The Effects of Exposure to Flowing Sodium on Vanadium Alloys in Stainless Steel Containment Systems

Abstract

In recent years considerable effort has been expended on the development of vanadium base alloys for fuel element cladding in sodium-cooled fast breeder reactors. This paper deals with the effects of exposure to sodium on several of the more promising vanadium alloys. The compositions that were studied are: V-9Cr-3Fe-1.3Zr-0.05C (VANSTAR 7) V-8Cr-10Ta-1.3Zr-0.05C (VANSTAR 8) V-6Fe-5Nb-1.3Zr-0.05C (VANSTAR 9) V-20Ti (for comparison) All alloys exhibited weight gains after exposure to flowing sodium containing less than lOppm oxygen in a Type 316 stainless steel loop system over the temperature range of 675–800°C. Removal of the interstitials, nitrogen, carbon, and oxygen from the stainless steel, and their absorption by the vanadium alloys is shown to account for the observed increase in weight after exposure. The kinetics of the corrosion process, the weight change data, and the mechanical property changes related to interstitial pick up from the stainless steel have been examined, using a layer chemical analysis technique. Interstitial concentration gradients in both the vanadium alloys and the stainless steel loop tubing have been used to obtain activation energies for corrosion and interstitial diffusion coefficients in these materials. In addition to weight change and metallographic results, data are presented on the effects of this interstitial absorption on the mechanical properties of the vanadium alloys. After 1500 and 2650 hours of exposure to sodium, 800°C tensile elongation values greater than 20 percent were obtained for the VANSTAR alloys, but the room temperature ductility was very much reduced. This paper discusses ways of reducing the interstitial absorption and mechanical property deterioration. Among these are the use of stabilized stainless steel loop construction at more realistic reactor operating temperatures, or modification of stainless to vanadium alloy surface area ratios.