CONSTRUCTION MATERIALS FOR VARIOUS HEAD-END PROCESSES FOR THE AQUEOUS REPROCESSING OF SPENT FUEL ELEMENTS

Materials of construction were evaluated for use in critical areas of head-end processes for the aqueous reprocessing of spent nuclear fuel elements. The SulfexThorex, Darex-Thorex, Darex, Zirflex, and Zircex processes were considered. The effect of varying heat treatments on the resistance of the materials was also evaluated. Dissolution of unirradiated fuel pins was carried out in vessels of promising materials. The corrosion rate of Ni-o-nel was about 5 mils per month during actual fuel-pin dissolution by the Sulfex-Thorex process. Stabilization and heat treatment are necessary to prevent intergranular attack at welds. Carpenter 20 Cb is subject to stress-corrosion cracking by the Sulfex decladding solution and Illium R behaves similarly to Ni-o-nel in Thorex solutions. Titanium shows promise as a construction material for a Darex-Thorex dissolver. However, several questions remain concerning a vapor-phase attack observed around certain weldments. Carpenter 20 Cb, Ni-o-nel, and Types 309 and 309S Cb stainless steel appeared worthy of further study for the Zirflex dissolver. Preliminary evaluations show that at least Ni-o-nel and Carpenter 20 Cb should be studied further as possible construction materials for a single vessel for Zirflex and Sulfex-Thorex processes. Illium R, Hastellcy C, and nickel were not attacked by hydrcchlorination conditions of the Zircex process. Minor attack was found on Inconel and Type S-816 alloy. The corrosion of titanium was negligible in the Darex dissolver and feed-adjustment system. The use of Type 304 ELC stainless steel lines for transterring Darex dissolver solution is not recommended. No corrosion of Type 347 stainless steel was observed after exposure for 2 years to chloride-contaminated Parex extraction systems at room temperature. (auth