The Sizewell 'B' reactor pressure vessel is
very large by industrial standards. It is 482 cm
in outer diameter, 1355 cm tall and 21.5 cm
thick, or thereabouts, weighing about 450 tonnes.
The interior surface which is in contact with the
reactor coolant is protected from corrosion by
two fusion weld deposited layers of stainless
steel with a minimum total thickness of a few
millimetres. The vessel has been constructed by
fusion welding forgings together. Forgings are
made by forming large pieces of steel when it is
hot enough to be relatively soft. Fusion welding
is itself a known cause of unreliability and in
addition it is known to be more vulnerable to
degradation of the physical properties by neutron
irradiation. For that reason a cylindrical
section of the Sizewell 'B' vessel is made in one
forged section only welded to the remainder of
the vessel at top and bottom edges, away from the
maximum neutron irradiation. In this review, only
failure of this central cylindrical section of
the vessel will be considered. The nuclear
reactor in Sizewell 'B' is situated centrally
within the cylindrical portion of the pressure
vessel.
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