Crystallisation Within Simulated High Level Waste Borosilicate Glass
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Crystallisation Within Simulated High Level Waste Borosilicate Glass Peter B. Rose, Michael I. Ojovan, Neil C. Hyatt and William E. Lee Immobilisation Science Laboratory, Department of Engineering Materials, University of Sheffield, Mappin Street, Sheffield, S1 3JD, U.K. [email protected]
ABSTRACT The devitrification products of two simulated high level radioactive waste (HLW) glasses have been investigated. Magnox waste glass contained RuO2 as its primary crystal phase, Pd-Te inclusions, a Cr-, Fe- and Ni-rich spinel phase, a Si- and lanthanide-rich phase and also a Zr-rich phase which incorporated Ce and Gd. Upon heat treatment the glass developed a zektzerite-type phase, CeO2, a strontium molybdate (containing Nd and La) as well as a Si-rich phase. 75/25 glass, comprising a blend of reprocessing waste derived from UO2 and Magnox fuels, contained RuO2 as its primary crystalline phase, CeO2 and a Si- and Ru-rich phase. Heat treatment of this glass resulted in the growth of the CeO2 crystals, the development of a strontium molybdate (containing Nd and La) and a Si- and lanthanide-containing phase. A sodium lanthanum molybdate with a powellite-type structure formed on the surface of both glasses after heat treatment in air.
INTRODUCTION In the UK, high level radioactive waste (HLW) arising from the reprocessing of spent nuclear fuel is currently immobilised in a mixed-alkali borosilicate glass matrix. Alkali borosilicate glasses are the matrix of choice for the immobilisation of HLW as they have good chemical durability and can incorporate a wide range of elements found in typical waste streams [1]. Crystal phases can form within the glass during its production which may result in corrosion of the glass melter and detrimentally affect the durability of the waste form. Also, the radiogenic heat produced by HLW retards the cooling of the glass canisters and may allow formation and growth of additional crystalline phases. The effect of devitrification upon HLW glass durability is an important issue, particularly with respect to partitioning of radionuclides into new phases. This study is concerned with characterising the phases present in two full-scale inactive simulant UK HLW glasses produced during commissioning of the Sellafield waste vitrification lines. Table I shows the composition of the simulant Magnox waste glass, this is representative of a HLW glass produced from waste arising from the reprocessing of Magnox nuclear fuel. Table II details the composition of a 75/25 “blended” glass. This is representative of a blend of waste arising from reprocessing of UO2 and Magnox nuclear fuels mixed in a 75:25 ratio (on an oxides basis, by weight). Both glasses have been investigated in the as-cast and heat treated state.
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Table I. Magnox waste glass composition (determined by XRF). Oxide Weight % Oxide Weight % Oxide Weight % Oxide Weight % SiO2 46.03 Na2O 8.28 NiO 0.37 Sm2O3 0.22 TiO2 0.01 P2O5 0.26 B2O3 15.87 CeO2 0.84 Al2O3 6.57 Cr2O3 0.58 Li2O 4.06 La2O3 0.48 Fe2O3 3.00 ZrO2 1.45 RuO2 0.7
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