loss-of-coolant accident (LOCA). It is shown that fine fragment production in integral LOCA tests performed in other independent investigations at Studsvik and Halden was generally well predicted using these thresholds of burnup and temperature. The NFIR investigations also reveal that the degree of pulverization and resulting fragment size are dependent on the temperature ramp rate. Moreover, they confirm that pulverization can be substantially reduced by the imposition of hydrostatic pressure.