Design of the facilities

In 1993 the Dutch Government added to the existing radioactive waste management policy by stating that final disposal must take place in such a way that the waste is retrievable for a lengthy period of time. To that end, in the framework of the METRO project, NRG has developed a new design for a repository for high level radioactive wastes in rock salt. At the same time, in the framework of the TRUCK project, SCK×CEN, NRG and KUL have proposed a design for a repository for the disposal of high level radioactive wastes in clay.

Both the METRO and the TRUCK disposal concepts are based upon the limited quantity of vitrified high level wastes (contained in approximately 320 'COGEMA' containers) from the reprocessing of the spent fuel from the two nuclear power plants in the Netherlands.

The general layout of the two concepts is similar (Figure 1). With respect to the detailed design of the disposal cell in clay, a watertight corrosion-resistant accommodation of the container is required. The containers can be retrieved after removing the backfill from the disposal cell. The procedure for retrieval is essentially the reverse of the procedure of emplacement.

An important implication of the Dutch policy is that the deposition tunnels (or drifts or galleries) should be accessible 'for a lengthy period of time'. In terms of 'traditional' operation of a disposal facility, maintaining the access to the deposition tunnels is a type of 'prolonged operation' of the facility.

Costs

Estimates show that the costs of building, operating and closing a geological disposal facility in rock salt, will be in the order of about 200-400 MEUR. The costs of the (optimised) TRUCK design are estimated between 400 and 800 MEUR. Optional retrievability does not significantly increase the cost of building and closing the facility.

Extending the operational phase for a lengthy period will cause additional costs, estimated 2-4 MEUR per year, on the basis of the technical effort needed. However, societal and political demands could translate to operating procedures which would lead to significantly higher costs.

Safety

NRG has screened its FEP catalogue (Features, Events and Processes) for items that threaten the proper isolation of the waste during the period of prolonged operation; after the waste has been emplaced in the disposal cells, but before the tunnels and shafts have been is properly closed and sealed. The result is given in Table 1.

The neglection scenario describes the evolution of the facility in the case of neglection after the waste has been emplaced in the disposal cells, but before the facility is closed and sealed. The first sign of neglection is that the maintenance ceases (or is insufficient). The neglection scenario consists of the steps in Table 2.

In case of neglection of a facility in rock salt, the waste will eventually be isolated due to drastic compaction of the plugs that seal the disposal cells. This is a result of a typical behaviour of the salt rock: due to the large lithostatic pressure and the plastic deformation of the rock salt formation all materials in the rock salt are squeezed. Due to this compaction, the salt plugs (that are used to seal the disposal cell) will eventually become impermeable. Figure 2 shows that the waste flow eventually stops due to the compaction of the plugs.

In low permeable clay, the nuclides are retained due to the adsorption capacity of the clay. This is shown in Figure 3. Because the adsorption is nuclide (species) specific, the maximum of each nuclide specific curve is found at a different time - the so-called retardation effect.

Acknowledgement

The METRO and TRUCK projects were conducted as part of the CORA programme, instigated and financed by the Dutch Ministry of Economical Affairs

Table 1: FEPs that threaten the isolation of the waste before closure of the facility

Table 2: Breakdown of the neglection scenario

Figure 2: Waste that escapes from the disposal cell to the flooded gallery after neglection of the facility (rock salt)

Figure 3: Nuclide release after neglection of the facility (clay)