Hydroceramics, a “new” cementitious waste form material for U.S. defense-type reprocessing waste

Abstract

A "hydroceramic" (HC) is a concrete which possesses mineralogy similar to the zeolitized rock indigenous to the USA's current "basis" high level radioactive waste (HLW) repository site, Yucca Mountain (YM). It is made by curing a mixture of inorganic waste, calcined clay, vermiculite, Na₂S, NaOH, plus water under hydrothermal conditions. The product differs from conventional Portland cement and/or slag-based concretes ("grouts") in that it is primarily comprised of alkali aluminosilicate "cage minerals" (cancrinites, sodalites, and zeolites)rather than hydrated calcium silicates (C-S-H in cement-chemistry shorthand). Consequently it microencapsulates individual salt molecules thereby rendering them less leachable than they are from conventional grouts. A fundamental difference between the formulations of HCs and radwaste-type glasses is that the latter contain insufficient aluminum to form insoluble minerals with all of the alkali metals in them. This means that the imposition of worst-case "repository failure" (hydrothermal) conditions would cause a substantial fraction of such glasses to alter to water-soluble forms. Since the same conditions tend to reduce the solubility of HC concretes, they constitute a more rugged immobilization sub-system. This paper compares leach characteristics of HCs with those of radwaste-type glasses and points out why hydroceramic solidification makes more sense than vitrification for US defense-type reprocessing waste.