6.1 Layout and radiological protection

As understanding and confidence grew with experi­ence, the widely separated reactors, turbine hall and auxiliary buildings exemplified by Hinkley Point A were replaced at the later magnox stations in the interests of capital economy and ease and efficiency of operation by a closely-integrated compact layout. This concept was also adopted for the AGR. AGRs also use prestressed reinforced concrete pressure vessels with boilers enclosed in an annular space protected by an internal shield as developed for the later mag­nox stations (Oldbury and Wylfa).

The basic radiological problems and needs of mag­nox reactors apply also to AGRs but there are some differences. It was deemed necessary to provide shield — mg over the reactor core to permit access to the ■ op dome area of the gas baffle. It is also necessary ■o dismantle irradiated fuel stringers and to carry out routine maintenance on fuel assembly components such as the plug units. During operation of the pro­totype AGR at Windscale, extensive deposits of active dust were discovered in the boilers. It was thought that a similar problem would arise in the CEGB re­actors and provisions were made to counter this. These included the installation of dust collectors (central inertial collectors) at the outlet of some reactor chan­nels and means for handling and storage of full col­lectors. Fairly elaborate facilities for handling and decontaminating gas circulators having significant de­posits of activity have been provided, but the active dust problem has not yet manifested itself.

Figure 2.1 shows the difference between the AGR В station and the magnox A station site arrangement at Hinkley Point. With the exception of the gas cir­culator workshop, the emergency generators (gas tur­bines) and waste incinerators, all ancillary services for the AGR reactors are inside the main building. This obviated the need for a controlled area fence and internal road system. The A station laundry was ex­tended to deal with the В station.

Although unique, it is worth noting the design of the boiler shield wall which comprises three layers of steel plate separated by steel tubes 89 mm diameter filled with calcium hydroxide. All the other AGRs, except for the ‘pod’ boiler arrangement at Hartlepool and Heysham 1, employ steel and graphite for this purpose.

Figures 2.70 and 2.71 show the disposition of most of the ancillary facilities in the Hinkley Point В sta­tion. The integral building includes a central shielded block^ between the two reactors and a reactor services building between the reactors and the turbine hall. The irradiated fuel cooling pond on the centre line of the building extends into an annexe beyond the reactor building which also houses the pond water and active effluent treatment plant. The central shielded block includes fuel handling and maintenance fa­cilities, waste stores, radiochemistry laboratory, CO2 coolant treatment plant and plant decontamination centre. The reactor services building includes the sta­tion control room and instrument rooms, main change room and ventilation plant. Although the control room is outside the controlled reactor area there is an emer­gency access route from the control room to the charge hall.

There are three extract ventilation plant rooms each with a separate discharge stack. There is one for each sub-pile cap interspace and one for the reactor ser­vices building and central shielded block.

Filtration comprises absolute filters with pre-filters. The areas covered include combustible active waste sorting, waste stores, radiochemistry laboratory, CO2 treatment plant, plant decontamination centre, reactor area workshop, charge machine maintenance, gas cir­culator penetrations, irradiated fuel handling, new fuel assembly and sub-change rooms. There is a separate

system for the cooling pond and effluent treatment plant areas.

Active waste stores are as follows;

• Irradiated fuel debris vault for reflector sleeves, tie rods and central inertial collectors,

• Maintenance cell debris vault for control rods and chains, tie rods and flux measuring heads. [19]

materials which are too active to burn, e. g., con­taminated clothing, filters.

• A ‘transient’ store for combustible waste.

• A ‘permanent’ store for combustible waste.

• A store for spent desiccant.

• Sludge and resin tanks.

• CO2 blowdown filter tank.

Within the main building there are four permanent sub-change rooms:

• One for the pond water and active effluent treat­ment plant, cooling pond and irradiated fuel trans­port flask area.

• One for the charge hall and contaminated ventila­tion plant extract rooms.

• One for the plant decontamination plant centre, reactor area workshop, charge machine service well and maintenance area.

• One for the new fuel assembly room and low-er new fuel assembly cell (primarily to maintain clean conditions).

Routes between the sub-change rooms and main change room can be isolated to prevent the spread of con­tamination.

Separate personnel control and change facilities are provided for the external circulator workshop and com­bustible waste sorting and incinerator facility.

For access to the pressure vessel there is a spe­cial penetration in the pile cap over which a tent is erected. Suction pumps discharge air from the ves­sel through the CO2 blowdown filters to ensure that no contamination escapes to the environment. The tent operates as an inlet filter to protect the core. There is a permanent gallery round the gas baffle from which access can be gained to permanent lad­ders between the boiler casings for inspection and maintenance. A system for the supply of air for pro­tective suit cooling and breathing from special com­pressors is provided.