Set some 50 years in the future, every spaceship in space is powered by a nuclear powered reactor and most of them came equipped with an ion propulsion engine for acceleration. I noticed that every spaceship's captain and engineers are drilled to wait for at least 60 hours before restarting the nuclear reaction again, especially after their ship had experienced a power phase out; it is clearly stated in the protocol and there will be a hefty fine for those who blatantly ignore it. I heard from some of those who has retired from the service mentioned that it could be to prevent nuclear poisoning, but come on, we are already in the space age and we still need to wait for the spaceship's engine to warm up? Why is that? any solution to expedite this?
Your coolant takes time to melt
Back in the days of the Soviet Union, there was a class of high-performance submarine called the Alfa class.
This submarine used a lead-cooled reactor. By doing so, it managed to compact the size of the reactor significantly, create enormous thrust allowing the submarine to even outrun torpedoes.
This caused the U.S. lots of anxiety, until it was realised the downside of such reactors was that the lead must be kept constantly in a liquid (molten) form and if the reactor stopped running, it would solidify and create a huge chunk of lead in the reactor - permanently disabling the submarine.
As a result, maintenance of the submarine was a major problem - for simplicity the reactor was kept on an 'always-on' mode, however for large periods of downtime there were significant problems with heating and pumping this type of coolant at coastal facilities.
Your spaceships could have similar design characteristics to the Alfa class, requiring any of the following:
- The need for high-performance reactor
- The need to keep reactor space minimal (easy to imagine in a cramped spaceship)
- The inability to keep a reactor 'always-on' and requiring coolant pumping and heating
Such constraints are easily conceivable in a spaceship with design trade-offs required to offset issues of power, size and weight.
A fission reactor works by maintaining a delicate and precise balance between creating and destroying free neutrons. When U235 splits, it releases neutrons which can hit other U235 nuclei and cause them to split.
If the growth rate of that process is greater than one, you have an explosion. If less than one, the process fizzles. If exactly equal to one, the reaction is sustained.
But, the reaction environment does not stay constant.
The U235 splits into other elements with varying half-lives and nuclear behavior. Xenon-135 is short-lived isotope produced from the decay of another fission product. Xenon-135 is an especially good absorber of neutrons, which makes it hard to get a sustained reaction. Doing so requires removing much of the deliberate neutron absorbing control rods, and makes the reaction, if it gets started, difficult to control. Here is a video explaining the problem, and the Wikipedia page: Xenon Poisioning.
It is a real problem, and could result in a loss-of-ship event.
A little more data inspired by comments
Xenon-135 is not formed immediately when Uranium is split. It is formed when Iodine-235 decays into Xenon-135. The half-life of Iodine-135 is short (Wikipedia reference), but because of that half-life the level of Xenon-135 takes a few hours to stabilize at new level. This creates a complex damping effect as a reactor's power (and thus the rate of fuel fission) increases, and instability as power is decreased.
Xenon-135 probably played a role in the Chernobyl melt-down and fire. The output of the reactor had been substantially reduced for a few hours, and then was ordered to be increased. The Xenon-135 load was higher that steady state for the reduced power level. To increase the output, the influence of the control rods needed to be reduced. It is likely that a localized reaction then grew out of control and ignited the graphite moderator.
This problem exists in any fission reactor that produces Iodine-135, and is not limited to a U-235 reactor. Thorium and Plutonium reactors must consider the levels of Xenon-135 in the control practices.
Because the reactor is sealed and inaccessible to crew whether operating or not, the maintenance cycle for a restart requires the use of dozens of tiny robotic drones to crawl through the pipeways and ductwork looking for and repairing minor cracks and faults. Although the chance that the damage to a reactor from a single phasedown would be so bad as to cause disastrous consequences without that maintenance pass is small, it IS nonzero, and it's definitely true that as maintenance cycles are skipped, the chance of a catastrophe skyrockets.
Thus, all prudent shipmasters allow the drones the full 60 hours that it typically takes to run a comprehensive cycle.