As mentioned in the last paragraph, temperature co­efficients of reactivity of the moderator in a magnox reactor and the fuel outer sieeve in an AGR become positive as fuel irradiation proceeds. The bulk mod­erator temperature coefficient of reactivity in an AGR is positive from start of life. Let us consider these three positive feedback effects together. As tempera­ture rises, this gives a positive change in reactivity which tends to increase the neutron power which in turn increases temperatures, and so on. The effect in the bulk moderator will be much less pronounced in an AGR than in a magnox reactor because moderator temperature is maintained largely constant by the re­entrant gas flow, but the principle is the same. This is a destabilising effect, causing the power to increase further. These effects have a time constant of 10-20 minutes.

A second destabilising effect is that of Xe-135, As reactor power increases, xenon is destroyed faster than it is created so the concentration decreases. Since xenon is an absorber of neutrons this results in a positive change in reactivity which tends to increase reactor power further. This initially reinforces the ef­fect of the graphite temperature coefficients mentioned in the previous paragraph.