Magnox and AGR cores are of fairly large dimensions, so it is possible for situations to develop in which power may be increasing in one part of the core and decreasing in another. Power oscillations with a periodicity of several hours may also occur. There are several modes of this type of instability, characterised according to the pattern of rising and falling power. To examine the processes involved in a power oscil­lation let us consider the simplest mode, called the ‘fundamental mode’, in which the power change is uniform across the core, і e., total reactor power rises and falls in an oscillatory manner. It must be emphasised that the reactor behaviour described in this section is of theoretical interest only, since the oscillation is not normally observed because the auto control system or the reactor control engineer will maintain stable conditions. However, in order that the auto control system designer and the reactor control engineer can assure stability, an awareness of the phe­nomenon is required.

Let us suppose that the reactor is operating at steady power with uniform conditions across the core when a uniform disturbance, for example, the raising of a group of bulk rods, causes the reactor power and temperatures to increase. Assuming that the auto control system on reactor gas outlet temperature is inoperative and that the reactor control engineer takes no action to arrest the rise in power and tempera­tures. The progress of the transient from this point onwards depends on a number of factors, some will have a stabilising influence and some will be desta­bilising; the timescales on which they operate are also significant.