The operating staff in the reactor control room;

• Supervise the reactor while it is shut down.

• Start-up the reactor.

• Control reactor output to match electrical genera­tion requirements and turbine-generator availability.

• Optimise reactor output when full load generation is required.

• Shut down the reactor.

At al! times the operating staff ensure that the reactor is operated safely and within the limits laid down in the Operating Rules.

Operating techniques vary from station to station, just as the type of plant varies from station to sta­tion. In this section the general and common features of operation are described.

To enable the operating staff to carry out the functions listed above, they are provided with a range of operating information and plant controls. The principal information presented includes:

• Neutron flux, calibrated in units of reactor heat output.

• Temperatures, including reactor gas inlet and outlet, fuel channel gas outlet, graphite moderator, fuel can surface, structural items.

• Reactor gas flow; this may be measured directly, or it may be inferred from such parameters as gas circulator speed or circulator inlet guide vane position.

• Reactor gas pressure.

• Control rod positions.

• Boiler water/steam conditions and turbine electrical generation.

The principal plant controls provided include:

• Control rods to regulate the neutron flux and hence the reactor power.

• Reactor gas flow controls to regulate the transfer of heat from the reactor core to the boilers.

• Boiler feedwater controls to ensure that the reactor heat can be disposed of. [33]

The temperature rise is from reactor inlet to outlet. This formula is useful when considering the effects on reactor power of changes in gas flow at constant temperature and changes in temperature at constant gas flow; specific heat is assumed to remain constant. Where the temperature rise is high, as in AGRs at full power, the variation in specific heat with tem­perature cannot be ignored if accuracy is important, so where an accurate result is required it is more usual to perform the calculation in terms of enthalpy — rise rather than specific heat and temperature rise. However for the purposes of this section the formula as expressed above is adequate.

Control rods directly influence the reactor power. If the gas flow remains constant, changes in neutron power will affect reactor temperatures; this is the usual situation during the early stages of start-up and at nominal steady power. On the other hand, if the temperature rise is to be kept constant, changes in neutron power will have to be matched by changes in gas flow; this is the usual situation during gross power changes.