Cooper and Caulcott Ltd

Environmental Modelling Consultants

Radioactive Waste Disposal

Radioactive waste is produced by nuclear power generation, and also by industrial and medical processes. In the UK such waste falls in to one of three catagories, depending on how radioactive it is: high level waste (HLW), intermediate level waste (ILW) and low level waste (LLW). At present the UK only has approved disposal facilities for LLW, with a national facility at the BNFL Drigg site near Sellafield in Cumbria. For more background on radioactive waste see the DETR Radioactive Waste Management pages

The disposal of radioactive waste is regulated by The Radioactive Substances Act 1993, which includes a requirement that 'Radioactive Wastes shall be managed in such a way that predicted impacts on the health of future generations will not be greater than relevant levels of impact that are acceptable today. In view of the long half lives of some radionuclides this implies safety for tens of thousands of years, which therefore entails consideration of future climate change.

Important components in a safety assessment of a disposal facility includes consideration of how the waste, containers, packaging, backfill and site environs(collectively called the "near field") will evolve over the coming millennia. This then provides a source term for models of groundwater flow and contaminant transport through aquifers and other geosphere media (the "geosphere" or "far field") . When contaminated underground water surfaces, for instance at a spring, it enters the "biosphere" where surface water processes are considered, with the final destination being in the ocean. The possibility of gaseous release and inadvertant human intrusion is also analysed. Typical calculations of dose to humans can then be calculated from possible consumption of contaminated crops, animals and water.

For undertaking a full safety assessment the future behaviour is simulated using numerical models of the near field , geosphere (far field) and biosphere. In view of modelling long in to the future there are a range of uncertainties that need to be taken into account. The most important of the six categories of uncertainty for such safety studies are parameter uncertainties (for instance in aquifer properties, and in the solubility of the radionuclides in the waste disposal site), scenario uncertainties (e.g. what will the climate and sea level be in 10,000 years time?) and conceptual model uncertainties (such as how to represent the processes in the intertidal tidal zone). Model bias can be significant, and is assessed by using a structured approach to model evaluation. Fortunately groundwater and biosphere transport only has limited amounts of turbulence and therefore does not contribute significantly to uncertainty in model results. There is very little observed data, most of which can is used qualitatively to help the conceptualisation of the flow in the region and is therefore not available for model evaluation.

In view of the size of important uncertainties (for instance sorption values and aquifer conductivities can be uncertain by more than an order of magnitude), in some cases radiological assessments use a 'Monte Carlo' approach, rather than purely using deterministic calculations with best estimate, or 'conservative' data values. In a Monte Carlo a series of runs (hundreds or thousands) are undertaken with chosen parameter values being sampled from their estimated PDF(Parameter Distribution Function). The results from all these runs are then combined, both to give single best estimates of future doses, and also to provide PDFs or CDFs (Cumulative Distribution Functions), which provide more information than a single value.

Such multiple runs also allow global sensitivity studies to be undertaken.

© 2000 Neill S Cooper
Last changed on June 12, 2000

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