19.5 Ventilation

Ventilation has to be carefully considered in turbine halls. The amount of heat given off by the plant is not excessive when compared with the volume of the building and a reasonably comfortable temperature is maintained except when extreme conditions prevail externally. The build-up of humidity can, however, be a problem and air at roof level cooled below dewpoint temperature causes condensation to form resulting in degradation of the protective coatings of the building fabric and corrosion of the steelwork roof members. Condensation can be kept under control by preventing the air being cooled below dewpoint by removal of the moisture-laden air. The application of insulation to metal and the use of double glazing reduce the possibi­ lity of condensation attack. Double glazing is very expensive and in most cases ventilation to remove air before saturation point is reached is the best method of limiting or preventing condensation. This is more easily carried out if the turbine hall is open to the de-aerator bay and air from the turbine hall can be taken directly from high level into the boiler house. If this is not possible then some form of ventilation must be intro­ duced at the highest level in the turbine hall.

Condenser design involves the provision of valves or diaphragms as protection against excessive pressure in the condensers. These valves or bursting discs are capable of discharging the full load steam flow, which for a 500 MW set is 450 m3/s. The ventilating arrange­ ments of the buildings must be capable of removing this quantity of steam sufficiently quickly to prevent it filling the building. To achieve this, some 40 nr of clear openings must be provided at high level in the turbine hall. As permanent openings of this size would result in excessive heat loss during normal operation, it follows that controlled ventilation must be provided. To func­ tion in an emergency, these ventilators must open fully in a few seconds and motor-operated louvres or open­ ing sections of patent glazing are suitable, the motors being controlled by a pushbutton in the control room. A high degree of maintenance is necessary if ventilators of this type' are to be kept in working order.

Normal ventilation of the turbine hall is provided by opening windows iri the walls and roof lights.

The boiler intake^ are normally located at the top levels in the boiler house and louvres are used at the lower levels in the walls (as shown for West Burton in Fig 3.53) to provide the necessary air inlet. The penthouses to allow the escape of boiler radiation losses are also shown in this figure. Approximately 100 000 m3/min of air is required for combustion in a 2000 MW station on full load.

In addition to this air which is required for combus­ tion, a similar amount must be allowed to pass through the boiler house to remove the boiler radiation losses making a total of 200 000 tn'/inin, of which half enters the boilers, the remainder pouring out of the boiler house at the high levels. Insulation is not normally

Reactor construction

provided to the boiler house wall cladding although it is usually used on the roof decking, mainly as an element in the composite weatherproof membrane of the roof.

Figure 3.57 shows the ventilation and boiler air intakes at Thorpe Marsh power station.

Hot smoke and smoke logging can be a major pro­ blem when fires occur in turbine halls and careful con­ sideration must be given to this matter in the venti­ lation design. Fire venting to reduce the temperature in the building and restrict the spread of flame must also be considered.

19.6 Floor and wall finishes

The turbine and boiler operating floors are usually finished with vitreous tiles or other high quality, hard wearing, non-absorbent floor finish. The basement floors to the turbine hall and the boiler house are provided with a quarry tile or granolithic finish and end-grain wood blocks are often used in the loading bays.

Large areas of the internal faces of walls in the turbine hall and boiler house are pointed and painted to provide a satisfactory finish. Plastering is only used in offices, toilets and other similar locations.

Insulated cladding is sometimes used for the walls of the turbine hall and large areas of windows, patent glazing or transparent sheeting are provided to give natural light.

Acoustic panelling is desirable in some areas for health and safety reasons.

Annexes to main buildings usually consist of a de­ aerator bay located between the turbine hall and the boiler house and an annexe to the boiler house on the opposite side to the turbine hall, which houses ash and dust plant with associated electrical plant.

20Reactor construction

20.1Reactors

The first reactors used in power stations had spherical pressure vessels constructed from welded steel plate about 50 mm thick. These reactors were located in a separate large cylinder formed of concrete biological shield walls about 1.83 m thick. Integral with the walls was the concrete forming the base and pile cap.

As the development of reactors proceeded it became apparent that if the concrete shields were increased in thickness and reinforced with prestressed cables, then the steel pressure vessel could be replaced by a comparatively light steel lining about 13 mm thick to the inner face of the concrete. This design allowed for the majority of the tension forces in the concrete due to gas pressure being resisted by the prestressing cables.

Models of pressure vessels of this type were mode

and tested, and from estimates and assessments of ' performance it became apparent that the advantages of

concrete pressure vessels over steel made them econ­ omically and operationally attractive. Some of the more important advantages of containing reactors in concrete pressure vessels are as follows:

• A prestressed concrete pressure vessel is cheaper than an equivalent steel vessel with a concrete biological shield.

