All the BCD systems installed, from the earliest mag­nox power stations such as Bradwell up to the last at Wyifa, were engineered from the same basic design concept developed for Calder Hall A (commissioned in 1956).

However, with the construction of large magnox reactors, the twin requirements of improving fuel sur­veillance and rationalising the pipework system (par­ticularly within the pressure vessel envelope) resulted in major design development on BCD components, such as standardised precipitators and improved high temperature selector valves. This greatly improved the BCD system overall performance. Examination of the BCD system currently in use has highlighted two dif­ferent design concepts, mainly involving the location of the primary selector valves:

• Positioning of selector valves on or adjacent to standpipe penetrations. These have been taken out horizontally and housed below the pile cap floor, outside the main primary circuit.

• Positioning of selector valves within the primary circuit above the reactor core. These have been taken out horizontally through a special side-mounted penetration.

1.7.5 Typical BCD layout The Wyifa BCD system has been chosen to illustrate generally the design intent of a typical BCD system installation. Like most of the BCD systems Wyifa has three independent operating facilities, each with its own performance regarding sensitivity and scanning frequency. Only the compressor unit is common to all three BCD systems. The complete BCD system pipework arrangement can be seen in the diagrammatic arrange­ment shown in Fig 2.27. The three facilities are com­monly known as bulk, group and single.
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2 77 -‘OURS З‘0 M:4JTES 32 SECONDS 9.8 SECONDS 3 0 SECONDS?:SECONDS ■ 4 SECONDS 4′ SECONDS ‘tSECONDS ‘ ’ SECONDS ’ SECOND	‘ 7 8 ViNUTES 1 5 4 MINUSES 2’4 M’NJTES ■ 4 MINUTES 8C SECONDS ShOPT ShOpt 32 MiNuTcS 95 UNITES Є6 SECONDS Short Short
Fig, 2.25 Relevant krypton and xenon isotopes