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Repository Designs

Various designs for a deep radioactive waste repository have been proposed.  The type of design is governed primarily by the nature of the wastes to be disposed and the geological environment in which the repository will be located.  In this article we will focus on the preferred design option in the UK for ILW (intermediate-level waste) in a "hard-rock" geology.  Granite is an example of a hard rock.  A feature of such rocks is that groundwaters present in the rocks tend to move through tiny connected fractures in the rock.

One possible high-level design for a deep ILW repository in hard rock is shown below [1].  This repository design would be located at a depth of between 600 and 700 metres below ground.

This is a very simplified representation of a potential deep ILW repository.  However, it can be seen that the repository is somewhat more than just a hole in the ground.  The vaults shown in the figure are basically rooms, about 300 metres long, that will contain waste packages.  The cross-section of the rooms is about 16 metres by 16 metres.  The access shafts and drifts allow access to the vaults and ventilation, and the roadways allow transport around the different components of the repository.  The plan area of the repository, in total is several square kilometres.  The repository is designed to hold several hundred thousand cubic metres of radioactive waste.

The following diagram (again from [1]) shows the inside of a typical vault in the repository.

Each of the vaults is filled with radioactive waste packages, made (most probably) out of steel or concrete.  This procedure is undertaken using a remote controlled overhead crane.  Once a vault is full (or at some other convenient point in time - see the next subsection), the remaining void space in the vault is "backfilled" with a concrete-like backfill material.  The preferred backfill material in the UK for LLW and ILW waste disposal is called NRVB (Nirex Repository Vault Backfill).

Repository Closure

At a suitable time after the repository has been filled, a decision may be taken to formally "close" the repository.  This is likely to happen some 50 years or more after the first wastes have been emplaced, as this is about the period of time that will be required to dispose of all the wastes that are scheduled to be placed in the repository.

The issue of closure of a repository is not straightforward, as there are some suggestions that once wastes are emplaced in the repository, future generations should be left with the option to remove the wastes, if desired.  Why would they want to do that?  There are two possible reasons.  One reason is that they may wish to reuse the nuclear materials.  The other is that they may discover alternative and safer ways to dispose of the wastes, and to apply other disposal methods it will be necessary to have access to the wastes.  The option of retaining access to the wastes is called "retrievability", and is a subject of some discussion at the present time.  Issues such as retrievability may impact on decisions about if and when to undertake final closure of the repository.

Concept of Barriers

The final subject we need to discuss in the context of repository design is the concept of "barriers".  Barriers are the individual components of the repository that act to prevent the  wastes from migrating back to the accessible environment.  It is an expectation that a radioactive waste repository system should provide a number of such barriers.  It is also expected that the disposal system should exhibit "redundancy".  That is, the overall safety should not depend unduly on the performance of any single barrier in the system.

The following figure (obtained from [1]) shows the repository barriers in the UK LLW / ILW deep disposal concept.

We can list the barriers that appear in this repository concept:

1.  The waste containers

2.  Grout materials that are applied to the wastes during waste processing

3.  Backfill materials in the repository

4.  The rock surrounding the repository.

Repository concepts that make use of multiple barriers in this way are referred to as "multi-barrier" concepts.  Each of the barriers acts to retain the radioactive wastes within the disposal system.  The waste containers provide physical containment of the wastes in the repository.  The grout and backfill provide chemical containment, since they are developed to have properties that favour the sorption of radioactive wastes to those materials.  The rock surrounding the repository provides physical isolation of the wastes from the accessible environment.  In a deep repository concept, the surrounding rock is usually the most important of the barriers in the repository system.

References

[1]  United Kingdom Nirex Limited, Generic Post‑closure Performance Assessment, Nirex Report N/080, 2003.

This report is freely available from the document library at http://www.nda.gov.uk.

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