RD&D-Programme 2004 - SKB

7 Canister – qualification
Before the encapsulation plant and the canister factory are taken into service, qualification of
methods for fabrication, welding and nondestructive testing of canisters must be carried out.
Qualification of the processes consists of documented investigations that ensure that the
finished canisters satisfy the ultimate requirement of being a reliable barrier in the deep repository.
Requirements will be specified for each process, after which the process will be qualified
to these requirements. A programme will be prepared for qualification and presented as a
supporting document for the permit application for the encapsulation plant.
The qualification work can be divided into qualification of methods for fabrication and welding
and qualification of methods for nondestructive testing. This is described in greater detail in
sections 7.1 and 7.2.
7.1 Qualification of methods for fabrication and welding
As is evident from Chapter 6, SKB is working in parallel on the development of two welding
methods at the Canister Laboratory: friction stir welding (FSW) and electron beam welding
(EBW). Prior to an application for a permit to erect the encapsulation plant, the methods for
sealing and nondestructive testing (NDT) must be determined.
The choice of reference welding method can be viewed as the first step of qualification.
A number of criteria have been formulated for this. The methods must fulfil these criteria
and be evaluated in reference to them. Some of the criteria are:
• Reliability. The risk that defects will arise that exceed the acceptance criteria for the sealing
welds must be low. It must be possible to control the welding process by means of variable
parameters.
• Robustness. The probability of disturbances that affect weld quality must be low. Small,
unforeseen or normal changes in welding parameters, equipment or the surrounding environment
must not affect the quality of the weld. The process window, i.e. the interval within
which the welding parameters must lie in order for the quality of the weld to be acceptable,
must be large in comparison with the tolerances of the process parameters.
• Repeatability. The same welding parameters and surrounding environment must give the
same weld quality. Changes in weld quality must be able to be attributed to changes in
parameters and the surrounding environment. After service and maintenance of the welding
equipment, the same welding parameters must give undiminished weld quality.
• Testability. The discontinuities that can arise in the process must be detectable with
sufficient probability by means of NDT. The forms for this evaluation are described in
greater detail in section 7.2.
SKB’s task is now to finish the development of the welding processes and equipment and verify
that the above criteria are fulfilled. Much of the fundamental development of the welding processes
and equipment has already been done. What remains to be done is to better quantify some
of the important variables and how they affect each other, and establish the optimal settings for
stable operation. Furthermore, SKB must demonstrate that it is possible to weld with sufficiently
high quality under conditions similar to those of serial production sealing in the encapsulation
plant.
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