RD&D-Programme 2004 - SKB

A3
Encapsulation
A3.1 Overview
Development of the technology for encapsulation is in an intensive phase. The work is now
focused on the supporting material for the permit application for the encapsulation plant. After
a permit has been granted, the focus will shift to construction, commissioning and trial operation.
According to SKB’s timetable, it should be possible to obtain a licence for operation of the
encapsulation plant by 2017.
The basic requirements on encapsulation are that the canisters to be deposited must be leaktight,
possess long-term corrosion resistance in the environment existing in the deep repository, and
be able to withstand the loads predicted to arise after deposition. They must be functionally
unaffected by the handling chain from sealing to deposition. The work of deriving quantitative
requirements regarding material quality and dimensions, as well as tolerances in fabrication and
sealing, from these basic requirements has been under way for a long time.
Figure 6 is a simplified illustration of the flow in the encapsulation process. There are different
methods for fabrication of the canister’s components. In cooperation with suppliers, SKB has
refined and tested several methods on a full scale. This work will be pursued further in order to
arrive at a finished industrial-scale fabrication process.
A key component in the production process is sealing, which takes place directly after fuel
assemblies have been placed in the canister. Sealing consists of welding a copper lid on the
copper tube that comprises the canister’s outer shell. Extensive development work is required in
order to be able to weld with sufficiently high reliability in the relevant materials and thicknesses.
SKB is developing two methods – EBW (electron beam welding) and FSW (friction stir welding).
In parallel with the development of fabrication and sealing methods, techniques are being
developed for quality inspection by means of nondestructive testing. In order to ensure that the
quality goals are achieved, a quality assurance system is being developed. The system stipulates
quality requirements and how they are to be satisfied and documented. It includes procedures
and quality plans, as well as requirements on testing methods and associated acceptance criteria.
Methods and suppliers for fabrication, sealing and inspection will be qualified, and a dialogue is
being held with SKI regarding the forms for this qualification.
Even though the production process is being developed towards the goal of very high reliability,
the risk that canisters that fail to satisfy the specifications will nevertheless occasionally be
produced and pass the inspection system must be taken into consideration. This risk must be
quantified and taken into account in the safety assessments. This quantification will be based on
trial production and statistical evaluation of the resulting quality. The risk that a canister will fail
to satisfy the specifications is dependent on the reliability of both the welding process and the
quality control system.
Clab
Supplier A
Spent
nuclear fuel
Canister factory
Canisters
Encapsulation
plant
Encapsulated
nuclear fuel
Deep repository
Supplier B
Figure 6. Overview of the production process.
376 RD&D-Programme 2004