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Selection of welding processes for nuclear waste encapsulation

Connect no.168

Schematic of canister repository
Schematic of canister repository

The safe and effective storage and disposal of high-level waste (HLW) and spent fuel (SF) arising from nuclear power generation is a global issue, posing major technical challenges. When TWI was asked by Nagra to advise on suitable welding technologies for hot cell encapsulation of HLW and SF, TWI was able to establish a multi-disciplinary team to investigate the best choice of welding processes.

Nagra is the Swiss national co-operative for the disposal of radioactive waste and is responsible for final disposal of all types of radioactive waste produced in Switzerland.

TWI was selected for the project due to significant experience with other long term dry cask and repository storage projects for the global nuclear industry such as the successful development of friction stir welded copper canisters for SKB and the hot cell welding procedures for closure of the canisters to be stored in Yucca Mountain.

From the outset it was clear that the Nagra approach to encapsulation would pose entirely new challenges to the welding processes. The chosen material for the canister construction was to be carbon steel, 190mm thickness with a minimum lifetime of 1000 years. The timescale for development also added challenges. With the weld production not scheduled until 2035, all processes had to be considered.

After brainstorming, literature reviews and preliminary investigations, TWI engineers reduced the list of potentially viable welding processes from over thirty to two: electron beam welding (EBW) and narrow gap tungsten inert gas (NG-TIG) welding. These processes were then researched and investigated extensively and all details of advantages, disadvantages and the technology readiness level were reported. This included in-depth analyses of potential metallurgical and residual stress issues arising from each of the potential techniques. In addition to the welding process technologies, methods of non destructively testing the welds in a hot cell environment were also investigated and reported.

Cross-section of hot wire, narrow gap tungsten arc welding in 180mm P91 steel
Cross-section of hot wire, narrow gap tungsten arc welding in 180mm P91 steel
Single pass, fully penetrating electron beam weld in 280mm thickness C-Mn steel
Single pass, fully penetrating electron beam weld in 280mm thickness C-Mn steel

The results of TWI's work were published in a Nagra report and presented at a meeting in Wettinngen, Switzerland. TWI's report was subsequently issued publicly, along with a previously published material selection report. This work formed a precursor for a multi-year study to develop designs for carbon steel canisters, which was issued as an open tender in March 2010.TWI (with Hitachi Zosen Corporation as a subcontractor) successfully bid for this competitive tender. Nagra evaluated the offers received on the basis of the financial offer, qualification and experience of the bidder, qualification and experience of the experts, proposed project work plan and an oral presentation made to Nagra.

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