The temperature raising phase will have been carried out at a low gas flow. Although reactor power will have increased considerably during this phase, the term ‘power raising’ is normally used for the phase in which gas flow is increased. This is carried out at nominally constant temperature, important on a magnox reactor where the number of variables must be kept to a minimum because of the effect of the moderator temperature coefficient of reactivity.

On a magnox reactor the blowers may be on pony motor drive at the start of the power raising phase. As gas flow is increased, initially by raising pony motor speed, the circulators are transferred to main motor drive at a stage appropriate to the particular station design. Gas flow is increased by the method appropriate to the particular station design, for ex — ample, by increasing motor speed where this is a variable (at Bradwell and Hinkley Point A, auxiliary turbine speed is increased; at Dungeness A and Old­bury, the blowers are steam-turbine-driven), by increas­ing blower speed at constant motor speed where a fluid coupling is fitted {Berkeley and Dungeness £). by manipulation of throttle and bypass valves at constant motor, blower speed (Trawsfynydd), by opening inlet guide vanes at constant motor/blower speed (other stations). Where the blowers are steam-driven (either directly, or indirectly via auxiliary turbine-generators), blower speed is dependent on steam generation.

As gas flow is increased the core temperatures tend to decrease (see the formula in Section 5.1 of this chapter relating power, gas now and temperature rise) therefore control rods are operated as required to ensure that reactor power is increased in step with the increase in gas How, so that reactor gas outlet temperature is maintained constant. Net rod movement is much less than in the temperature raising phase because moderator temperature changes are small. During this phase the reactor gas outlet temperature will be on auto control on those stations which have the facility, but temperature control cannot be left entirely to the regulating rods because the reactivity changes required as the xenon concentration builds up would cause the regulating rods to run outside their useful range of operating height, therefore bulk rods are used for overall control and regulating rods for local trimming. On the stations without auto control the principles are the same. Xenon build-up is rela­tively slow’, taking two days to completion, and is therefore easily allowed for.

As gas flow is increased, reactor gas inlet tempera­ture will tend to rise and is allowed for in the re­actor by control rod movement to maintain constant gas outlet temperature.

When full gas flow has been achieved the power raising phase is complete. At various stages during the start-up other checks and operations may be car­ried out. for example, checks on the BCD readings for failed fuel as power increases (fission product activity increases with neutron power), transfer from intermediate to high range flux instrumentation as appropriate, changes in protection devices as shutdown and start-up protection gives way to full-power pro­tection. The start-up will be halted and the reactor power held steady while these checks and operations are carried out. The plant and start-up procedure are designed such that these checks and operations are grouped at convenient stages, for example, the tem­perature raising phase is very demanding on the re­actor control engineer so it is arranged to proceed ‘-moothly with minimum interruptions.

On AGRs, however, although the temperature rai­ding and power raising phases are easier from the reactor point of view they are more difficult from the boiler point of view. As mentioned before, there is no clear distinction on an AGR between tempera­ture raising and power raising phases. The operating regime required on AGR boilers strongly influences the reactor start-up procedure in as much as the target rate-of-rise of temperature is determined largely by the start-up technique required on the boilers. The rate-of-rise must, of course, be within the limits im­posed by the plant and flux and temperature pro­tection as mentioned in preceding paragraphs. The turbine can also influence the start-up procedure, see Fig 3.26 which shows two routes for the start-up of Dungeness В depending on the turbine casing temperature.

Fig! 3.26 Reactor start-up at Dungeness В
The start-up procedure on an AGR is influenced by
the state of the main turbine. In the example shown,
Route A is for a turbine casing temperature below
150°C and Route D is for a turbine casing
temperature above 250°C. The temperature raising is
shown as a steady linear rise for the purpose of
showing the differences in circulator speed-raising
stages, it is not intended to imply that the actual
temperature raising proceeds in this way.