Hot-Pressed Barium Sulphate Ceramic Waste Forms for Direct Immobilisation of Medium Level Magnox Waste

Abstract

ABSTRACTA 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 BaSO4 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 BaSO4 , MgO and Fe3O4, 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, and leach rates ∼ 4 × 10−7 kg/m2/sec are found. In contrast, pure BaSO4, 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 × 10−8 kg/m2/sec.