more difficult to balance. There is no easy way to consider all the different environmental impacts together to determine what measures are justified to reduce them, or to rank the various proposed power station sites. The search for better aids to judgements is essentia) but unending.

Figure 1.19 shows the Dinorwig pumped storage power station built deep inside a mountain in Snow­ donia and which is the largest in Europe. It represents a supreme example of a generating station in harmony with the environmental and ecological needs of a national park.

2.5 Site selection

After the sort of investigation described, some sites may prove to be unsuitable on technical or environ­ mental grounds and will therefore be rejected. Sites found to be acceptable but not already owned by the CEGB may be purchased for either immediate or later development. However, if the site is likely to remain available for use, then the CEGB may decide to delay purchase until development is required.

When detailed investigations have been completed, the overall cost of constructing and operating a parti­ cular size and type of station on the site is calculated. Alternative sites can then be evaluated and a recom­ mendation made to the CEGB Executive as to the site or sites which give the best balance of technical and economic considerations on the one hand and impact on the environment on the other. Sites which have been found to be suitable for development but which are not requited immediately are placed in reserve as potential sites for later reconsideration.

3 Site layout — thermal power stations

3.1 General

It is not possible to say that there is a recognisable site layout pattern which is adopted at most stations, but like the design of the station itself, the problem of site layout is to find an optimum arrangement which results in minimum overall cost consistent with ease of erec­ tion, ease of operation and good appearance within the limits of the available site (see Fig 1.20). There are always conflicting requirements to be reconciled and alteration of one feature often has repercussions on others so that every major aspect of the design must be considered in relation to the others. The main factors influencing site layout are discussed in the following paragraphs.

3.2 Foundations

When selecting a site the general foundation conditions are determined by sinking exploratory boreholes. The boreholes must then be extended to cover the whole

Site layout — thermal power stations

site in detail before foundation design and site layout decisions are made. The tests determine the most economic location of the main buildings and indicate any geological features which might limit the area in which the power station can be built. All other things being equal, a situation would be chosen for the main power station buildings where foundation costs would be at their lowest, as long as a good layout is obtained.

It is also necessary to establish whether there are any geological faults that could interfere with the location of the power station. A fault is the result of some long previous disturbance of the earth’s crust where there has been a vertical sliding movement; along the line of this the underlying strata at each side of the fault line will be found at two different levels. Where such a condition exists the station should be constructed clear of the fault line and not across it, as this could give rise to a subsequent differential settlement in the founda­ tions. In the case of nuclear stations, the seismic safety requirements insist that the safety related buildings are at a minimum distance away from any fault. This is assessed for specific sites depending on the significance of the particular fault.

A piled foundation is usually adopted where the subsoil is inadequate to carry the required loading, and it is necessary to penetrate to some depth to reach a load bearing strata.

A raft foundation is adopted where ground condi­ tions are suitable to carry the required loading. The raft, which can be of solid concrete or of cellular construction, that is with cavities or tunnels within -the mass concrete, is used to distribute the point loadings equally over the ground.

Assuming that there is no marked preference because of cost, the cellular raft has the advantage in that some accommodation is available for running pipes and cables below basement level.

Also associated with the strength of the ground is the question of coal mining. Where underground coal mining is expected to be carried out in the vicinity of a power station during its lifetime, it is necessary to prevent subsidence of the main components on site by refraining from mining within a closely defined volume of ground below the power station. The shape of this volume is known as a pillar of support and varies according to the surface area which is to be supported and the geological characteristics of the ground. This shape is formed by extending outward sloping lines, below ground level, at approximately 35 degrees to the vertical from the extremities of the foundations on the pillar. The power station designer should aim to keep this area to a practical minimum. The CEGB has an agreement with British Coal to restrict this supported area and so reduce the quantity of any coal which becomes unobtainable. Since some compensation to British Coal is payable under the agreement, the area is usually limited to the main buildings and cooling towers (if any) and sometimes the switch compound, but not the coal store and rail sidings.

3.3 Site and station levels

The main factors governing the choice of power station and site levels are:

•The need to protect the power station against the risk of flooding.

•Capital cost of civil works.

•Cooling water pump running costs.

•Ease and speed of construction.

For economy and general convenience on the site, the power station basement, roads and rail siding are gen­ erally constructed at the existing ground level. This avoids the necessity of extensive excavation and removal of soil, or the importation of filling material. If a site is above the predicted maximum sea or river level, the necessity for protection against flooding does not arise. If the site is below flood level there are two main methods of providing prolection. The surest way is to lift the power station basement and all other installations essential to the safe operation of the power

station, above flood level. This method is adopted where the cost of the required filling material is cheap and cost of pumping cooling water is not materially increased.

Where the filling material and pumping costs are expensive, flood banks are relied upon along those sides of the site below flood level.

The level of a power station with a closed cooling tower system is not dependent on the absolute sea or river level and. in consequence, the choice of power station basement level is little influenced by pumping costs.

