Creep and Creep-Rupture Properties of the Nuclear Fuel Cladding Tube for Fast Breeder Reactor in High Temperature Sodium

Abstract

As the basis for designing the fuel assembly for a sodium cooled fast breeder reactor and for determining the service condition of the reactor core, it has been required to obtain creep and creep-rupture properties of the domestic seamless tubes of AISI Type-316 stainless steel with the nominal dimension of 6.3mm outside diameter and 0.35mm wall thickness in flowing sodium environment under various conditions.
For the examination in sodium environment, creep testing apparatuses were designed and incorporated with the two sodium loops for material tests. The sodium handling technique and the testing method up to 700°C have been established.
The effects of oxygen level in flowing sodium on creep-rupture properties of tubular specimens of Type-316 stainless steel have been examined. At the oxygen levels of 5 and 10 wppm in flowing sodium, the values of creep-rupture strength under uniaxial tension at 600°, 650°, and 700°C up to about 2000hr (the longest rupture time) are not significantly different.
The rupture strengths under internal pressure and under uniaxial tension are significantly lower in flowing sodium containing 10 wppm oxygen at 700°C than in air at the same temperature. Furthermore, the decrease of rupture strength is greater as the exposure time in the sodium is longer, and it is related to the tendency of the relative increase in the minimum creep rate.
This apparent reduction in rupture strength is accompanied by the significant decarburization of specimens tested, and the formation of a degraded layer in the neighbourhood of the specimen surface exposed to sodium.