The initial effect of the xenon change is to reinforce the power change, but on a timescale of hours the effect of the power increase is to increase the xenon production rate which contributes a negative change in reactivity (see Section 2 of this chapter). At first sight this may appear to be a stabilising effect, reinforcing the effects of neutron leakage and the negative tem­perature coefficients of reactivity. However, due to the delay between the change of power and the consequent change in xenon production rate, and provided this effect is of a magnitude which can dominate over the destabilising effect of the positive temperature coeffi­cients of reactivity, the increase in power is terminated and the power is driven downwards.

The destabilising effects of the positive temperature coefficients of reactivity now reinforce the effect of the change in xenon production rate and the power continues downwards until the decrease in power shows an effect in decreasing xenon production rate and the trend is reversed a second time. Thus an oscillation is set up. An oscillation has been observed at Berkeley in a test carried out during the early months of op­eration, at a core irradiation of approximately 900 MWd/t. The oscillation is shown in Fig 3.28, mani­fested as a change in fuel channel gas outlet tempera­ture. This was an oscillation in one of the higher modes, as described in the next Section 5.6.4.