Analysis of the behavior of 0Kh16N15M3BR steel BN-600 fuel-element cladding at high burnup

Post-reactor investigations have been performed on BN-600 fuel-element cladding, made of 0Kh16N15M3BR steel, after irradiation to maximum burnup of 10% h.a. and higher. It is shown that the highest degradation of the operating properties of the fuel-element cladding is observed in the zone of maximum increase of its diameter and is expressed as total embrittlement of the cladding material and appearance of cracks of substantial depth on the inner surface. The processes resulting in the degradation of the properties of fuel-element cladding are directly related either with swelling or with radiation-induced segregation, occurring in the same temperature range and under the action of the same driving forces as swelling. The most important stresses, from the standpoint of the serviceability of fuel elements, turn out to be those arising in cladding as a result of the gradient of the swelling along the thickness of the cladding. The level of these stresses is also determined by the form of the temperature dependence of the swelling of the steel used for the fuel-element cladding.