COMPLETE DOCUMENT (1862 kb) - OECD Nuclear Energy Agency

are placed in deposition holes drilled below the floors of tunnels at about 500 m depth in the crystalline,
granitic bedrock (Figure II.29). Each canister is surrounded by blocks of compressed bentonite. When
the bentonite absorbs water from the surrounding bedrock it will exert an intense swelling pressure and
completely fill all void space in the near vicinity of the canister. The clay barrier will contribute to the
isolation by preventing or delaying access to the canister by dissolved corrosive species that may exist in
minor amounts in the ground water. The clay will also provide some mechanical protection for the
canister. The tunnels will eventually be backfilled by some material like a mixture of crushed rock and
bentonite.
For a repository to dispose of all spent fuel from the present Swedish programme (about
8 000 t or 4 000 canisters), the underground facilities will need some 30~40 km of tunnels and cover an
area of about 1 km 2 .
Figure II.29 Swedish repository concept
Encapsulation of spent nuclear fuel
A necessary facility is a plant for encapsulating the spent nuclear fuel. The plant would take
fuel assemblies from interim storage and transfer them to canisters made of copper with a steel insert,
change the atmosphere to inert gas, put lids on the canister and seal the lids by electron beam welding.
The quality of the filled and sealed canisters would be checked by non-destructive examination (NDE)
methods – ultrasonic and radiographic – before shipping to the repository.
Each canister can hold 12 BWR fuel assemblies or 4 PWR assemblies. The copper thickness
will be about 50 mm and the steel thickness as a minimum about 50 mm (Figure II.30). The copper
must be thick enough to prevent corrosion from penetrating the canister as long as the spent fuel
radiotoxicity substantially exceeds that in a rich uranium ore. The combined thickness of steel and
copper should be enough to prevent any significant radiolysis of water outside the canister after
deposition in wet bentonite clay. The steel insert is designed to withstand the normal mechanical loads
that will prevail in the repository, such as hydrostatic pressure and the bentonite swelling pressure. The
total weight of a canister with fuel will be about 25 t.
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