Disposal : The Engineered Barrier System
A multi-barrier approach ensures the long-term isolation of radioactive waste and is the adopted solution of preference for radioactive waste management. The Engineered Barrier System (EBS) is one part of this passive multi-barrier system approach and it refers to all barriers introduced through technological (engineered) activities. It is complemented by the natural barrier, also referred to as the geological barrier or geosphere.
Most of the experiments running at Grimsel are focusing on using in-situ tests to analyse and characterise the near field of a nuclear waste repository. The near field includes the Engineered Barrier System (EBS) and the immediate rock surrounding a repository. In Nagra's concept, the disposal tunnels and caverns are placed deep underground (from around 600m) and in stable rock formations. Slightly different disposal methods are envisaged for the high level waste (HLW - spent fuel rods from reactors or vitrified waste) and the intermediate to low level waste (I/LLW - industrial waste). These two concepts are explained below.
Nagra's concept for an underground nuclear waste repository
1. High level waste (HLW) tunnels
The high level waste is contained within large steel disposal canisters.
These canister are placed on bentonite pedestals within the disposal tunnel at regular intervals, and the tunnel backfilled with bentonite.
Nagra's high level waste (HLW) disposal concept - cutaway
Nagra's high level waste (HLW) disposal concept - partly backfilled
Grimsel experiments which are relevant to the high level waste (HLW) concept are :
- CFM - Colloid Formation and Migration experiment
- C-FRS - CRIEPI's Fractured Rock Studies
- ESDRED / TEM - Test and Evaluation of Monitoring Systems
- LCS - Long Term Cement studies
- LTD - Long Term Diffusion experiment
2. Low level waste (LLW) caverns
Low level waste is contained within steel drums, and these steel drums are placed into concrete disposal casks, and filled with mortar.
The disposal casks are stacked into the low level waste disposal caverns which are lined with concrete. When full, the caverns are backfilled with mortar.
Nagra's low level waste disposal cavern concept - cutaway
The low level waste (LLW) disposal cavern is shown below during the operational phase, and when it has been backfilled with mortar and sealed.
Nagra's low level waste (LLW) disposal concept - operational
Nagra's low level waste (LLW) disposal concept - backfilled
Grimsel experiments which are relevant to the low level waste (LLW) concept are :