Object

Feasibility of retrievable storage of heat-producing vitrified radioactive waste in deep lined boreholes in rock salt based on 'proven technology'.

Torad-B facility design (Fig.1)

• Bore-hole liner of low-alloyed stainless steel (St-52) (even corrosion behaviour, corrosion rate: 0.11 mm/a)
• External tube diameter 700 mm
• Tube thickness 60 mm
• Upper and lower lids 100 mm
• Borehole sealing
• 10 m pre-shaped rock salt plugs
• Low-alloyed stainless steel overpacked COGEMA containers (Fig.2)
• External diameter 508 mm
• Overpack thickness 20.6 mm
• Wrinkle zone 200 mm
• Upper and lower lids 20 mm

Torad-B feasibility checks

• Static and dynamic strength of container overpacks
  • Maximum static stress well below the critical yielding stress
  • Wrinkle zone capable of deceleration of a container at 100 m/s
• Radiation shielding capacity
  • 10 m rock salt plugs yields total radiation shielding

• Temperature development
  • Maximum temperature in rock salt (16 years) well below 180° C
  • Maximum temperature in access drifts about 45° C (DT: 10 K)
• Liner stress and deformation development
  • Maximum stress (central boreholes) 295 MPa
  • Maximum horizontal tube deformation < 1 mm
  • Maximum horizontal tube displacement 100 mm

Torad-B placement and retrieval procedure (Fig.4)

Overpacked containers are lowered in a lined borehole through a sliding slit construction, providing the necessary radiation shielding, on top of the stack of containers already stored. On top of the stack several steel containers are placed to provide the shielding during the welding of the upper sealing lid, after which a borehole is sealed using pre-shaped rocksalt plugs. If deemed necessary, the access drift is sealed with a dam.

The retrieval operation in essential is the opposite of the placement procedure. The tube is exposed either by re-drilling the upper 10m of the borehole or by excavation of a new access drift. After removal of the welded lid the containers are retrieved using the same equipment as for the placement procedure.

Torad-B design accident

• Preventive design features (Fig.5):
  • Protection against hoisting-wire failure (spring-forced brake)
  • Mechanical parachute to decrease impact velocity (air brake)
  • Periodical narrowings of the liner (wall friction brake)
  • Fluid filling of the borehole (viscosity brake)
• Development of alternative grab mechanisms:
  • Wedge bolt with hydraulic cylinders
  • Grab beaks with air-cushions
  • Twist grip and hoisting-eyes
  • Chemical anchor

Further investigations

• Development of monitoring procedures
  Monitoring equipment in short electrical heated boreholes parallel to the storage boreholes.
• Probabilistic performance assessment
  Early groundwater intrusion scenario (neglection scenario).

Acknowledgement

The Torad-B project was conducted as part of the CORA programme, instigated and financed by the Dutch Ministry of Economical Affairs.

Fig.1: Torad-B facility design

Fig.2: Container and overpack

Fig.3: Repository, in two galleries with vertical boreholes

Fig.4: Placement and retrieval procedure in the Torad-B concept

Fig.5: Design features of container and overpack in Torad-B concept:
spring-forced brake (1), mechanical parachute (2) to decrease impact velocity (air brake), periodical narrowings of the liner (wall friction brake, 3), fluid filling of the borehole (viscosity brake, not shown)