CEGB to maintain a ‘pool’ of potential sites from which suitable candidates can be chosen as necessary. This pool is made up of the following three types of site:

•Existing power station sites capable of further development.

•Pieces of land already purchased by the CEGB for future development.

•Pieces of land not owned by the CEGB, but that have been identified as potential sites.

The identification and investigation of potential sites is usually divided into two phases: area of search and detailed investigation.

Although the Supergrid allows the transfer of large amounts of electricity from one part of the country to another, its capacity to do so is limited by both technical and economic constraints. Therefore, when the need for new generation is foreseen, transmission considerations combined with other factors, such as fuel sources, usually indicate in which part of the country it would be best to locate the station. The type of station required (nuclear, coal or oil) is dictated by such factors as the relative costs, the desired overall balance between fuels, and environmental considera­ tions.

Having identified the need for new generation in a certain region, a large area, perhaps covering several hundred square kilometres, is studied to find out its potential. Any known sites are also reviewed. Govern­ ment Departments are invited to draw attention to any places of special concern to them. Bodies such as The Countryside Commission and the Nature Conservancy Council, who have responsibility for preserving areas of natural beauty or of scientific value, are also notified and discussions are held with officers of the Local Planning Authority.

Information is gathered and analysed on technical matters such as water resources, geology, population distribution, road and rail system; as well as on

features and recreational areas. Much of this informa­ tion can be obtained from ordnance and geological survey maps, local and county plans, aerial photo­ graphs, Admiralty charts and other published material. These studies may take upwards of a year before a shortlist of sites thought worthy of detailed investiga­ tion can be prepared.

2.3 Detailed site investigation

Prospective sites may be identified through the area of search work or because changes in land-use give new opportunities, e.g., the closure of defence installations. Before detailed site investigations are started, the bodies previously consulted are notified, the owners

and occupiers are approached, and announcements are made in national and local newspapers.

It can take over two years to carry out the necessary detailed studies to prove the viability and determine the optimum capacity of each of the alternative sites being considered. During this period consultations take place with the authorities concerned with planning, environ­ mental protection, transport, water supply, flood pro­ tection, fisheries, safety and other relevant subjects. A careful study is made of the technical and amenity aspects of power station siting. The main topics covered for a typical nuclear power station site are shown on Fig 1.5. The major aspects of the studies are described as follows.

2.3.1Preliminary station layout

In order to assess the suitability of a particular site for the type of power station being considered, it is necessary to establish the initial basic station design. This includes the disposition of the major plant or groups of plant in the main station buildings, leading up to the determination of the shape and size of the build­ ings and then the grouping of the various individual buildings, and external plant items to produce a co­ ordinated station design which achieves the lowest capital cost, ease of construction and efficient oper­ ation and maintenance of the power station.

The preliminary station layout enables the on-site geological work to proceed and assessments to be carried out on the proposed site level, disposition of construction contractors’ plant and storage areas and environmental aspects.

are described in Chapter 2 of this Volume.

2.3.2Land requirements

Sufficient land will be required not only for the station when it is in operation, but also to provide adequate areas during the construction period.

The area occupied by an 1800 MW tower cooled, coal-fired station may be up to 100 ha (excluding ash disposal areas). The station buildings will take only a portion of the site. The remainder depends on (he needs of the coal store and railway sidings.

A 1200 MW nuclear power station will require 16-20 ha for operational purposes.

A considerable area will be required during the construction of both coal-fired and nuclear power stations. Typically 28-34 ha would be required to provide adequate working and storage areas for the contractors and for the construction car and bus parks. In addition, storage space will be required for topsoil removed during excavations (the area required would

£1 °n the Particular site) and for excavated material required for backfill.

nuclear station about 60 ha. Some further off-site land may also be required to provide areas for planting or landscaping to screen the station

_________________ Site selection and investigation

Figure 16 shows the typical land requirements for a

pressurised water reactor (PWR) station location next to an existing nuclear station.

