RD&D-Programme 2004 - SKB

6 Canister – sealing
Two different methods are being considered for sealing the canister: friction stir welding
(FSW) and electron beam welding (EBW). SKB is studying the methods in parallel. The results
obtained to date show that both methods provide welds of sufficiently good quality. Our intention
is to choose a reference method for sealing in March 2005. This will give us enough time to
finish designing the encapsulation plant prior to applying for a permit. Section 6.1 deals with the
overall planning prior to an application. The programmes for electron beam welding and friction
stir welding are described in sections 6.2 and 6.3, respectively. A more detailed description of
strategy and methodology for choosing a welding method is presented in Chapter 7.
When the canisters have been sealed, they have to be inspected by means of different methods
for nondestructive testing (NDT), for example radiographic and ultrasonic inspection. The
application of the NDT methods differs depending on which welding method is used. Thus, it
is not possible to detect discontinuities by means of ultrasound in an FSW weld using the same
procedure as for an EBW weld. SKB’s work with nondestructive testing of the sealed canisters
is aimed at:
• Developing existing methods.
• Testing new methods.
• Verifying the reliability of the methods.
The programme for nondestructive testing is presented in section 6.4.
6.1 Planning
At the time of an application for a permit to build the encapsulation plant, SKB must have
chosen methods for sealing and testing. The reason for this is that the methods influence the
design of the encapsulation plant. At this time, a programme for qualification of the methods
must also exist. The overall milestones for the work on sealing technology and nondestructive
testing are:
• Verifying of the welding processes, i.e. confirming that the important process parameters
that determine weld quality are identified and establishing the limits within which they can
be allowed to vary. This programme will be carried out during 2004. Further information is
found under the relevant welding method in sections 6.2 and 6.3.
• Demonstrating the welding processes, i.e. showing the weld quality that is achieved under
production-like forms and showing that the different welding processes can be used under
industrial conditions. This programme will be carried out during 2004 and early 2005.
• Establishing how reliable the nondestructive testing is, i.e. determining the probability that
we can detect discontinuities of different types and sizes. This programme will be carried out
during 2004–2005. Since the welding methods are based on different principles, the NDT
methods that are applied are tailored to the particular welding methods, see section 6.4.
• Choosing a reference method for welding the canister. The time for choosing a reference
method is linked to the timetable for designing the encapsulation plant. For further information
on choice of welding methods see Chapter 7.
• Obtaining data for the safety assessment SR-Can. A programme aimed at determining the
probability that canisters containing critical defects are delivered from the encapsulation
process has been started, see further section 7.1. Other research on the canister’s long-term
safety is dealt with in Chapter 16.
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