1.1 Layout and radiological protection

In common with fossil fuel fired plant one of the
prime objectives in deciding the layout of magnox stations was to minimise capital cost without preju­dicing reliability, safety, efficiency and ease of opera­tion. For the early magnox stations perceived safety needs and, to some extent, construction needs dictated that each reactor building in a two-unit station and the turbine-generator building should be separate. This led to separate housing of many of the common re-

r services with long interconnecting routes and ‘■onsiderable ground area requirements. The aim of ^his section is to indicate the effect on station layout of radiological protection needs.

Radioloeical hazards may be categorised as follows:

• Direct radiation lrom radioactive materials.

• Inhalation of active gases and dusts.

• Inaestion of active material primarily through the food chain or from contaminated hands.

• Skin contamination.

Л number of UK statutory regulations exist to safe­guard both operators and the public. In particular, design and operation must comply with the nuclear site licence and the authorisations to discharge radio­active substances which are granted by UK govern­ment departments for each station. The CEGB Safety Rules (Radiological) were produced to ensure that stations are operated in accordance with statutory requirements and stations must be so designed that they can be operated in accordance with the rules and requirements (see Chapter 4).

Insofar as station layout is affected radiological protection needs may be considered in five categories:

(a) Plant and structures required specifically for radio­logical protection.

(b) Plant and equipment required for reactor protection.

(c) Special maintenance requirements.

(d) Active waste storage (see Chapter 4).

(d Control of personnel and protection against con­tamination.

In category (a) the most significant items are:

• Shielding to give protection against direct radiation lrom the core, irradiated fuel, active coolant and components and active waste.

• Filters tor removing active particulate matter from reactor coolant and coolant blowdown.

• Filter for removing active iodine from CO: blow­down tollowing an accident.

• ^ emulation and filtration plant to prevent the spread of airborne particulate activity arising outside the reactor and to prevent the accumulation of toxic voneerurations of CO: in accessible areas. [14]

Category (b) includes such items as emergency gen­erators and batteries, reactor protection systems, burst cartridge (or can) detection (BCD) equipment, control rod motor generators, etc.

Considering category (c), there are a number of plant and equipment items which become activated or contaminated in normal operation and which require routine maintenance. For magnox stations it was not considered necessary to provide facilities specifically for maintaining equipment which becomes highly ac­tivated in the core. However, it was considered ne­cessary to be able to maintain contaminated plant such as gas circulators, pond equipment, active liquid pumps and valves, refuelling machines, etc. For this purpose a decontamination centre and special work­shops are needed. A specially equipped radio-chemistry laboratory is needed for handling active gases and liquids and a special laundry for cleaning contaminated protective clothing.

With regard to category (d) shielded storage faci­lities are required for any parts removed from irra­diated fuel before it is despatched from the station, for discarded grabs and other fuel handling equipment and for discarded in-core instruments. Storage is re­quired also for spent desiccant from the CO: drying plant, spent resins and sludges from pond cooling and active effluent treatment plant, discarded CO: filters, shield cooling air filters and ventilation filters, con­taminated gas circulator lubricating oil, clothing, tem­porary coverings and swabs. Of a somewhat different nature is the three month store needed for the decay of spent fuel before despatch from the site.

Considering category (e), the plant must be arranged and shielding provided so that normal operations can be carried out without exceeding the maximum per­missible dose to operators or the public. Arrange­ments must be such that only authorised persons can gain access to radiation zones. Where infrequent access is required to high radiation zones these must be normally locked-off. For protection against surface and airborne contamination, potential contamination zones must be environmentally isolated from neigh­bouring areas by suitable enclosures and a ventilation system which ensures that no air can escape from the enclosure except through a filtered extract. Facilities for changing into and out of protective clothing with washing, showering and personal monitoring facilities must be provided. As in the case of radiation zones, access to potential contamination zones must be con­trolled so that only authorised persons can enter.

The requirements for control of access, the protec­tion of personnel against contamination and the pre­vention of spread of surface contamination are met by segregating the radiation and contamination zones from the remainder of the plant and by providing only one controlled point for entry and exit. At this point a main change room, with shower and monitoring equipment is located with the appropriate number of dean and dirty clothes lockers and an office for a

‘guardian’. A ‘sub-change’ room with washing facilities connected to the active drains is placed adjacent to each potential contamination area to which access is required. Where only infrequent access is needed it is sufficient to provide a space with water supply and active drains in which temporary change facilities can be erected.

Where the station layout demands the movement of activated or contaminated equipment outside a building to, for example, workshops, decontamination centre or waste stores it is necessary for the intercon­necting roads to be within the controlled area and therefore fenced off from the remainder of the site.

Hinkley Point A is one of the earlier magnox sta­tions and has separate reactor and turbine room build­ings with some of the common reactor services also in separate buildings. Figure 2.1 shows the Hinkley Point site plan. The disposition of the A station reactor and turbine room buildings can be seen together with the separate buildings for new-fuel store, emergency generators (diesels), decontamination centre, active workshop, irradiated fuel cooling pond, active waste store, cooling pond water and active liquid effluent treatment plant and other services. The station control room and the main change room are both situated at ground level in an annexe on the reactor side of the turbine hall on the station centre line. Figure 2.1 also shows the A station controlled area fence and the road system within.

Special ventilation plant is provided to ensure that any leakage of CO2 is safely dispersed from areas such as the circulator houses, boiler rooms, bypass filter and drier rooms, burst can detector (BCD) pre­cipitator and compressor rooms, CO2 safety valve, vacuum pumps and filter rooms. Filters are provided in the shield cooling air discharge and the ventilation discharge from areas such as the reactor equipment building, fuelling machine maintenance bay and maga­zine filling pit, the irradiated fuel discharge shaft and decontamination building. Figure 2.2 shows the loca­tion of the BCD precipitators, CO2 filter room, shield cooling air fans and filters, fuelling machine main­tenance bay and irradiated fuel discharge shaft.

There are four solid active waste stores:

• Pile cap mortuary holes into which discarded con­trol rods and charge chutes are lowered by the fuel machine.

• Voids between the primary and secondary shields near the fuel discharge points into which the fuel machine discharges control rod cables and connec­tors, flux flattening elements, control rod stools, grabheads, fuel element bottom support struts and thermocouple leads and ion chambers. [15]

• A store with airlock type access to prevent release of dust for fuel element splitter vanes.

1.2 Fuel

The first magnox reactors at Calder Hall started op­eration in 1956; in 1962 the CEGB commissioned the Berkeley and Bradwejl plants which represented the first of the eight Mk 1 Nuclear Power Stations to be operated.

When fuel for these reactors was first designed, little information was available on endurance, and as a result the initial fuel cycles were based on reject irradiation of 1750 MWd/t (channel average) and three years’ irradiation time.

The plants were designed for on-load refuelling by means of a system whereby individual fuel elements could be removed through standpipes which had ac­cess to groups of channels. Low burn-up limits in­volved on-load refuelling rates which were at the limit of charge machine capacity.

There was therefore an urgent need to gather in­formation on fuel endurance in order to raise these limits to provide improved plant margins and operating economics.

Although oxidation of certain steel reactor circuit components has subsequently necessitated some re­duction in output, the fuel endurance has been pro­gressively improved allowing fuel discharge limits to be raised to levels in excess of 5500 MWd/t (chan­nel average) and nine years’ dwell, with experimental quantities now exceeding 6500 MWd/t (channel aver­age) and eleven years (1986 targets).

The success of the magnox fuel can be judged against the irradiation of several million fuel elements to date with only limited failures which have caused negligible lossiof generation.