2.3.3Cooling water

The total cooling water (CW) required depends on the ultimate station capacity planned. Typically for a coalfired station a 900 MW turbine requires a main Sv

AnnWJwe °f.about 24 m3/s- For a PWR station a 600 MV, turoine requires about 23 m3/s. Allowing for

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—— I XIS I ING Cl GH OWNI IlSHIP BOUNDARY

AREAS 1. B. 10 ANO 11 WOULD BE RE INSTATED AFTER COMPLETION OF CONSTRUCTION AND POSSIBLY RETURNED TO AGRICULTURAL USE

other cooling water requirements this means that an 1800 MW coal-fired station would require about 52 m3/s and a 1200 MW PWR station about 50 m'/s.

As the cooling water flows through the condenser tubes, its temperature is raised and this could typically vary between 8°C and 12°C. This warmed water must then be cooled using cooling towers or, in the case of direct cooled stations, by discharge to the water source and be dispersed in such a manner as to minimise its recirculation back into the cooling water intake with attendant loss of steam cycle efficiency.

The use of cooling towers requires that a suitable make-up water supply be identified which would typi­ cally amount to 2% to 3% of the total cooling water flow. Whilst the actual flow would be influenced by the

site-specific water quality aspects, it is usual for about two-thirds of this abstraction to be returned to the water source as purge to maintain the concentration factor within the cooling system. This water would be about 10°C warmer than the ambient water temperature.

If such a water supply is to be obtained from a river,

return on the environmental well-being of the river system. In the UK, water authorities often hold long term records of water flows and details of licensed abstractions. A seasonally variable ‘minimum pre­ scribed flow’ is often applied to rivers, which prohibits abstractions if the actual river flow fails to the specified level.

The preferred location for a power station from the cooling water viewpoint, is near a large river, estuary or sea coast to obtain the large volume flows at lowest temperatures. One of the key problems facing the cooling water system designer is therefore to provide the optimum location and separation between the cooling water intake point and the outfall. Another important requirement is to design a system which has the minimum effect on marine ecology. In this con­ nection it is necessary to ensure that warm water is adequately dispersed to avoid harmful effects on marine life. The acquisition of information on currents and water temperatures over a large area is necessary for these cooling water studies.

With a direct cooled system abstracting from and discharging to the sea or estuary, the eventual loss of heat to the atmosphere is a lengthy process, and in the intervening period the dispersion of warm water dis­ charged from a station outfall can be identified in a number of separate stages. The first, or near-field stage, is represented by the immediate mixing of newly discharged warmed water into the ambient sea.

Site selection and investigation

After a brief transitory period, a second or midfield stage is represented by a buoyant plume of warm water lifting towards the surface and spreading outwards at a rate determined by gravity currents, momentum effects and the action of the tidal stream. A midfield plume can eventually reach several hundred metres in width and can extend in length for 1 km to 2 km in the direction of the tidal stream (see Fig 1.7).

The normal practice is to minimise recirculation by physical separation of the intake and outfall structures. Civil tunnelling costs may limit the degree of protection that can be afforded by this practice, but additional protection can be sought by designing the intake structure to minimise drawdown from an overhead plume, to ensure maximum possible depth of water over the period of coverage, and to minimise the period of coverage.

CEGB surveys have identified a third stage in the heat dispersion process at a number of sites. During periods of calm weather conditions it has been observed that sequential flood and ebb movements of the midfield plume alongshore over a period of several

days can develop a far larger pool of warm water: Fig 1.8 shows this condition al Sizcwcli on the Suffolk coast. It can be seen that the spatial spread extended a full tidal excursion alongshore and several kilometres offshore. This far field plume is also moved alongshore by the reversing tidal stream and an amount of secon­ dary recirculation cannot be avoided, in the example shown, as the cost of separating the intake and Outfall structures exceeds the recirculation penalty. It has been found that a far field plume is quickly dissipated with increasing wind strengths.

It is important that survey operations should be conducted over a long period to ensure that the eventual design of the cooling water offshore works is founded upon a data base that sufficiently represents the variable meteorological and tidal current conditions local to the site. It is equally important that the survey period should include the calmer and warmer condi­ tions of the summer months when the natural and artificial thermal fields are most likely to reach a combined maximum temperature.

