1.4.1 Pressure circuit — primary circuit

Function

The function of the reactor pressure circuit is to contain the coolant gas which transfers the core­generated heat to the boilers. An increase in gas pressure increases the rate of heat removal from the core and decreases the circulator power. As successive magnox stations were developed, gas pressures were increased.

At the stations with steel pressure vessels, the de­sign limited the gas pressure which could be used for a given size of core and hence reactor output. This was the primary incentive for adopting the pre-stressed concrete pressure vessel at Oldbury and Wylfa (Table 2.1).

Standards and design

Safety considerations require that the components of the primary coolant circuit have the highest integrity. When the first stations were designed, it was generally agreed that the existing standards for the design, con­struction, inspection and testing of these components were not adequate. Supplementary requirements to the existing standards were agreed with the Independ­ent Inspecting Authority and specified for primary coolant circuit components. These supplementary re­quirements were mainly concerned with the detailed analysis of the design and the quality of the fabrica­tion and inspection of the reactor components. They now form part of the relevant British Standards.

The Nuclear Installation Inspectorate (Nil), has a condition in the Station Site Licence which requires the CEGB to appoint an Independent Inspecting Authority.

Steel pressure vessel — reactor pressure circuit layout The reactor pressure vessel containing the core is con­nected to the boilers by ducts. Typical arrangements of the pressure circuits are shown in Fig 2.8, the number of boilers per reactor varying from station to station.

The location of the boilers relative to the reactor is governed by a number of considerations which include:

• The need for adequate shielding between the core and the gas circuit components.

• Gas distribution to and from the core.

• Economics (capital cost and gas circuit pressure drop).

• The requirement to have a good natural circulation round the gas circuits. This influences the relative heights of the boilers and the core.

Each gas circuit consists of inlet and outlet ducts connecting the reactor to the boilers; gas valves to isolate the reactor from the boilers; bellows units to cater for component thermal expansion; a gas circu­lator to circulate the coolant gas round the circuit and a bypass circuit which connects the outlet of the gas circulator to the boiler, i. e., bypassing the reactor.