Nuclear Reactor Design Variations

Fissile Fuel

There are in practice three fissile isotopes available for nuclear reactions, whether civilian or military. These are 233U,235U, and 239Pu

U is uranium. Pu is plutonium, and the leading superscript is the atomic mass of the isotope. Only 235U occurs in usable quantity in nature, but it is absurd to regard the other two as "unnatural", unless you're willing to regard steel as unnatural too, or even dogs.

Neutron capture by thorium 232 produces 233U after two radioactive "decays", and the common isotope 238 of uranium likewise transmutes into 239Pu.

Fast and Thermal Neutrons

For a chain reaction that is sustained and steady, one neutron from every fission event is needed to cause another. For a chain that breeds new fuel to replace what each fission uses up, we need a second neutron to turn a "fertile" nucleus into a fissile one. Neutrons are captured by the "strong" nuclear force which normally binds the neutrons and protons of a nucleus together, in spite of the electrostatic mutual repulsion of the positively charged protons. This is easier if the neutrons are moving at about the speed that corresponds to the temperature of the atoms of the reactor. The energy given by the force of attraction is usually enough to disrupt the temporary nucleus so created. But some of the captured neutrons merely create 236U from the 235U, without splitting it. Fast neutrons need a higher density of fissile atoms, but are energetic enough to split all or nearly all of the nuclei that they strike.

Moderators

For low enrichment uranium fuel, a moderator is what is required to reduce the speeds of neutrons to thermal energy quickly. The classical mechanics of elastic collisions apply even to nuclear particles. If an object hits another of exactly the same mass, head on, the second mass receives all of its kinetic energy. A larger mass will receive less, and the smaller will bounce away. For less direct impacts, a similar rule applies. So a "thermal" reactor, designed to work with low values of fuel enrichment, requires a moderator the atoms of which do not capture too many neutrons, and are fairly comparable with their mass. The commonest nucleus of ordinary hydrogen is a proton, the same mass as a neutron. Water is therefore a very good moderator. It has the disadvantage that a proton can capture a neutron and become a deuteron, the nucleus of deuterium, 2H. Deuterium oxide is called "heavy water" and is so good a moderator that it can be used with natural uranium. Very pure graphite, with atoms of mass 12, is still quite a good moderator. It was used in the earliest reactors. But whereas water, when it gets hotter, expands enough to diminish its moderating power and provide negative feedback to the reaction, graphite does the opposite, which was the worst feature of the doomed Chernobyl reactor.

Coolants

Liquid water obviously is an excellent coolant, but it boils at 100 Celsius at atmospheric pressure. The most popular reactor presently deployed is a PWR, a Pressurised Water Reactor. The water stays liquid at quite high temperatures. It has the disadvantage that the steel piping, and its welding, must be of very high quality to prevent leaks. When the reactor is shut down, by its neutron-absorbing control rods, the fission power disappears, but the coolant pumps need to be run until the radioactive energy of the fission products, which drops exponentially, is low enough. This requires auxiliary electricity, provided either by the grid or by emergency diesel.

A fast reactor requires the absence of a moderator, so water won't do. The answer is molten sodium, or in some cases lead. Sodium melts at the (atmospheric) boiling point of water, and does not boil at any temperature that a reactor is allowed to reach. It is of course sealed under an atmosphere of the inert gas argon. At atmospheric pressure there is no excess in the coolant pipes, and indeed a pool-type reactor design is appropriate. In such a reactor, there is no need for auxiliary pumping, because convection in the bulk liquid sodium is entirely enough to deal with the relatively small heat of the fission product radioactivity.

Finally, in the case of reactors where the fuel is dissolved in a liquid, that liquid is the coolant.

Ceramic, Metallic, and Liquid Fuels

Thorium, uranium, and plutonium are somewhat pyrophoric. They oxidise spontaneously on the surface. So an easy way to avoid this is simply to use the oxide itself as fuel. Alternatively, the nitride or carbide. These fuels are ceramic, not good conductors of heat. In a reactor, ceramic fuel pellets get significantly hotter on the inside than at the surface, in contact with the coolant. This means that when shutdown is wanted, the reactor fuel stays hotter longer than metal would.

In a fast reactor, the most successful design uses a metallic alloy that is resistant to oxidation, and besides, it is immersed in a liquid metal coolant. The conduction of heat, from the fission event's energy to the coolant that takes the energy to the generator's heat supply, is swifter in metal than in ceramic.
In terms of stability and safety, the huge advantage of metal (or liquid) fuel over ceramic is greater thermal expansion. When the reactor core gets hotter, the nuclei are farther apart, which is exactly the effect we need to slow down the reaction. The apparent target for the neutrons is smaller per unit area. If the load increases on the secondary circuit, it will cool the reactor circuit, and the desired fission activity will automatically respond.
The other advantage is that we don't want the moderating effect of carbon, nitrogen, or oxygen on the neutrons.

There is also the option, invented for the thorium to uranium breeder design, of an actual liquid fuel, a solution of thorium and uranium fluorides in lithium and beryllium fluoride. Liquid fuel is obviously immune to meltdown, and the temperature necessary to boil the liquid is far higher than fission products could generate. There is also the dump plug, a fusible block that melts if the temperature exceeds a given limit, and dumps the entire fuel load into a chamber filled with neutron-absorbing rods of boron or cadmium.