The design of the ITER fusion reactor is of a doughnut shaped vacuum vessel approximately 15m high, 9m wide and with an outside diameter of 20m. The walls are of double skin construction, each 60mm thick and up to 750mm apart in316L stainless steel. The doughnut structure is made up of nine sectors to be welded together on site and one of the processes being considered is electron beam (EB) welding; access for welding is available only from inside the vessel.
The requirements for the EB equipment are:
- Compact, fully mobile electron gun capable of operation in a vacuum environment
- Welding vacuum about 0.1mbar because of large volume and local sealing arrangement
- Capable of penetrating 60mm stainless steel
- Capable of operating at short (<100mm) and long (750mm) working distance
- Capable of operating in any attitude
The standard Reduced Pressure electron gun column developed by TWI (pictured above) fits most of the above requirements; work has been progressing for the last three years both on modifications to the electron gun so that it fits the requirements exactly and also on developing welding conditions for penetrating up to 60mm deep in stainless steel at both short and long working distance at all welding positions. From flat (IG/PA) to just above vertical up position (3G/PF) sound welds in the full thickness are possible while from 3G/PF to overhead (5G/PE) the maximum possible sound weld using single pass autogenous conditions taper reduces to 25mm.