Development of Gd-enriched alloys for spent nuclear fuel applications - Part 1: Preliminary characterization of small scale Gd-enriched stainless steels

The influence of Gd additions on the microstructure and hardness of type 316L stainless steel was investigated by various microstructural characterization techniques. This work was conducted as a first step toward the development of Gd-enriched alloys for spent nuclear fuel applications. Small (∼10 g) gas tungsten arc melt buttons were prepared to produce 316L stainless steel with Gd levels from 0.1-10 wt.% Gd. Electron microprobe measurements showed that Gd is essentially insoluble in the austenite/ferrite matrix. All of the alloys formed an interdendritic (Fe, Ni, Cr)₃Gd intermetallic, and the amount of the (Fe, Ni, Cr)₃Gd phase increased with increasing Gd concentration. Depending on the P and O levels, various amounts of Gd phosphides and oxides were also observed. The relatively high Ni concentration (∼28 wt.% Ni) and low Cr concentration (∼3 wt.% Cr) of the (Fe, Ni, Cr)₃Gd phase led to Ni depletion and Cr enrichment in the matrix which, in turn, affected the matrix stability. Alloys with 0.1-6 wt.% Gd exhibited a two-phase ferrite/austenite matrix. Alloys containing 8 and 10 wt.% Gd exhibited a fully ferritic matrix due to extensive Ni depletion/Cr enrichment and concomitant stabilization of ferrite. Hardness increased with increasing Gd concentration due to the formation of the (Fe, Ni, Cr)₃Gd intermetallic and the change in matrix structure from austenite to ferrite. A mass balance procedure is presented that accounts for changes to the matrix composition with Gd concentration. This procedure can be used to determine the nominal alloy composition required to produce a 316L-type matrix composition for any Gd level.