With successive magnox designs, reductions in capital and operating costs were achieved by increasing the reactor sizes and coolant gas pressures. As stated pre­viously, there is a limit to the reactor size and coolant gas pressure if steel pressure vessels are used to contain the core.

Safety could be improved if the boilers were con­tained in the same pressure vessel as the core, thereby eliminating the gas ducts, and if an alternative could be found to the steel pressure vessels. Steel pressure vessels have a single load path to withstand the gas pressure and care has to be taken in the design, selec­tion of material, welding, inspection and testing to ensure that possible failure by fast fracture can be discounted.

Towards the end of the magnox station programme, prestressed concrete pressure vessels were being de­signed and models built and tested. For the last two magnox stations at Oldbury and Wyifa, prestressed concrete pressure vessels were used to contain the core and boilers (Table 2.1).

The Oldbury vessels are cylindrical in shape, with a top and bottom slab, whilst the Wyifa vessels have a spherical internal shape.

The boilers are arranged round the core and are protected against irradiation from the core by a boiler shield wall. The vessel walls contain a large number of penetrations of differing diameters; for fuelling the core, for control rods, gas circulators, for feedwater and steam to and from the boilers, for safety valves and
connections to the auxiliary gas circuits, coolant gas make-up and gas blowdown systems (Fig 2.11).