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.