Emplacement of wastes, backfilling, monitoring and repository sealing

The overall layout of the underground facilities is shown in Fig. 4.5-1. The major part of the facilities is the main facility with the emplacement tunnels for SF and HLW canisters. Prior to

70 A canister of 3.25 m length containing two HLW flasks is also an option.

105 NAGRA NTB 02-05

emplacement of SF / HLW canisters in the main repository, some canisters are emplaced in the pilot facility, which consists of a few short emplacement tunnels and an observation tunnel, but is otherwise constructed along the same principles and with the same materials as the main facility. The pilot facility is designed to permit monitoring of near field rock and the emplaced engineered barriers (see Chapter 2).

Fig. 4.5-6: Emplacement containers for ILW Dimensions are in mm.

Emplacement of SF / HLW canisters

The emplacement tunnels for SF / HLW have a diameter of 2.5 m and are optimally aligned with respect to the regional stress field in order to reduce excavation-induced damage arising from stress relief. Design studies based on field measurements of geotechnical properties of Opalinus Clay suggest that under these conditions, the rock will be self-supporting, although a light reinforcing mesh and rock bolts will be required to ensure operational safety. Nonetheless, the possibility that the SF / HLW tunnels will require a liner cannot be completely excluded. In such a circumstance the interaction of liner material (e.g. concrete or synthetic polymer) with bentonite and Opalinus Clay would have to be evaluated.

Canisters of SF / HLW, contained in a transport shield, are moved by rail into position at the entrance of emplacement tunnels. Here the canisters are transferred onto bentonite blocks on an emplacement wagon, which is moved by rail to the final emplacement position (Nagra 2002b).

The distance between canisters is 3 m.

2438 25

2438 2400

25 25

2438 20

2438 2000

20 20

WA-BNF- 4

WA-BNF-2 /7

WA-COG-2

WA-COG- 6

WA-COG-4A/6A

EC2-MA-25

EC2-MA-20

Emplacement containers for ILW

NAGRA NTB 02-05 106

Backfilling

The highly compacted bentonite blocks which support the canisters have a dry density of

~ 1.75 Mg m^-3 (emplaced at an assumed initial moisture content of ~ 10 %). The region around the canisters is backfilled with granular bentonite using a pneumatic or conveyor system. The granular backfill has a moisture content of ~ 2 % (Naundorf & Wollenberg 1992), and com-prises ~ 80 % by volume very dense granules (~ 2.1 – 2.2 Mg m^-3)and 20 % bentonite powder.

Upon emplacement, the material is expected to have an average dry density of 1.5 Mg m^-3 (Röski 1997). An important consequence of using high density, low moisture content granules is low thermal conductivity⁷¹, as discussed in Section 5.3.2. The sequence of operations involving excavation, waste emplacement, backfilling and tunnel sealing ensures that a given emplace-ment tunnel is open for only one to two years, thus avoiding significant alteration of the Opalinus Clay at the tunnel periphery. The emplacement tunnels at the completion of backfilling are illustrated in Fig. 4.5-7. Note that both the terms buffer and backfill are used to describe the bentonite backfill surrounding SF / HLW canisters.

Fig. 4.5-7: Longitudinal section through disposal tunnels for canisters of HLW (top) and SF (bottom) at completion of canister and backfill emplacement

The tunnel diameter is 2.5 m.

Emplacement of ILW

The ILW emplacement tunnels are approximately 9 m × 7 m in cross-section (open space after lining) and are supported by a concrete liner and rock bolts. They are located at the end of the operations tunnel, ~500 m away from the SF / HLW tunnels (see Fig. 4.5-1). Cross-sections

71 If required, the thermal conductivity of the bentonite can be significantly increased by the addition of 5 % graphite, which would likely decrease the maximum temperature at the canister/bentonite interface by ~ 20 – 30 °C.

3 m

3 m Bentonite blocks

(compacted) Bentonite pellets

Bentonite blocks

(compacted) Bentonite

pellets 2 m

4.6 m

107 NAGRA NTB 02-05

through the ILW emplacement tunnels are shown in Fig. 4.5-8. A smaller diameter ILW emplacement tunnel (ILW-2) for disposal of ILW that have significant concentrations of low molecular weight organics and other complexing agents is located nearby, but well separated from the ILW-1 tunnels (Fig. 4.5-1). After emplacement of the ILW, the void regions within the emplacement tunnels are filled with a cementitious mortar. Most of the ILW would be emplaced during the first phase of repository operation, at the same time as wastes are emplaced in the pilot facility.

Fig. 4.5-8: Cross-sections of ILW emplacement tunnels at the end of waste emplacement, with different types of waste emplacement containers

Monitoring phase

The status of the disposal facility after the completion of emplacement of all wastes is shown in Fig. 4.5-9. At this time, all wastes are in backfilled and sealed emplacement tunnels and the main operations tunnels are sealed. The wastes in the pilot facility are in backfilled emplace-ment rooms that are accessible to monitoring using the adjacent observation tunnel and monitoring boreholes. The shaft is sealed with ~ 40 m of highly compacted bentonite, above which is backfill and an additional bentonite seal is placed where the shaft exits the Wedel-sandstein Formation, to prevent nuclides that may have migrated through the Opalinus Clay from taking a shortcut path through the overlying confining units to the overlying aquifer. The SF / HLW and ILW operations tunnels and the construction tunnel are backfilled with a 30 % bentonite/70 % sand mixture and sealed with ~ 40 m of highly compacted bentonite (Sitz 2002).

Closure and final sealing

Final closure of the facility would involve emplacement of two ~ 40 m long seals of highly compacted bentonite and backfilling the ramp. The main seal at the repository horizon is to be placed at the construction branchoff of the access tunnel (Fig. 4.5-10), the second where the ramp intersects the overlying Wedelsandstein Formation. These long-term seals are designed

NAGRA NTB 02-05 108

with the objective of ensuring that the main tunnels and access ramp have hydraulic properties similar to those of the undisturbed host rock. This will be achieved by the following steps:

1. The concrete tunnel liner and steel rails are removed within the seal section.

2. A ring of Opalinus Clay is excavated to a depth of ~ 1 m to remove the excavation-damage zone, which may have been altered by contact with the concrete and with air.

3. A bulkhead is installed across the tunnel.

4. An approximately 40 m section of highly-compacted blocks of bentonite is installed, keyed into the Opalinus Clay. The blocks would have a density sufficient to give a swelling pressure of 9 MPa, to counteract tunnel convergence and the formation of a new EDZ.

5. The second bulkhead is emplaced; the ramp is then backfilled with a mixture of bentonite and sand.

Details of the design of seals are given by Sitz (2002).

Fig. 4.5-9: Status of the repository during the monitoring phase, when waste emplacement is complete, but before final sealing and closure of the facility

Note that seals are considered to comprise highly compacted bentonite, along with a con-crete bulkhead. Plugs at the entrances to ILW emplacement tunnels are composed of concrete.