Some hydrographical information will be available for proposed sites near to an existing power station. However, the increase in size of new developments, the need to place the new offshore works in correct juxta­ position to existing structures, and the need to ensure that eventual combined discharges will not adversely "affect local ecology, will still require additional survey operations.

A survey will comprise an array of moored instru­ mentation to icconl continuous data of How pallci ns and water temperature changes throughout the survey period, and a number of individual operations gen­ erally limited in time to a single tidal excursion. The moored array can include current meters, tide gauges and thermistor stringers which, together with an on­ shore automatic meteorological station, provide an overall record of data to improve understanding of the results from individual survey operations. These indivi­ dual operations can include float tracking, intra red photography from helicopters or satellites, dye release, thermal plume profiling, and temperature/current/ salinity profiling. Alongside these activities, which are mainly designed to assist in evaluating thermal plume behaviour, the survey will contain the necessary echo sounding, side scan sonar, seismic work, seabed sediment sampling and wave recording to supply the information required by civil engineers for designing the station structures. The data is also used for asses­ sing the movement of materials on the sea bed and beach under the influence of winds and tides.

Thermal images of offshore coastal waters or estuaries can be obtained using infrared cameras on satellites. Contours of temperature may be marked by different grey tones for each temperature band step on black and white image presentations. Alternatively, a colour sliced image may be obtained, as shown in Fig 1.9, by the choice of individual colours for each contour.

Improvements in survey operations and measuring equipments have been paralleled by development of mathematical modelling techniques. I'xpetiencc how­ ever has clearly demonstrated the complex problems involved in both modelling the separate temperature fields that make up the thermal structure of a body of water, and of estimating the relative contributions of these, temperature fields at different sites and under differing tidal and meteorological conditions. Conse­ quently, despite some good correlation between survey results and model predictions, the CEGB continues to regard the hydrographic survey as the primary tool in present investigations. In particular, it provides a validation source for the flow predictions of a mathe­ matical model and the only satisfactory means of identifying the natural temperature fields. Hydraulic and mathematical models in which conditions such as tides, water and silt movement can be reproduced, are often used when planning and designing the cooling water works and jetties for power stations to ensure that they will not be adversely affected under these conditions.

Figure 1.10 shows a typical prediction of current circulation patterns for a coastal power station location.

The cooling water studies described here, which may take several months, need to be carried out before the final selection of a site can be made.

Chapter 1

2.3.4Transmission

A route must be available for the liansinission lines Ironi the proposed power station site Io a suitable point on the 400 kV supergrid system or major load point on the Area Board system which can accept the power station’s output. Increasing opposition from the public, amenity societies and planners to overhead lines makes routes increasingly difficult to obtain, and sometimes the only solution is to put sections of the line under­ ground. There are tremendous financial penalties and engineering difficulties for underground cabling, how­ ever, and so their use can have a major effect on the selection of sites when considering the economics.

Studies of the pattern of power flows which will occur after the commissioning of the proposed station are carried out to discover if any reinforcement of the grid system (such as uprating or addition of transmission lines) will be needed.

2.3.5Geology

Modern power stations, both coal-fired and nuclear impose very heavy loading on the subsoil which must be able to support it with suitably designed foundations.

The general nature of the soil can usually be obtained from records and maps of the geological strata. Before making a final decision on a site, however, a detailed

By analysing the results from float tracking over a tidal cycle and from longer term monitoring with moored current meters it is possible to predict the current circulation pattern at different states of the tide. TNs diagram shows the situation one hour after high tide on a spring tide. Similar complex patterns occur at other stages of the tide.

To minimise recirculation of the warm water discharge without having to discharge into the strong currents 1km offshore which would incur significant costs it is possible to discharge into the small bay to the north of the station. Discharge into the bay to the west would obviously cause problems with recirculation.

survey of the subsoil conditions must be carried out to determine the ability to carry the loads; the costs of suitable foundations, which can vary widely, can then be estimated. Where the proposed cooling water works require tunnels to and from the sea or rivers (usually a major item) subsoil investigations are necessary. These could include permeability tests and groundwater tests to enable the feasibility and cost of building the tunnels to be determined.

