This whole process is regulated via neutrons(or maybe protons, not 100% on that). Put the rods of plutonium closer together, get more fission, more heat. I think they use graphene to regulate things too.

The gist of it is that the fuel rods are designed in such a way as to maintain about the right level of neutron emission, with further refinement using a neutron moderator* (which slows down neutrons and this increases the reaction rate, this is generally water or graphene) along with adjustable control rods, which can be inserted to slow down the reaction.

There are two kinds of decay leading to neutron emission: prompt neutrons are emitted immediately, while delayed neutrons take time to be emitted because of the decay path that the excited atom has to take. In order to maintain control of a reactor, the number of prompt neutrons must be lower than the level needed to reach criticality, with the additional delayed neutrons being enough to push it over the edge. The delay is what gives you time to control the reaction; if the reactor becomes prompt critical, then it begins to melt down.

*Not all reactor designs use neutron moderators, but fast reactor designs are generally military or research reactors, while moderated ('thermal') reactors are typical for civilian power generation. Here in Canada, we use the CANDU reactor design, which is moderated using heavy water