The other copper items include lower seal and a lid, both of which will be electron-beam welded to the copper shell. The gap will close as the ductile copper squeezes inward under the pressures imposed by groundwater and swelling bentonite. The copper canister is fabricated either as two tube halves formed from rolled plates that are welded together or as large seamless tubes that are made either by extru-sion or by piercing and drawing. Copper lids and bottoms are machined from for-gings. Forging produces a near-final shape, reducing the amount of material to be machined off.
Forging also results in a fine grain structure and increased strength.
EB welding, which is carried out in a vacuum chamber, creates very narrow, high-precision joints while not adding excessive heat to the canister. The picture below shows two half-tubes being assembled prior to welding. After welding, the tube is annealed to remove residual stresses. After annealing, the tube is machined to its final dimensions, as shown in the subsequent picture. Welded canister being machined to finished dimensions.