Enhancing isotope mixing in U-10Mo downblend castings with electromagnetic stirring

• Published in: Journal of nuclear materials Vol. 555; no. C
• Main Authors: Huber, Zachary, Athon, Matthew, Shen, Steve, Conte, Elise, McCoy, Kaylyn, Lavender, Curt
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier 01.11.2021
• Subjects: Uranium, electromagnetic stirring, UMo, casting, isotopes
• ISSN: 0022-3115; 1873-4820

Uranium alloyed with 10 weight percent molybdenum (U-10Mo) is the proposed fuel for use in the United States’ high-performance research reactors. The U-10Mo fuel is fabricated by a two-step casting process that downblends (dilutes) highly enriched uranium with depleted uranium (DU) or natural uranium (NU) and subsequently alloys the resulting high-assay, low-enriched uranium (HALEU) with Mo. The ingot formed by this process undergoes multiple processing steps to fabricate the desired monolithic plate fuel. Currently, a two-step casting process is needed to meet the ingots’ 235U enrichment homogeneity specifications. This work demonstrates the ability to provide more homogeneous 235U distributions during downblend casting by melting the metal in zirconia crucibles rather than graphite crucibles. When graphite is used as a crucible, the electromagnetic field produced by the induction heater couples directly with the graphite and then heats the metal charge indirectly by conduction. Zirconia is nonconducting in the composition frequency range used in this study, and therefore does not couple with the field. The induction field couples directly with the metal, and the field causes electromagnetic stirring (EMS) in the molten metal pool. Eight downblend casting experiments were carried out in this work. Four were performed with zirconia crucibles and four with graphite crucibles. The 235U enrichment was measured at nine discrete points in each casting using laser ablation multi-collector inductively coupled mass spectrometry (LA-MC-ICP-MS). The enrichment homogeneity was approximately eight times better in zirconia crucible castings than in graphite crucible castings as measured by the overall plate enrichment range and enrichment coefficient of variation. The results show a single casting step could be used to meet enrichment specifications if electromagnetic stirring is present during the casting process. A related but separate investigation was carried out to determine whether it is appropriate to measure enrichment on as-cast ingots or whether heat treated specimens should be used to accurately characterize enrichment. The results show that the as-cast microstructure plays a significant role in enrichment homogeneity. Hence it is recommended that all enrichment measurements be done after the homogenization heat treatment typical of U-10Mo processing.