The subsoil investigation usually requires a number of exploratory boreholes to be sunk, some perhaps over 120 m deep, and trenches dug to expose geological features. Samples are tested both in situ and at soil mechanics laboratories to determine the thickness, strength and other physical properties of the strata under the site. More detailed information about the subsoil conditions between the boreholes can be obtained by the use of seismic reflection techniques. This involves vibrating the ground with either hand­ held or vehicle-mounted machines. The resulting shock waves reflect off the various layers of strata and are picked up by instruments set out on the ground. By moving the vibration source along a line and measuring the time taken for the waves to reach the detectors, a fairly accurate picture of the strala along that line can be obtained. In order to build tip an understanding of the geology of the site and surrounding area a grid of such lines is traversed.

In industrial areas it is essential to know the previous uses of the land; old foundations, mine workings, or similar features must be located.

Having established the general geology of the area, further boreholes arc then sunk to cover the whole site in detail before design and station layout decisions are made (sec Section 3.2 of this chapter).

2.3.6Site and station levels

A site should be reasonably level, not liable to flooding and not so high above the source of cooling water that excessive pumping power is required to supply water for cooling purposes.

A site requiring extensive filling to bring the level above the general flood level, or excessive excavation for the purposes of siting buildings, greatly increases the site preparation costs. However, in order to provide for the major capacities now required, such additional cost can often be justified. For base-load stations, where subsoil conditions are suitable, it can be econ­ omic to excavate deeper for the turbine house founda­ tions to save on pumping power.

2.3.7Access

Access to a power station is required for construction materials and plant, fuel supplies and employees.

Good road access is essential for construction, and rail and sea facilities are useful advantages. Direct access to a main trunk road to bring in heavy loads is desirable.

Site selection and investigation

Modern transformers and generator stators for large units can weigh up to 350 tonnes and, therefore, impose point loads unacceptable to many local authorities, through whose areas the heavy transporters pass on their way to the power station from the manufacturer’s works. The CEGB has to bear the cost of any bridge strengthening necessitated by the heavy loads.

A design of trailer with the facility to spread its load when negotiating bridges or unsuitable load heaping roads was therefore an attractive idea. A hovercraft principle of floating the trailer on a cushion of air was • adopted for transport over those roads where a weight restriction was in force. This vehicle, which is now in regular operation, is known as air cushion equipment. It relieves individual axle loadings on the road and saves the CEGB a great deal of expense reconstructing and strengthening weak links along the route (see Fig 1.11(a)).

An alternative to transporting a load the whole distance by road is to have it shipped from the port nearest to the place of manufacture to the point nearest to the site which special shallow-draught, end-loading vessels can reach. The CEGB owns some specially designed ships of this type known as roll-on and roll-off vessels. These have a capacity of 1000 tonnes and enable a 600 tonnes single load, inclusive of vehicles, to be shipped (see Fig 1.11(b)). Very large components can also be shipped by large sca-going barges.

The construction of a heavy load berth suitable for barges and roll-on/roll-off vessels adjacent to the site allows very large prefabricated components to be delivered, thereby reducing the amount of on-site » fabrication. Road traffic can also be reduced by deliver­ ing aggregate by sea.

While a power station is being built, traffic is greatly increased and so local roads adjacent to the site are often reconstructed and re-routed to avoid undue inconvenience or risk to other road users.

The overriding consideration for a conventional power station in full operation is access for its fuel supplies. The site therefore, must be conveniently situated either close to a main railway line to accept rail-borne fuel or, in areas remote from the coalfields or refinery, on an estuary or the sea coast to enable it to take its fuel from colliers or tankers.

For nuclear stations there must be a rail link on or near the station for transport of the flasks of irradiated fuel.

2.3.8 Water supplies for make-up and domestic purposes

Town mains water is used for all services where water has to be fit for domestic purposes, or where clean water is required for make-up or control purposes. However, it is the CEGB’s policy to increase the security of supply by providing, where possible, an alternative source. This could be by suitable treatment of seawater or from a river, lake or borehole.

