system
Given the following post, respond as if you're commenting on Reddit:
human
What is the process of "turning on" a nuclear reactor?
gpt
Starting up a reactor from a shutdown state is a very precarious and difficult procedure for something that seems rather simple.

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If the fuel rods aren't new, starting up a reactor involves raising the control rods and turning on the coolant pumps, albeit in a very controlled manner. Reactor control rods act as neutron absorbers, absorbing neutrons emitting from the decaying fissile material, and since neutron collisions are what catalyze the fission of atoms, the control rods effectively slow down or stop nuclear fission. By releasing the control rods, the plant operators decrease the number of neutrons that are absorbed and increase the total number and energies of the propagating neutrons, more formally referred to as neutron flux. As neutron flux increases, more atoms of the fissile material go through fission, increasing temperature, neutron flux, and pushing the reactor towards criticality, or the point where a sustained fission reaction takes place (every fission event causes one or more fission events to occur). The reactor pumps are also turned on to increase decay heat in the reactor vessel, bringing it to operational temperature.

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As the control rods are released and the reactor approaches criticality, then comes the most dangerous part of reactor startup. The exact position of the control rods at which criticality occurs must be carefully calculated and approached. At this point, the rods must be raised very, very slowly, otherwise a sudden increase in temperature or prompt neutron flux may occur, resulting in prompt criticality. The moderator-temperature coefficient must also be considered, which shows how the increasing temperature, and therefore decreasing density, of the neutron moderator causes decrease in the production of thermal neutrons, slowing down the reaction. Once criticality (or, more accurately, delayed criticality, a reaction sustained by delayed neutrons) is achieved, the reactor finally achieves a fission chain reaction, and is in a state of "minimum load." From here, especially if it is a graphite-moderated reactor, the reactor must quickly increase its temperature not only to ensure criticality is sustained, but also to eliminate energy built up in the moderator due to the Wigner effect.

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If the fuel rods are fresh, an extra step is added. In the first part of the process, when the control rods are raised and the pumps started to cycle heat, a startup neutron source must be added. These are radioactive elements that provide a sufficient source of neutrons to achieve minimal neutron flux for the fuel rods to begin the reaction. Such elements may include californium-252, or Plutonium-238. These startup neutron sources are necessary because fresh fuel rods do not have a decay rate high enough to produce a sufficient neutron flux, as they have not yet begun creating more shorter-liver, energetic fission products.