For the purpose of describing the principles of the kinetic behaviour of reactor parameters in this sec­tion, a blower/circulator failure has been used as an example. Let us now consider controlled gas flow changes, for example, a reduction in gas flow to effect a reduction in reactor power for whatever rea­son. In most cases this can be carried out at a rate determined by the reactor control engineer, so many of the potential problems outlined in the preceding paragraphs should not occur. If a large reduction in power is intended, it will probably be necessary to estimate the magnitude of the increase in xenon worth which will result from the power reduction, to es­tablish whether there is sufficient positive reactivity available from pulling the control rods in order to maintain criticality. The reactivity available is often referred to as ‘xenon override capability*. Such an estimate is necessary on AGRs as well as magnox reactors, particularly if the reactor has a ‘backlog’ of refuelling so that the regulating rods are less deep­ly inserted into the core than normal. In this cal­culation, account can be taken of the duration of the load reduction, since if the operators are confi­dent that load will be restored before the xenon peak is reached, a larger load reduction can be tolerated because the load restoration will curtail the rise in xenon concentration.

Change in reactor gas inlet temperature with gas flow к has been assumed so far that reactor gas inlet tem­perature remains constant. In the case of a blower/ circulator failure, requiring the associated gas circuit to be taken quickly out of service, the reactor gas inlet temperature returned by the healthy gas cir­cuits will remain normal, while any deviation in the gas temperature from the failed blower/circulator will become irrelevant as the gas circuit is taken out of service. In considering gas flow changes on healthy gas circuits, for example, in a planned load reduction, on AGRs and at Oldbury and Wylfa (magnox reactors with once-through boilers), reactor gas inlet tempera­ture is held constant by auto control loops. On mag­nox reactors with drum boilers, however, no such auto control loops exist, and the thermodynamics of the boiler give a reduction in reactor gas inlet tempera­ture as gas flow is reduced. In addition there is a reduction in the temperature rise across the blower as blower power is reduced. The combined effect amounts to about 20°C for a 50^0 reduction in gas flow. Reactor gas inlet temperature can be controlled manually, see Section 5.5.4 of this chapter. A reduc­tion in reactor gas inlet temperature causes a reduc­tion in moderator temperature which is a disadvantage from the point of view of reactivity, so it is desirable for reactor gas inlet temperature to be held constant. This is also desirable from the reactor control engi­neer’s point of view because the fewer the number of variables which are changing the easier it is to control the reactor.