RD&D-Programme 2004 - SKB

Siting
SKB’s main alternative is to build the encapsulation plant adjacent to Clab so that it can be
coordinated with existing activities. The spent fuel can then be transferred directly from the
storage pools in Clab to the pools in the encapsulation plant. At Clab there are also personnel
with expertise in and experience from radiological work.
SKB has conducted a feasibility study of a standalone encapsulation plant /1-1/ as well as a
comparison between different alternative sitings /1-2/. The standalone encapsulation plant is
assumed to be located near the deep repository’s surface facility. The primary technical difference
between the planned encapsulation plant at Clab and the one at the deep repository is the
way in which the fuel is handled and prepared prior to encapsulation. In the latter case, pools are
not used to receive the spent fuel. The fuel is instead stored under dry conditions before being
encapsulated.
1.3.2 Deep repository
SKB’s reference method for disposal of spent nuclear fuel is called the KBS-3 Method, where
the abbreviation KBS stands for kärnbränslesäkerhet = Nuclear Fuel Safety. The method
involves encapsulating the fuel in copper canisters which are then deposited, surrounded by
a buffer of bentonite clay, in deposition holes in a tunnel system at a depth of approximately
400–700 metres in crystalline bedrock, see Figure 1-6.
The purpose of the three barriers (canister, buffer and rock) is to isolate the radionuclides in the
fuel from the surrounding environment. Only if the radionuclides are brought up to the surface
by the moving groundwater do they become harmful to man and the environment. In the deep
repository it is primarily the canister that provides the isolating function. If radionuclides should
escape from a leaky canister, their transport must be retarded. All barriers contribute to the
retarding function. A partially damaged copper canister can effectively contribute to retardation
by impeding inflow and outflow of water. The bentonite buffer has the capacity to retain many
of the long-lived radionuclides, since they adhere to the surfaces of the clay particles. The rock
contributes to the retardation by virtue of the low water flux at such great depth. Furthermore,
radionuclides can adhere to fracture surfaces or penetrate into microfractures containing
stagnant water.
Cladding tube
Spent nuclear fuel
Bentonite clay
Surface portion of deep repository
500 m
Fuel pellet of
uranium dioxide
Copper canister
with cast iron insert
Crystalline bedrock
Underground portion of
deep repository
Figure 1-6. The KBS-3 method is based on multiple barriers (canister, buffer and rock) that
prevent the radionuclides in the fuel from harming man and the environment.
26 RD&D-Programme 2004