In a reactor with a negative fuel (fast) temperature coefficient any rise in fuel temperature is accom­panied by a drop in reactivity. This tends to reduce the rate of rise of temperature thus stabilising the reactor. There will be a consequent rise in moderator temperature but the time scale for such changes in the graphite is long and the de-stabilising effect of the positive moderator is easily dealt with.

2.3.1 Positive fuel temperature coefficient

If a reactor were to have a positive fuel feedback co­efficient it would be highly unstable to temperature perturbations. It would be possible to control such a reactor only with a highly sophisticated, fast-acting, auto-control system. It is important, therefore, to main­tain the fuel coefficient at a suitably large, negative, value.