BEHAVIOR OF BREACHED MIXED-OXIDE FUEL-PINS DURING OFF-NORMAL HIGH-TEMPERATURE IRRADIATION

A test containing 19 mixed-oxide fuel pins was operated in the Experimental Breeder Reactor II (EBR-II) at peak cladding temperatures near 800-degrees-C. Two test pins that had been designed to fail at approximately 5 at.% burnup and two low-burnup environmental pins failed and then were operated in the run beyond cladding breach mode for 22 days. Very high delayed neutron signals occurred during the irradiation of the test, and it was terminated as a result of high delayed neutron signals and evidence of plutonium in the coolant. Each of the four pins exhibited multiple breaches in the upper half of the fuel column. Measurements of fuel trapped on the filter section of a deposition sampler that was located above the test indicated that approximately 2.7 g of fuel was lost during the irradiation. Postirradiation examination of the pins indicates that most of the fuel was lost from a single pin. The fuel loss resulted in an increase in the background delayed neutron signal but had no other deleterious long-term effect on the operation of the EBR-II.