The smallest amount of fissile material in which a fission chain reaction can take place is called the critical amount. Most of the neutrons produced in it during fission continue to participate in fission. This allows the reaction to run continuously. The critical amount depends on the type of fissile material, its concentration and the shape in which it is arranged. In the case of pure uranium-235, the critical mass would be ball with a diameter of about 18 cm and a weight of about 50 kilograms.

But the uranium sphere is not a reactor. We expect the reactor to deliver a certain amount of power stably for a reasonably long time, perhaps at least a year without changing the fuel.

If we started the fission reaction in a critical amount of material, it would not last long. After the first few fissions, the amount of material becomes subcritical and the reaction gradually stops. We therefore need more material. But so that the reaction does not start too quickly, we have to control it. This will be taken care of by the absorption rods, which will absorb the excess neutrons. Furthermore, we need any medium that will remove the generated heat from the reactor so that we can use it for our needs. If our reaction depends on slow, thermal neutrons, we will have to intersperse the fissile material with their retarder called a moderator. This again increases the size and weight of the reactor. So we won’t build a nuclear power plant the size of a matchbox, but we will build a reactor of a size of a dishwasher. Current trends are, among other things, towards micro-reactors — small modular reactors that can be transported entirely on a truck and whose output ranges from 1 to 20 MW.

The smallest operating reactor built so far was the SNAP-10A with an output of 500 W designed for operation in space. The reactor itself was a cylinder 40 cm long and 23 cm wide, containing 37 fuel rods. Zirconium hydride served as the moderator and potassium sodium as the coolant. The generated heat was directly converted into electricity by a thermoelectric converter. The reactor was launched in 1965 into polar orbit and operated successfully for a month and a half until its operation was terminated by a malfunction in the non-nuclear part of the rocket. Other small nuclear reactors include the Russian EGP-6 reactor with an output of 12 MWe, serving in the Bilibino nuclear power plant in Chukotka, or the floating nuclear power plant Akademik Lomonosov equipped with two KLT-40S reactors with an output of 35 MWe.