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

Abstract

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.The initial period of operation of the BN-600 reactor showed that the main factor limiting the service life of fuel assemblies is inadequate radiation resistance of the structural materials. This has intensified efforts to develop a new type of steel, first and foremost, for fabricating fuel-element cladding and fuel-assembly jackets [1,2]. Post-reactor studies of the standard and experimental BN-600 fuel assemblies which had reached different degrees of burnup were performed in parallel. The investigations were performed in two stages: primary -in a hot chamber at the Beloyarskaya nuclear power plant [3] and detailed -in specialized hot laboratories. In the first load of the BN-600 core, 0Kh16N15M3B (EI-847) steel was used first in the austenitic and then in the cold-worked state. The service life of the fuel elements with such cladding did not exceed 9% h.a. [4]. A new steel 0Kh16N15M3BR (EP-172), differing from EI-847 by elevated boron content [5], was developed to increase burnup. A series of experimental fuel assemblies with cladding made of this type of steel was irradiated in BN-600. The present article presents the main results of the investigation of several such fuel assemblies and analyzes the factors resulting in lower serviceability of fuel-element cladding.Character of the Damage to BN-600 Fuel-Element Cladding. For the purpose of designing fuel elements for the first fast reactors, it was believed at the time that their serviceability will be determined by the long-time mechanical properties of the cladding material and its corrosion resistance with respect to the fuel and coolant. It was thought that the pressure of the gases inside a fuel element will increase continually as a result of the accumulation of fission-product gases under the cladding, which ultimately will destroy the cladding as a result of thermal creep. The corrosion rate and hence the thinning of the cladding will increase with temperature and reach a maximum...

Swelling and radiation creep of austenitic corrosion-resistant steel irradiated by neutrons in wide dose and temperature ranges

“…Post-reactor studies showed that the maximum increase of the diameter and swelling of the EI-847, EP-172, and ChS-68 cold-deformed steel cladding for fuel elements occurs in the temperature range 420-460°C and for austenitized steel to 500°C [4][5][6][7][8][9]. On the basis of these data, the appearance of the low-temperature maxima in Figs.…”