CEMENTITIOUS SYSTEMS FOR ENCAPSULATION OF INTERMEDIATE LEVEL WASTE

摘要

Encapsulation in cement is the favoured method in the UK for disposal of intermediate and low level radioactive wastes. It is usual to use composite cement systems incorporating blast furnace slag (BFS) or pulverised fuel ash (PFA) as these offer several advantages over Portland cement, notably a lower heat of hydration. The use of these mineral additions utilises a waste product which would itself need a disposal route and, because of the decreased amount of Portland cement used, provides a reduction in cost and energy consumption. Cementitious systems have many attributes which make them suitable for encapsulation and immobilisation, including: 1. Inexpensive and readily available 2. Assist immobilisation of radionuclides by: a) acting as a diffusion barrier b) providing sorption and reaction sites c) maintaining a high pH which in turn decreases radionuclide solubility 3. Provide radiation shielding which is not degraded by the radiation 4. Controllable permeation and diffusion characteristics over a wide range via selection of constituents and components. Where physical adsorption is a significant factor for immobilisation, the calcium silicate hydrate gel (C-S-H) formed on hydration of a Portland cement is advantageous as it has a high surface area and large micropore volume. Composite cements based on blast furnace slag will produce a higher proportion of C-S-H than ordinary Portland cement increasing the sorption capacity, and reducing the capillary porosity so that the diffusion resistance is increased.