• The concrete is subjected to its greatest compressive stresses during the prestressing operations, for when the reactor is operational, the gas pressure has the effect of reducing the compressive stresses induced in the concrete by the prestressing operations.

•The steel prestressing wiies or strands which carry the tensile forces induced by the gas pressure are protected from overheating by a good thickness of concrete.

•There is relative immunity to failure from local weakness in the tensile steel wire used for the cables, as each of many tendons is anchored independently.

•The demand for skilled labour on a prestressed, concrete vessel is less than for a steel vessel.

•Using prestressed concrete construction, the size of

the pressure vessel and the contained pressure can be increased without the fabrication difficulties asso­ ciated with thick steel plates. It therefore became possible to introduce the boilers into the pressure vessel, thereby eliminating the gas ducts and the awkward duct-to-vessel intersections.

• The load-carrying tendons can be examined during the life of the vessel and if external or ungrouted tendons arc used, they can be retensioned or replaced if necessary.

• A concrete pressure vessel could not fail catastrophi­ cally if reasonably designed.

Figure 3.58 shows the arrangement of a typical cylindri­ cal prestressed concrete pressure vessel in which the ends and walls are reinforced with prestressed tendons. There are no special requirements for the concrete used in concrete vessel construction, normal good quality concrete with a minimum crushing strength of 41.37 MN/m2 at 28 days being suitable. The arrangement of the tendon ducts for the prestressing wires in the ends and side of the cylinder is such that the concrete will not

be subject to cracking with gas pressures up to 1.65 limes the winking pressure. The precompression of the concrete due to the stressing of the cables results in compressive stresses of between 1724 kN/m2 and 2068 kN/m2 in the concrete, and the loading of the tendons by the jacks induces stresses up to 965 MN/m2 in the steel. The use of the spiral layout of the cables in the walls of the cylinder ensures a uniform prestress therein, the basic angle of the spiral establishing the relationship of vertical tb hoop prestressing. Alternat­ ing spiral layers are needed in both directions. Local tendon path deviations ensure there is no weakness at any of the penetrations in the walls for the entry of steam and feed pipes or control rods.

Figure 3.59 shows the arrangement of a spherical prestressed concrete reactor on which a large propor­ tion of the prestressing tendons are fixed externally.

Over the years, the methods of prestressing pressure vessels have varied considerably from a simple threedimensional tendon arrangement at Dungeness B to the use of a vertical and a wire winding system at Hartlepool and Heysham 1. At several of the more recent stations a sophisticated variation of the spiral layout of tendons in the walls has been used that eliminates the need for end cap tendons. The spiral system has been used successfully on 50% of the UK pressure vessels. The principal advantage of this arrangement is that the anchorages of the prestres­

sing tendons are readily accessible for load checking and replacement, if necessary, when the reactors are operating.

The arrangement of the stressing gallery for tendons in the wall of an AGR pressure vessel is shown in Fig 3.60.

20.2 Reactor buildings

Layouts of the reactor buildings and turbine halls for stations using prestressed concrete cylindrical and spherical reactors are shown in Figs 3.61 and 3.62 respectively. Generally, turbine halls are very similar to those used for coal and oil-fired stations. Although the first reactor buildings were constructed largely from concrete, structural steelwork being only used on the framing for the structures enclosing the boilers and the roof over the charge face, the more recent ones have used structural steelwork on an increasing scale for economic reasons. Figure 3.63 shows the reactor build­ ing at Oldbury power station.

The most recent stations at Heysham 2 and Torness have the turbines and reactors contained in one inte­ grated building. The form of the main structures and the principal dimensions are shown in Figs 3.64 and 3.65. They are generally framed in reinforced concrete, with the exception of the charge hall and turbine hall

above crane rail level, which are framed in structural steel.

The nuclear island buildings are all founded directly on rock, with the PCPVs, central control building and fuel handling building foundations being separate rafts, with the remainder of the structure founded on strip or isolated foundations.

21 Ancillary buildings ‘21.1 General

In the past there have been many views held on the desirability of including ancillary buildings as part of

the main structure, grouping them in annexes to the main buildings or locating them away from the main building. The present trend is for buildings, especially those occupied by personnel for long periods, to be located away from the main buildings. This minimises noise nuisance and also enables the architect to obtain more natural lighting in the buildings.

One big disadvantage with providing accommodation for ancillary functions in the main buildings or annexes is that the steelwork must be designed in conjunction with the main building steelwork and this takes place at a very early stage in the project. Due to their size, the time of construction for ancillary buildings is less than

Fig. 3.60 Lower stressing gallery for AGR pressure vessel (see also colour photograph between pp 242 and pp 243)

buildings Ancillary