When radioactive atoms decay or are hit by neutrons, they produce lighter elements and release energy in a process known as fission. Fission can produce large amounts of energy over a long period, which makes it an ideal heat source for nuclear thermal rockets, where radioactive fuel is contained within a reactor core and cooled by propellant.
The heat produced by fission is also ideal in power generation, which can be done through the use of a turbine or magnetoplasmadynamic generator.
Fuel
Specific isotopes of elements such as uranium (in the case of the Yama-SSR series) and plutonium are the chosen fuel for nuclear reactors because they are fissile. While many radioactive isotopes are fissionable, meaning that they undergo fission when hit by neutrons of any energy level, fissile materials are especially responsive to low-energy "thermal" neutrons. Thermal neutrons are the result of high-energy neutrons scattering and losing energy in a moderator, which absorbs and dissipates their energy as heat.
Thorium may also be used (as is the case for the SO6 fuel rod series), but the most common isotope of thorium is not fissile; it is, however, fertile. When exposed to neutrons, fertile materials absorb them to create fissile materials, which can then undergo fission to produce energy.
Radioactive fuels last far longer than chemical fuels due to their incredible energy density. Fuel rods can operate continuously for months or even years before needing to be replaced, depending on how they're used.
Controlling the Reaction
Fission can be controlled by adjusting the parameters of the reactor. Different approaches include:
- Control rods that absorb neutrons can be inserted between the fuel rods, reducing the amount of neutrons available to trigger fission.
- The fuel rods can be removed from the reactor, reducing the amount of fissile material for neutrons to hit.
- In liquid core reactors, the liquid fuel can be pumped away from the moderator, reducing the amount of thermal neutrons that can trigger fission.