(140b) Accelerated Carbonation Study for the Prediction of Long Term Performance of the Nirex Reference Vault Backfill in Radioactive Waste Disposal

United Kingdom Nirex Limited has developed a Phased Geological Repository Concept for the long-term management of solid intermediate-level and some low-lever radioactive waste. In this concept, the packed waste would be placed in the purpose-built vaults excavated deep underground which would be backfilled with a specially designed cementitious material, the Nirex Reference Vault Backfill (NRVB). The NRVB acts as chemical barrier within the repository by conditioning the in-flowing groundwater to high pH over long time scales, as well as absorbing radioactive carbon dioxide (14CO2) generated from certain organic radioactive wastes. To prevent the release of 14CO2 from the repository, two factors should be satisfied. One factor is that there is sufficient NRVB in the vault. The other factor is that the gas generated has enough time to react with the backfill.

The objectives of this work are to determine the carbon dioxide absorption capacity of the NRVB and develop a kinetic model which can be used for the prediction of the carbonation rate, i.e. the rate that the lime component in the NRVB material reacts with carbon dioxide and water to form solid carbonates. One-dimensional accelerated carbonation experiments on 25mm NRVB cubes have been carried out under selected reaction conditions to simulate the underground environment. At present, the effects of temperature, pressure and saturation degree on the carbonation of the NRVB are being investigated. Reaction rate curves and the effects of these conditions will be presented.