Burst tests were conducted at 900°F on specimens prepared from 75% UO2 −25% PuO2 fuel pins clad with 0.250-in. o.d. × 0.016-in. wall Type-304 stainless steel (PNL 1-3 and 1-6) that were irradiated in EBR-II to ∼8500 MWd/MTM at 8 to 9 kW/ft. Argon gas was used internally to pressurize the fuel pin sections. The fuel was left intact in order that the effects of the fuel and fission products on the properties of the cladding could be determined. Specimens taken from above the midplane but within the fuel column exhibited brittle, intergranular fracture at drastically lower stresses (∼30 000 psi) than would be expected from Type-304 stainless steel irradiated under similar conditions. The measurable strain in these specimens was <1%. Fueled specimens from below the midplane and specimens from the fission gas plenum exhibited a ductile, transgranular fracture at stresses and strains normally expected of Type-304 stainless steel irradiated under these conditions, i.e., ∼60 000 psi and ∼9% strain. Post-test examination of the burst test specimens included diameter measurements, optical metallography, and electron microscopy. Fueled specimens from above the midplane contained M23C6 and sigma phase at the grain boundaries of the cladding whereas the cladding of specimens from below the fuel midplane was free of grain boundary precipitates. From the test results and examinations it is concluded that:

  1. Sections of irradiated fuel pins may fail at much lower stresses and strains than would be expected from mechanical tests conducted on unfueled cladding material
  2. The premature failure in certain sections of the pin is associated with grain boundary precipitation in the cladding
  3. The precipitation is believed to promote chemical attack along the cladding boundaries where the chemical specie is either a fission product or a reaction product produced from their interaction with the hot cell environment.