A controlled shutdown occurs when it is dear that the reactor must be shut down but the severity of the problem does not require an instantaneous shut­down.

Magnox

On a magnox reactor the ideal procedure is in three parts. First, temperature is reduced by a few degrees to ensure that in the subsequent stages of the shut­down, when the uniformity of temperature distribu­tion across the core may be disturbed, Operating Rule temperature limits are not exceeded locally. Second, gas flow is reduced at constant reactor gas outlet temperature to achieve a substantial reduction in re­actor power. By maintaining high temperatures dur­ing the power reduction, the main turbines can be kept on load (their load being reduced to match the reduction in reactor power) to dispose of the reactor heat. Third, with gas flow at a low value, the control rods are run into the core to complete the shutdown.

This is the ideal procedure, but for various reasons it rarely occurs. The initial temperature reduction may not be necessary if the reactor is operating already several degrees below the Operating Rule temperature limits, for example, if it is operating to corrosion control limits which are more restrictive than Operat­ing Rule limits. Indeed it may be highly inadvisable to attempt a temperature reduction; if the bulk rods and sector rods are only lightly inserted into the re­actor core, for example, because of operation at high mean fuel irradiation, there may be inadequate over­ride capability to offset the negative reactivity change arising from the overall positive temperature coeffi­cient of reactivity.

Xenon override is also considered. As neutron power is reduced, particularly during the gas flow re­duction where the majority of the power loss occurs, the Xe-135 concentration will increase. This will in­troduce additional negative reactivity which must be balanced by control rod withdrawal, and where this is limited by the circumstances outlined in the previous paragraph then the shutdown will become uncontrolled and the reactor will shut itself down. However, xenon changes occur over a longer timescale, so unless the shutdown is prolonged it is unlikely to be a problem.

A useful source of positive reactivity for tempera­ture and xenon override is described in Section 5.5 °f this chapter, namely increase in reactor gas inlet temperature, but this is only used for power reduc­tions where the aim is to keep the reactor on load.

If the objective is to shut down the reactor com­pletely, there is tittle to be gained from a controlled shutdown (rather than a trip) except the advantage in being able to perform the necessary operations on the boilers, turbines and various auxiliary plant on a less-rushed timescale. As far as the reactor is concerned, it may as well be tripped, in fact this is preferable to an uncontrolled shutdown as would occur if the reactor ‘poisoned out’ on temperature or xenon. The usual procedure is to effect a tempera­ture reduction of a few degrees, then to trip the reactor w’hen the operating staff on the plant are prepared.

A controlled shutdown in which control is main­tained throughout is an advantage if it is has been initiated by a plant deficiency or abnormality which, if sustained, requires a reactor shutdown, but which may be rectified on a timescale that will permit the reactor power to be restored. Such a situation may occur, for example, if the burst can detection (BCD) system became inoperative; in this situation the Op­erating Rules require that the reactor be shut down within a specified period of time (typically half an hour), so the shutdown is started in accordance with the Operating Rules while emergency maintenance is carried out in an attempt to restore the BCD system to service.

AGR

With sufficient care an AGR can be reduced to about 30-40% power, as is current practice for refuelling, and it could then be tripped from that power; how­ever, only in very unusual circumstances would the reactor be shut down in this way.

On an AGR the conclusion on shutdown proce­dure is the same as on a magnox reactor, i. e., the reactor may as well be tripped as soon as the operat­ing staff are prepared. However the reasons for it differ. Temperature poisoning is not a problem, first because the bulk moderator temperature is main­tained largely constant, second because there is more reactivity worth invested in the grey rods and it is unlikely that they will be only lightly inserted into the core. Xenon build-up occurs but, as mentioned for magnox reactors, this is on a longer timescale. The principal reason for preferring a reactor trip is the limited regime in which the boilers must be operated, the difficulties in maintaining operation within the allowable regime and the consequences for the plant of operating outside that regime.