It's complicated. Nuclear fission generates a load of by-products, and some of those are known as fission "poisons", mainly Iodine-135 decays to Xenon-135, whic

It's complicated. Nuclear fission generates a load of by-products, and some of those are known as fission "poisons", mainly Iodine-135 decays to Xenon-135, which very easily absorbs neutrons and inhibits fission. During normal operation, there are plenty of neutrons flying around, so there are enough for some to be absorbed by Xenon-135, and the rest to continue the fission reaction. However, when the power output is reduced, the Xenon-135 can inhibit the nuclear reaction. This can make it less safe to restart the reactor, because the operator would need to increase the reactivity of the system - if they increase it enough to overcome the inhibition caused by the Xenon-135, then the increased nuclear activity can burn off the Xenon-135, and then you have excessive reactivity and the reactor could go out of control. For that reason, when a reactor is shut down, you typically need to wait several hours for the Xenon-135 to decay by itself - this is called the "Xenon dead time". Failure to do this was partially the cause of the Chernobyl disaster.