Weighing some 1800 tons and measuring 70ft in diameter each reactor vessel comprised an outer sphere of 3in thichsteel together with an inner steel vessel and supporting plates for the reactor core.

After construction on the site - by the welding together of sections weighing 300 tons or more - each completed vessel was heated uniformly to a dull red heat for stress relieving. When undertaken this was the biggest electrical heating job ever tackled in the world.
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