Hot-Pressed Barium Sulphate Ceramic Waste Forms for Direct Immobilisation of Medium Level Magnox Waste
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HOT-PRESSED BARIUM SULPHATE CERAMIC WASTE FORMS FOR DIRECT IMMOBILISATION OF MEDIUM LEVEL MAGNOX WASTE
A. BRIGGS, D.V.C. JONES AND G.B. COLE Materials Development Division, AERE Harwell,
Oxon,
OXI1 ORA, UK
ABSTRACT A possible method of treatment for Magnox cladding waste is by dissolution in nitric acid and precipitation of barium sulphate-based floc with which radioactive ions are co-precipitated. The floc could then be immobilised in a matrix material such as cement or bitumen to give the waste form, or alternatively can be converted directly into a waste form by hot pressing. This paper describes the direct conversion of barium sulphate floc, containing simulated radwaste, into a synthetic, ceramic version of the natural mineral barite by a hot-pressing route. By variation of the parameters pressure, temperature and time, optimum conditions for consolidation of the floc to > 90% theoretical density on a laboratory scale are found to be 22.5 MPa, 900*C for 10 minutes. Using a pressure of 15 MPa, at 900'C for 30 min., hot-pressed billets of BaSO 4 have been made on a 5 kg scale. In going from the Magnox waste to the hot-pressed barium sulphate a volume reduction factor % 18 is achieved. The principal phases in the product are found to be BaS0 4 , MgO and Fe 3 0 4 , and the degree of consolidation achieved depends on the MgO content. The leaching behaviour of the hot-pressed materials in 100%C, 3 day Soxhlet tests also depends on the MgO content, and on the consequent level of open porosity. If there is porosity accessible to the leach water, MgO at the internal surfaces is converted to Mg(OH) 2 , which deposits within the pores, and a weight gain is registered in the Soxhlet test. If, however, there is no open porosity, a weight loss occurs, 2 and leach rates u 4 x 10-7 kg/m /sec are found. In contrast, pure BaSO 4 , hot-pressed to similar densities, shows no variation in leaching behaviour over a wide range of o en porosities, and gives Soxhlet leach rates . 8 x 10-8 kg/in/sec.
INTRODUCTION Barium sulphate, BaSO4 , is found in nature as barite, in the form of crystals or as the fossilising medium in some invertebrate fossils [1]. Their survival in near-surface strata for many millions of years indicates the resistance of barium sulphate to ground water leaching, and this observation is supported by well-documented data on the low solubilities of both natural barite and precipitated powders [2]. In Britain, cladding waste from the reprocessing of Magnox fuel elements is currently stored under water in silos, and during storage much of the Magnox alloy is converted to a hydroxide sludge. A proposed method of treatment for the sludge and residual alloy, to concentrate fission product contamination, is by dissolution in nitric acid, followed by co-precipitation with barium sulphate. During this floc precipitation process, Ba, Ca and Sr are co-precipitated as
542 sulphates; Cs forms a complex with sodium nickel ferrocyanide by ion exchange, and also precipitates; some of the Ce and Ru is adsorbed ont
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