The Nuclear Decommissioning Authority’s (NDA) waste management directorate published its Generic Disposal Safety Case (DSSC) for geological disposal of radioactive waste on February 23. Use of multiple barriers is the key to the NDA’s case for safe disposal of radioactive waste using a deep underground disposal facility which will need to keep the radioactive materials isolated from the environment for hundreds of thousands of years.

In July last year, the NDA published its Geological Disposal: Steps towards implementation report which set out the steps required to bring such a facility into existence. The latest suite of scientific and technical reports cover all the safety factors that the Radioactive Waste Management Directorate (RWMD) will consider when submitting an application to the nuclear regulators for permission to operate such a facility.

The reports, collectively known as the Disposal System Safety Case (DSSC), cover safety issues including:

* the waste to be disposed of and how it will be packaged;

* the safety of the transport to and within the disposal facility;

* the operational safety of such a facility and safety to the environment during all of these phases and into the future after the facility has been closed.

No site has been selected yet, (although a process of voluntarism and partnership with potential local communities who may be willing to host such a facility is underway), hence the Disposal System Safety Case is “generic”.

To date the only area which has volunteered to discuss the possiblity of hosting a repository – for which a ‘community benefits package’ would be available, is West Cumbria. A decision as to whether it wishes to participate further is expected to be taken early in 2011.

Bruce McKirdy, RWMD’s Managing Director, said in a statement,

“The Disposal System Safety Case assesses the safety and environmental implications of all aspects associated with the geological disposal of higher activity radioactive waste in the UK. Our preliminary, generic assessments have helped highlight areas where further research is required and the uncertainties that will need to be resolved during a site investigation programme.”

The wastes which would require disposal in a GDF are described as higher activity radioactive waste. This includes high level waste (HLW) and intermediate level waste (ILW) as well as a small quantity of low level waste (LLW).

The wastes are made passively safe by being mixed with molten glass (HLW) or cement (ILW) and stored until a disposal facility becomes available. These waste packages are all assessed and given a “Letter of Compliance” showing they are suitable for disposal in a GDF.

The fundamental basis to the safety of a geological disposal system is described as the multi-barrier system and this approach is common in all of the international geological disposal facility concepts.

The multi-barrier comprises the following:

* The waste form into which waste is conditioned to make it suitable for disposal.

* The waste container provides a physical barrier and enables the waste to be transported and handled safely during interim storage and then for emplacement in a GDF.

* The buffer or backfill is the material that is placed immediately around the waste containers providing physical protection of the waste containers and in some cases a chemical barrier.

* Mass backfill is the material used to fill the excavated access tunnels, shafts or drifts in a disposal facility.

* Sealing systems compliments the mass backfill and controls the movement of fluids along the excavated access tunnels, shafts or drifts.

* The natural geological barrier is the host rock in which the facility is constructed and its surrounding rocks.

The engineered barriers (packaging, vaults and backfill) and the natural barrier (the rock cover) will work together to ensure the necessary levels of safety. They will provide specific safety functions contributing to isolation (siting the waste away from the biosphere) or containment (the ability of a barrier to hold the radioactivity within it) over different timescales. Each of the barriers will provide specific safety functions ensuring isolation or containment of the radioactivity over long timescales.

Bruce McKirdy said:

“An important objective of GDF design development and safety analysis is to provide assurance that the majority of shorter-lived radioactivity will decay in situ and that the release of any longer lived types will be spread over such a long period of time that they will not lead to significant amounts getting into the environment.”

The reports published today show how we are approaching all of the safety concerns in relation to the transport of waste to the facility, the operations of the facility and how we can insure protection to the environment over hundreds of thousands of years.