The earlier AGRs were not designed to the ‘CEGB Design Safety Guidelines’, but as this document is largely a statement of requirements which arose out of the Board’s experiences with these stations, there are many similarities in the design of the post-trip heat removal, systems. None of them, however, have the extensive provisions made at Heysham 2.

Hinkley Point В is the basis of the station design for Heysham 2 and the systems for post-trip heat removal are similar, but without the extensive redun­dancy and segregation. The main heat removal route is via the main boilers, with steam venting to atmos­phere. The boilers are fed either by the two starting and standby boiler feed pumps or a station-based emergency boiler feed system comprising three 10% pumps drawing from the reserve feedwater tanks. The gas circulators only run at full speed, thus only one of the two circulators is normally operated in each quadrant, with the inlet guide vanes partially closed. Automatic sequencing equipment is provided to estab­lish post-trip cooling and organise the electrical sup­plies, which are provided from a conventional station and unit boards system, backed-up by four 17.5 MW gas turbines for emergency duties. The decay heat boilers and the feed system are introduced manually about 20 minutes after the reactor trip; a single feed and condenser system serves both reactors.

Hartlepool and Heysham I

These two stations are quite different in concept to Hevsham 2 and do not have the benefit of decay heat boilers. The main post-trip heat removal route ■ ^ja the main boilers, fed either by the single start — in(T ancj standbv feed pump or two 8% emergency boiler feed pumps, these latter mav draw water from the de-aerator, the RFTs or the condensate polish — jne plant outlet. The steam is dumped into a single dump condenser which serves both reactors. The gas circulators are driven at low speed through pony motors. Automatic sequencing equipment establishes these post-trip heat removal systems and the essential electrical system. The latter comprises two unit trans­formers supplying two 11 kV boards, with a generator switch to enable them to remain energised with the unit shutdown; emergency power is provided by four

17.5 MW gas turbines. In addition there is an entirely separate back-up cooling system which is initiated solely by operator action; this is to cater for certain hazard conditions which may result in total loss of the normal systems. The system comprises two HP diesel-driven pumps supplied from RFTs to provide feed to the main boilers, and two LP diesel-driven pumps drawing from towns water reservoirs to supply the primary sides of the PVCS coolers and reactor auxiliaries cooling system coolers; gas circulation is achieved through natural convection.

Dungeness В

Dungeness В again is without a decay heat boiler sys­tem. The main boilers fed by either the single starting and standby feed pump, a 15% steam-driven emer­gency boiler feed pump or two 2,5% electrically-driven emergency boiler feed pumps, all drawing from the de-aerator. The gas circulators, operating at low speed through a pony motor drive, provide the main post­trip heat removal route. The electrical system is a conventional station and unit transformer system, with lour 3.5 MW diesel generators suppling 3.3 kV station boards. Again an entirely independent back-up cool­ing system is provided, similar in concept to Hartle­pool. However, the Dungeness В reactors do not have sufficient natural convection capability, so three diesel generators and associated distribution boards are provided to supply the gas circulator pony motor drives and three electrically-driven HP and three LP emergency cooling water pumps.