RD&D-Programme 2004 - SKB

In the subsequent phase, verification, the intervals in which the welding parameters must lie to
ensure the desired quality of the weld are being studied. The intervals must be large enough so
that small, unforeseen or normal changes in welding parameters, equipment or ambient conditions
will not affect weld quality. This phase will be concluded in 2004.
This will be followed by demonstration of the welding methods, to such an extent that the
results can comprise input data to the safety assessment SR-Can. Results and experience are
presented and reported. During this phase, the possibilities of running the process under conditions
equivalent to serial production are studied.
Development of methods for nondestructive testing of the canister’s sealing weld is also
pursued at the Canister Laboratory. Procedures for testing of canisters sealed by EBW have
been developed, and similar work is being pursued for FSW. In order to determine the reliability
of non destructive testing (NDT), development work is being conducted at BAM (Bundesanstalt
für Materialforschung und -prüfung) to quantify the risk that a canister weld that does not
satisfy the acceptance criteria will escape detection.
The criteria for the choice of welding method will include experience from the development
work, evaluation of the robustness of the welding methods, data on achieved quality in serial
welding, and expected rejection frequency in production. If for some reason it is not possible to
select a main method at the planned point in time (2005), the choice can be postponed. If both
welding methods are deemed suitable at the time of application, it is possible to specify one
method in the application but continue to pursue development work for both. The final choice
can then be made later, provided that the design of the encapsulation plant is kept open for both
alternatives.
A3.3.2 Canister components
Fabrication technologies for the canister components (canister insert, steel lid for the insert,
copper tube, copper lid and bottom) have been under development for a number of years. The
work has mainly been pursued at suppliers inside and outside Sweden, with support from trade
associations, universities and research institutes, see Chapter 5.
Four methods have been tested for fabrication of copper tubes. The development status of the
methods varies, but all are judged to have potential for meeting the quality requirements. In
order to guarantee future deliveries and stimulate supplier competition, the aim is to continue
the development of different methods in parallel. There is also an ambition that future serial
production will engage several suppliers and perhaps also different methods.
Development of fabrication technology for the canister’s nodular iron insert is proceeding with
different casting methods at different suppliers. As is the case with the copper tubes, the aim is
that the future serial production of inserts will engage several suppliers and methods.
A large number of copper lids and bottoms have been fabricated by forging. This part of the
fabrication chain works very well, and the components meet the stipulated quality requirements.
Work on the canister’s lid and bottom has thus come far. In the reference design, the wall
thickness of the copper tube is 5 centimetres. Thinner copper shells have been discussed. The
choice of copper thickness is a question of optimization with respect to e.g. short- and long-term
safety, technology, environment and resource-effectiveness. Trial welding of the bottom onto
the copper shell by means of EBW has previously been conducted at TWI, and quite recently
FSW trials were conducted at the Canister Laboratory. A welding method that satisfies the
quality requirements for lid welding can also be used to weld bottoms. Compared to lid welding,
bottom welding is simpler to perform and inspect, since it is done at an earlier fabrication stage
without insert or fuel. If the copper tube is made by pierce and draw processing, bottom welding
can be omitted.
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