Fuel Failure Behavior of Unirradiated Fuel Rods under Reactivity Initiated Accident Conditions

An in-reactor experimental research on fuel behavior under reactivity initiated accident conditions has been performed in the Nuclear Safety Research Reactor (NSRR). Unirradiated test fuel rods were exposed to pulse-power at the energy depositions of 39-433 cal/g·UO2 in water environment with ambient pressure and temperature.Oxidation and deformation of the cladding commence at the energy deposition of about 140 cal/g·UO2, and increase almost in proportion to the energy deposition.There are two types of fuel failure which are characterized by the circumferential cracks in the cladding or guillotine type break, and by the accompaniment of fuel fragmentation. The threshold energy depositions for them are about 260 and 380 cal/g·UO2, respectively.The former is the brittle fracture of the cladding. The cladding material becomes thinner because of melting of the cladding inner surface, and becomes brittle by oxidation. The failure is caused by the prevention of cladding contraction at or after the time of quenching of the brittle cladding due to strong binding between the pellets and cladding.The latter is the failure caused by the melting of the pellet accompanied with the considerable loss of mechanical strength in the cladding, resulting in the generation of mechanical energy.