Presented at: 2001 International High - Level Radioactive Waste Management Conference, Las Vegas, Nevada, USA, 29 April - 3 May 2001
Publisher: American Nuclear Society
Colin N Ribton and Richard E Andrews
Two welding processes are being investigated for the sealing of copper vessels that form the outer barrier of Sweden's spent fuel assembly disposal canisters. TWI Ltd in the UK has developed Reduced Pressure Electron Beam welding and Friction Stir Welding for 50mm thick copper. This paper describes some of the investigations and compares the techniques.
Sweden has chosen to manage spent fuel assemblies by direct encapsulation and storage in a deep level repository. The canister has been designed to ensure there is no leakage of radioactive material into the ground water. The outer canister is manufactured from 50mm thick copper and the manufacture and sealing of this vessel has required extensive study and development of joining techniques. The Welding Institute (TWI) in Cambridge, UK have worked closely with Svensk Kärnbränslehantering AB (SKB) on the development of two joining techniques: electron beam welding and friction stir welding. This paper describes the techniques and compares the processes for high level waste encapsulation.
II. The Swedish method
There are a number of varying philosophies concerning the treatment and management of high level nuclear waste. In Sweden, SKB has been tasked with the management and disposal of high level waste produced by the country's nuclear power plants. A policy of safe long-term disposal of the fuel assemblies has developed and SKB have a substantial programme examining the feasibility of all aspects of this policy. The estimated time scale for this work is:
- 2002 Site investigations for a long term, deep repository site
- 2007 Approval of encapsulation process
- 2008 Approval of the deep repository
- 2012 Commissioning of first stage of encapsulation plant
- 2015 Deposition of the first copper canister in the deep repository
The spent fuel will be lifted from fuel storage ponds, dried and placed into canisters. These will then, after sealing, be transported to the repository. The repository will be some 500m deep in granite and the canisters will be packed in bentonite clay to provide an environment where they will contain the waste for at least 100,000 years, see Fig.1
III. Canister design
The design of the canister consists of a cast nodular iron inner container with cavities to support the spent fuel assemblies. This is loaded into a copper canister that provides the corrosion barrier. The copper canister is 50mm thick, 5m high and 1m diameter (approximately).
Sealing of the lid is across a minimum joint plane thickness of 50mm, this thickness being the least necessary to maintain the integrity of the canister wall. Copper is a material that can only be readily welded with a few techniques in thick section. Joint depths of 50mm have only been addressed by one of the two techniques below.
IV. Electron beam welding
Electron Beam Welding (EBW) has been used industrially for more than 40 years. Originally the technique was carried out in a vacuum chamber and was limited in power capability. Progressive improvements in gun design have led to higher power systems being available, and variants of the process have been developed where the vacuum chamber is only operated at a coarse vacuum or where the electron beam is brought out into the atmosphere (non-vacuum Electron Beam (EB)). TWI has carried out some more radical developments - in 1972 producing the world's first 75kW 150kV system and in 1986 producing a 100kW diode gun. (Almost all commercial equipment uses triode guns and may be prone to high voltage flashover that can produce large weld defects.) In 1991 TWI produced a variant of the process (Reduced Pressure Electron Beam (RPEB)) operating at a chamber pressure of 0.1 to 100mbar. This addressed some of the outstanding problems for EBW of the copper canisters. Such large pieces would be difficult to pump to the low pressures needed for conventional (in-vacuum) EB, and obtaining more than 50mm penetration in copper was proven to be impossible with the highest power and voltage non-vacuum EB system available at the time.
Welding work carried out throughout the duration of a 10-year development programme has produced a distinct fusion zone shape for the canister lid seal weld, see Fig.2. It has proven necessary to produce this shape to ensure that root defects are not produced in the fusion zone. Copper welding is particularly prone to this type of defect as the material has a high thermal conductivity, causing the fusion zone to narrow, and the copper vapour has a focusing effect upon the beam further narrowing the fusion zone.
Fig.2. Typical EB weld section profile
V. Friction stir welding
The invention of Friction Stir Welding (FSW), a solid phase joining process, in 1991 at TWI, has provided a second means for making the high integrity welds in 50mm thick copper. In 1997 SKB commissioned TWI to develop FSW for thick section copper, and specifically to aim at applying the process to welding bases onto canisters. FSW is considered by SKB to be a relatively simple machine tool technology that could provide an alternative or complementary method for canister production and repair.
Over the past 3 years a full size canister welding machine has been designed and built. For the past 12 months this equipment has been employed for development of FSW tool technology and optimisation of weld parameters. Welds in 50mm thick copper have been produced in 120° segments of approximately 1m length, see Fig.3. Much of the recent work has concentrated on the development of tool forms and tool materials. The results being achieved using FSW have been very encouraging and development of the technique will continue at SKB and TWI.
Fig.3. Friction Stir Welded segment in 50mm thick copper
There are marked differences between FSW and EBW. FSW is a solid phase process and a hot forged microstructure is produced at ~850°C in copper without melting. From the process viewpoint, FSW is a contact technique and tool breakage, if it occurred, would be likely to give rise to repair difficulties. EBW produces a cast microstructure, which can give rise to porosity and other weld defects. EBW development at TWI and SKB has concentrated on producing high integrity EB welds where incidence and size of such defects are reduced to acceptable levels.
The performance of EB welds and FSW joints differs as a result of the differing microstructures produced by the processes. The finer grained microstructure typical of FSW gives properties similar to the parent plant. FSW being a forge process, the material is also consolidated to some extent. The coarser grained microstructure of EBW gives rise to difficulties in ultrasonic examination, and to date SKB also have used X-radiographs to search for fine pores of 1mm diameter or less.
SKB and TWI have investigated EBW extensively for the fabrication and sealing of copper canisters for high level nuclear waste encapsulation. This has been demonstrated on full size canisters using the Reduced Pressure machine developed at TWI. More recent developments in FSW technology have enabled a second joining process to be investigated and this programme is producing very encouraging results. Joints of up to 1m length have been produced in segments that simulate the canister lid weld. Specialised tools have been developed for the welding of such thick sections in copper.
Sweden has an advanced programme for long-term disposal of high level waste. TWI continues to work closely with SKB in optimisation of copper canister welding.
The work at TWI has been funded by SKB, who have kindly given permission for this publication. Allan Sanderson (EB) and Dick Andrews (FSW) lead lid welding activities at TWI.