However, with direct cooled systems (see Section 3.8.1 of this chapter) the water has to be pumped from the adjacent sea or river; in these cases the height between the power station level and water level is very important. This water level is itself constantly changing because of tidal variations; the maximum pumping head necessary is associated with the lower water level.

Because of high pumping costs, advantage is taken at direct cooled power stations of the principle of siphonic action. Atmospheric pressure acting on the surface of the sea or river is sufficient to lift the water to a height of about 10 m; this means that the cooling water pump has to generate a head sufficient to overcome only culvert and condenser friction before entering the outfall system.

There are sites, however, with the power station basement at site level, where the top of the condenser would be considerably more than that equivalent to 10 m head of water above the lowest level from which the water had to be pumped. If this is the case, one of two alternative arrangements is usually adopted. Either the condenser is lowered relative to the site level to reduce the height, or it becomes necessary to construct a seal pit or weir to restrict the length of the siphon leg. The choice is usually made by carefully examining the advantages and costs of both schemes (see Fig 1.21).

3.4 Main buildings and orientation

New power stations within the CEGB are now planned from the outset to be completed with a given number of units all of the same rating and over the years the layout of major thermal stations has developed into a general standard pattern. The boiler house and turbine hall arranged side by side can be regarded as a central feature. Very often between these two a mechanical annexe accommodating auxiliary plant items is inter­ posed. The building is usually completed by an auxili­ ary switch bay with generator transformers on the side of the turbine house and a bunker bay on the side of the boiler house. The other items required to complete the concept of this standard pattern are the electrostatic precipitators (on coal-fired plant), the induced draught fans, the flue gas desulphurisation (FGD) plant (where fitted) and the main gas flue and chimney. These features are illustrated in Figs 1.22 and 1.23.

It is currently proposed that future coal-fired stations should be based on the use of 2 x 90(1 MW(e) units, and the details of a reference design have now been established. Similar standardisation is also occurring on nuclear power station layouts, and it is intended, for example, that future PWR station buildings would be based on the station design used for Sizewell B. It can be se^n therefore that it is intended that future site layouts for conventional and nuclear plants will be based on the use of reference design arrangements

always conflicting requirements to be reconciled when deciding the best location and orientation for the main buildings. It is almost always impossible to satisfy every requirement perfectly. If a cooling water system is as short as possible, then the connection between the generator transformers and-the switchgear may be increased in cost; if coal conveyors or fuel pipelines arc reduced to minimum length then some other services may suffer, and so on. The emphasis which can be placed on each factor in terms of money, operational convenience and amenity value depends on the type of power station, the site and its environment.

Within these restrictions the orientation of the main station complex is chosen to give the best compromise of the following factors;

•Making the most economic use of the ground conditions by placing the heavier conventional boiler or nuclear reactor loads on the best ground.

•Minimising generator transformer cable and trans­ mission routes.

•Minimising pressurised cooling water culvert and outlet culvert routes.

•Minimising fuel supply and ash and dust removal routes.

•The adequacy and convenience of construction areas.

•The ability to place the administration buildings in a reasonable environment and close to the main access to the site.

•Meeting the requirements of the appointed Architect with respect to the important off-site views of the complex and its relation to other develop­ ments. particularly any existing power station on the site.

Figure 1.24 shows the layout of a proposed coal-fired reference design station.

3.5 Ancillary buildings

Ancillary buildings can be broadly categorised into two groups, those that are directly related to the main plant

N3

183 m

NOTE

ALL LEVELS ARE RELA’ '. £ TO GROUND LEVEL V.i- C~ IS 68 73 m AeO'»E ORDNANCE DATUM

Fig. 1.23 Section through 660 MW unit oil-tired power station

statii power thermal — layout Site

operation and those that are needed for accommodat­ ing personnel, or providing a service function. The first group would typically include:

•Town water reservoirs.

•Fire fighting pumphouse.

•Make-up water treatment plant.

•Bulk chemical stores.

•Hydrogen production plant.

•Hypochlorite production plant.

•Gas stores.

•Auxiliary boiler house.

)

Additionally on a nuclear site they would include:

•Diesel generator buildings.

•Active waste management building.

•Decontamination workshop.

•Radioactive fuel store.

•Reserve ultimate heat sink.

The locations of these buildings arc to a large extent dictated by their functional relationship to the’main

plant. As far as possible they are positioned to mini­ mise the length of pipework and cable connections and to provide good operational access routes.

As the reference designs for the PWR and new coalfired stations are developed, it is the intention that the relative locations of these buildings will as far as possible be fixed within the constraints of a particular site. This applies particularly to the buildings associated with safety aspects of the PWR.

The second group covering personnel accommoda­ tion and services includes:

•Administration building.

•Welfare building.

•Canteen.

•Fire station.

•First aid and medical centre.

•Workshops.

•Heavy and light stores.

Consideration of the routine and emergency require­ ments for the power station as a whole suggests that the location of the above items relative to each is signil'i-