Where water for fire fighting is to be taken from the town mains, allowance should be made either to duplicate the supply or to provide adequate storage capacity to ensure 100% availability; this is most important during the commissioning period of a boiler when the demands on the supply are heavy. Where the Water Authority permits, arrangements should be made for a bypass round the meter so that in cases of extreme emergency, a full flow is available for fire fighting.

11 a heavy, uneven demand is anticipated which is likely to exceed the maximum capacity of the water main, suitable storage capacity should be provided to give continuity of supply and to reduce the rate of draw-off from the main.

During the construction period, water consumption depends on the size of the labour force, the nature of the civil engineering works, e.g., aggregate washing, water jetting of piles, concreting, etc., and plant testing. For example, the demand for water during the construction of a 1200 MW nuclear station would usually be about 1100 m1 per day but could rise to three times this amount during commissioning. The maxi­ mum consumption during operation would be about 2200 m3 per day.

2.3.9Ash and dust disposal

The CEGB is the major consumer of the lower grades of UK coals which are being produced with an increasing ash content.

During the early years of operation an 1800 MW station, when operating on base load, can produce nearly a million tonnes of ash each year. If this were consolidated it would cover about 12 ha of land to a depth of 10 m in one year. When selecting a site for a coal-fired power station, very careful consideration must therefore be given to the provision of suitable economic ash disposal, either on low-lying ground or worked out mineral workings which can be filled by the creation of landscaped hills, or by the sale of pulverised fuel ash (PFA) to the construction industry.

Figure 1.12 shows ash disposal at the reclamation site which takes ash and dust from the Drax power station. Here, a hill is being constructed of PFA which is progressively top-soiled, grassed and landscaped. Alternative schemes involve reclaiming worked-out gravel pits which can then be used for agriculture or for amenity purposes, including fishing and sailing.

2.3.10 Flue gas desulphurisation byproducts

An 1800 MW coal-fired power station fitted with the limestone-gypsum type of flue gas desulphurisation (FGD) plant will produce about 500 000 tonnes of gypsum per year. This gypsum could be used to make building materials such as wallboard. The amount produced would be sufficient to supply an average size wallboard factory, and it is therefore possible that such

Site selection and investigation

a facility could be built adjacent to the site, otherwise the gypsum would be removed from site for use else­ where or disposal.

The byproducts produced by regenerative processes would have to be transported to a suitable chemical works for use. It is difficult and undesirable to store large quantities of products such as sulphuric acid or elemental sulphur and therefore a regenerative process would only be used where a reliable market for such products existed.

2.3.11 Detailed investigations related to nuclear safety

When considering the suitability of a site for a nuclear power station, additional safety-related aspects need to be studied. These can be divided roughly into the following four categories, the first three of which are potential sources of hazard to the station, and the fourth involves the safety of the public in the event of an incident:

(a) Earthquakes (seismicity) The geology of the site and the surrounding area is investigated to find out the local faulting pattern.

1 listoric research is carried out in order to estimate the location and size of any earthquakes that may have occurred in the general area in the past. For some sites it may also be necessary to place sensitive detectors at various places in the locality of the site to monitor the occurrence of very small seismic events. All this information can be put together to assess the probability of earth­ quakes of various sizes at the site. Nuclear stations are designed to safely withstand a certain size of earthquake. However, it is necessary to satisfy the Nuclear Installations Inspectorate (Nil) that the combination of design and siting is such that the risk of an uncontrolled release of radiation as the result of an earthquake is as low as reasonably acceptable (ALARA) that is, less than once chance in ten million years.

(b) Other natural hazards Studies are carried out into the potential hazard from other natural sources such as extreme weather conditions or flood. In this country it is unlikely that extreme weather conditions would present an appreciable hazard, however, the positioning of nuclear power stations on the coast means that the possibility of flooding must be carefully investigated. Not only must extreme sea levels, caused by a combination of tide and wind be evaluated, but the possible erosion of the coast must also be taken into account. From this work the need to raise the level of the site and/or provide coastal defence works can be assessed. It must be borne in mind, however, that raising of the site not only increases cooling water pumping costs but requires the delivery to site of large amounts of fill material.