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Decision made - it's official

Connect, no. 138, September/October 2005, p.1

Friction stir welding in action, joining the lid to the canister
Friction stir welding in action, joining the lid to the canister

Swedish nuclear waste specialist selects friction stir welding as front runner for nuclear waste encapsulation in thick section copper canisters.

Twenty years of joint development between SKB and TWI has produced two novel welding processes that can meet the highly demanding requirements for sealing 50mm thick copper canisters for nuclear waste disposal - friction stir and Reduced Pressure electron beam welding.

Having compared the processes in detail, SKB will now put forward plans for the encapsulation plant to use FSW to seal the canisters.

Friction stir welding entered the contest in the mid nineties and was immediately considered a serious contender for closure and sealing of the 5m long, 1m diameter, 50mm wall thickness copper canisters that will provide the main physical barrier in the waste storage system.

The announcement has been made following close collaboration between SKB and TWI on the assessment of alternative welding processes for the task.

In the early eighties SKB raised the question of which of the available welding processes was most applicable to welding of thick section copper. As anyone who has tried fusion welding copper will know, it is a tricky process because heat is conducted away rapidly from the fusion zone. This makes it difficult to establish and maintain a molten pool of any significant size. This severely limits the process's capability as material thickness increases.

At the time, the only process that showed promise was electron beam welding which, due to the high power density in the focused beam, was capable of efficient coupling with the workpiece and the generation of narrow, deep welds by the keyhole welding process.

In parallel with the EB work, TWI researchers were busy developing the innovative friction stir welding process that has made such an impact in welding of aluminium alloys for the aerospace and transportation industries.

Early results on flat plate copper material were sufficiently encouraging for SKB to commission the design and build of an experimental prototype FSW machine at TWI to weld circumferential parts representative of full sized canisters.

This machine proved capable of welding 50mm thick material and the experience gained was used to specify a bespoke machine to be installed in Oskarshamn for further tests.

A period of further process proving followed. The two candidate processes went head-to-head in a test to produce a score of full diameter welded joints in production-like conditions in early 2005.

Both processes were found to be readily capable of producing high integrity welds and performed admirably in the test. However, it was SKB's view that the FSW process, which produces a solid state weld - no melting takes place - can be regarded as a highly robust, machine tool process. It offered the potential to be the more reliable and reproducible in the welding of thick section copper, which is recognised as being particularly difficult to weld using fusion techniques.

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