High-level radioactive wastes (HLW) remain problematic for very long periods of time (thousands or even hundreds of thousands of years) so it is important that they remain isolated for such time-scales.

A multi-barrier approach ensures this isolation, combining a series of planned engineered barriers (EBS) situated deep below the surface within a suitable stable geological location (Deep Geological Disposal). The EBS is one part of this passive multi-barrier system approach.

The Grimsel projects run equipment, methodology and model tests in the equivalent underground conditions to those found in the near field of a real repository.

The Multi-Barrier Safety System has several defined zones: Near Field - The near field encompasses the Engineered Barrier System (EBS) and disturbed zones due to tunneling work carried out to create a repository. Far Field (Geosphere) - The far field is the geological mass surrounding the near field. Biosphere - The biosphere is the near surface, organic zone that contains life. It is expected that most radionuclides will decay in the EBS (e.g. it is conservatively expected that the steel canister (see below) used in Nagra's current design will last for over 1,000 years).

The EBS is a series of engineered barriers that work together to passively contain the radioactive waste. The EBS for HLW consists of the following engineered barriers:

1. Vitrified waste

The vitrified waste is a solidified glass mix of molten glass and processed radioactive products.

2. Steel canister

The massive steel canister encases the vitrified waste.

3. Sand/Bentonite backfill

A sand/bentonite clay mix is used as a buffer between the tunnel wall and the steel canister. In Nagra's concept compressed sand/bentonite pellets are sprayed around the steel canister. In almost all repository concepts it is assumed that the geosphere surrounding the EBS is saturated with water, and this groundwater will gradually flow back into the tunnel and the EBS after the tunnel is closed. When saturated, bentonite swells and seals the tunnel, restricting almost all flow of water inside the EBS.

The Grimsel FEBEX project has been examining the emplacement of a steel canister within a bentonite backfilled tunnel.

1. Steel Drums

The low level waste is immobilesed in concrete within a 200L steel drum.

2. Concrete Containers

Large concrete containers are filled with the steel drums and the gaps between the drums filled with a porous concrete mix.

3. Concrete cavern

The concrete containers are stacked within a concrete lined disposal cavern.

As the EBS for L/ILW waste is constructed mostly from concrete, the effect of such a large cementitous mass on the rock and water systems is being investigated directly by in situ Grimsel projects such as GMT, HPF and LCS

Picture/anim of LLW coming !