Behavior of Discrete Plutonium-Dioxide Particles in Mixed-Oxide Fuel During Rapid Power Transients

Transient tests were conducted on nonirradiated oxide pellet-containing fuel pins at SPERT to investigate the possible effects of large single 550-μm diam PuO₂ particles on transient behavior. Results show that the effect of the large PuO₂ particles was to reduce slightly the cladding failure threshold energy from the range of 225 to 274 cal/g of fuel to the range of 200 to 213 cal/g of fuel. Clad perforation occurs by localized melting caused by the expulsion of PuO₂ particles through the cladding. The presence of single 550-μm diam PuO₂ particles in mixed-oxide fuels does not appear to affect significantly the cladding failure threshold energy from that of mixed-oxide fuels with the normal PuO₂ particle size and distribution. Therefore, product specifications which limit the maximum PuO₂ particle size to 550-μm diam in mixed-oxide fuels do not appear warranted from the standpoint of transient fuel performance considerations.