Interactions of Simulated High Level Waste (HLW) Calcine with Alkali Borosilicate Glass
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,QWHUDFWLRQVRI6LPXODWHG+LJK/HYHO:DVWH +/: &DOFLQHZLWK$ONDOL%RURVLOLFDWH*ODVV S. Morgan, R. J. Hand, N. C. Hyatt and W. E. Lee Immobilisation Science Laboratory, University of Sheffield, Mappin St, Sheffield, S1 3JD, UK $%675$&7 This study looks at the interactions between simulated calcined high level waste from fuel reprocessing and mixed alkali borosilicate glass frit in the early stages of melting, and the possibility of the formation of yellow phase during these stages. Simulant “calcine” from a full scale inactive trial (Magnox: oxide “blend” 25:75) was pre-mixed with alkali borosilicate glass, to achieve a 25wt% waste loading, and melted at 1050ºC at various times. It is shown that dissolution occurs in two separate stages; the first involves formation of a low density CsLiMoO4 fluid, which separates and forms a yellow/green layer on the surface of the melt, accompanied by some dissolution of rare- earth elements (Nd, Ce, Gd) and Zr from the waste into the glass matrix. The second stage entails more extensive migration of these rare-earth elements into the glass, and the disappearance of the surface layer on the melt. The glass appears more homogenized at the later stages of melting, but still contains undissolved particles of calcine after 16 minutes. ,1752'8&7,21 Borosilicate glasses are used extensively as host matrices for immobilisation of high level radioactive waste (HLW) for many years [1]. Borosilicate glasses have become the most popular choice for immobilisation due to their good durability and their ability to incorporate a wide range of elements found in a typical waste stream. The formation of crystalline phases such as molybdates, refractory oxides within the glass and anything which is immiscible in the melt may adversely affect the long term durability of the glass wasteform and also lead to enhanced melter corrosion [1-3]. In the UK, HLW arising from nuclear fuel reprocessing is vitrified in a two stage process. The spent nuclear fuel (SNF) is dissolved in concentrated hot nitric acid and the remaining U and Pu removed. The aqueous raffinate is then stored in stainless steel tanks and, prior to calcination, combined with sucrose and a solution of lithium nitrate. The addition of sucrose reacts with the free nitric acid in the system to reduce the potential for the ruthenium to oxidize and form RuO4, which is extremely volatile, and lithium nitrate is added to prevent the formation of refractory oxides of Fe and Al, which are difficult to dissolve within the melt. The Highly Active Liquor (HAL) is then passed through a rotary calciner and emerges as a granular solid. The calcining process has four heating zones within the rotary calciner, and it is estimated that the temperatures which the waste experiences are
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