b'This project has made significant advances in understanding the difficulties that arise from nuclearfuel waterproofing efforts and has resulted in selection criteria that will be useful in evaluating futurewaterproofing concepts.Composite pellets of UN with Y,dates might react with nitrogen.Cr, Si, and SiC were fabricated byThis diagram is shown in Figure 2 mixing powders of the aforemen- which indicates that Zr, Al, Ti, and tioned materials. Yttrium dihydrideY metals will absorb nitrogen from (YH2) powder was used in lieu ofUN, while Si, Cr, and Fe should Y metal powder due to the brittleremain metallic. nature of the hydride, allowingConversely, Si and SiC appeared mechanical milling with the UNto react with the material. In the prior to dehydriding at an elevatedcase of silicon, the resultant pellets temperature. Composites with Y, Cr,comprised of primarily uranium and Si were heated to temperaturessilicides. It is not yet clear what above the melting points of thereactions occurred with silicon respective metals (1526, 1857, andcarbide, but the system is being 1410 C) with the aim of evaluatingstudied further due to interest in chemical interactions and promoteunderstanding the interactions liquid-phase sintering. Compositesbetween these two compounds. The with SiC were heated under temper- composites with chromium showed ature profiles used for two-step,little-to-no remaining chromium pressure-less sintering of SiC. metal due to the high vapor pressure Results showed that yttriumof chromium at the temperatures absorbed nitrogen from therequired for liquid-phase sintering. uranium, resulting in the formationAs a result of these various of yttrium mononitride (YN) andoutcomes, the several selection U metal. An Ellingham diagram forcriteria have been proposed to assess nitride formation was developedfuture waterproofing concepts.using ThermoCalc 2019a to deter-mine which waterproofing candi-2019|AFC ACCOMPLISHMENTS 57'