Deep geological disposal

The latest issue of the ENS newletter has a report on deep geological disposal of nuclear waste and spent fuel.

Isolation in deep geological formations (geological disposal) is now considered the safest of all long-term solutions to the problem of dealing with long-lived nuclear waste. Particular attention has been given worldwide to the high-level radioactive waste resulting from the spent nuclear fuel after discharge from nuclear power plants, as it constitutes more than 98% of the radioactivity generated in these plants. This high-level radioactive waste can be in vitrified form for those countries with reprocessing options. Alternatively, the spent fuel itself can be considered as a form of waste.

Potential disposal locations in Europe, in granite, clay or salt formations, are stable for tens of millions of years and are characterised by very slow groundwater movement. Research at international level has been on-going for more than 30 years. Large experimental data bases have been generated to simulate the long-term interaction of groundwaters with different types of nuclear waste glass and spent nuclear fuel. This includes glass with compositions similar to those produced in reprocessing and associated vitrification plants in La Hague (France), Mol (Belgium), Sellafield (U.K.) and Karlsruhe (Germany), as well as low and high burn-up spent fuels from PWR, BWR, HTR and MTR reactors.

A considerable effort has been made to develop descriptive and predictive modelling procedures. The long-term stability of the waste solids in groundwater must be deduced from short-term laboratory experiments with a duration of a maximum of a few years. As a result, the evaluation of the performance of the spent fuel and glass over different geological time periods relies largely on the development of waste/groundwater interaction models and more general safety assessment models. This allows one to assess the long-term impact of glass and spent fuel/water interactions on the overall level of risk associated with the nuclear waste repository. This assessment has to take into account the inherent uncertainties in the understanding of the processes considered in the model, the parameter uncertainties and the uncertainties in hydrogeological boundary conditions. The repositories are planned for siting in places characterised by very slow geological evolution, but the presence of waste disturbs the systems in place there and the return to natural conditions may take hundreds of thousands of years. Also uncertainties regarding the scenarios for this evolution over various geological time periods must be taken into account.

The GLAMOR and MICADO projects, which coordinated actions carried out recently by the European Commission, have assessed the uncertainties in the models and  described the dissolution processes of nuclear waste glass containers and spent nuclear fuel in a repository over geological time periods. These projects were special in the sense that a common set of existing experimental data and existing models were selected. These models were applied in each project and the results discussed. Care was also taken that participants in the projects should present the various scientific opinions.

The GLAMOR project, coordinated by the Belgian Nuclear Research Centre, SCK•CEN, and carried out between 2002 and 2006, assessed the principal hypotheses and uncertainties of models for the dissolution of nuclear waste glass in pure water systems without repository near field materials (such as bentonite, metallic corrosion products etc.). Key European partners (SCK•CEN, CEA, SUBATECH/ARMINES) joined forces for the project with experts from leading US laboratories (SRNL, PNNL). Two reference analytical models selected by the partners were jointly evaluated using a common experimental data set.

You can read the full report here.