The first is Light-water reactors that include pressurized water, boiling water and supercritical water reactors. This is opposed to heavy water reactors. The vast number of operating reactors today are light-water reactors. Fuel is suspended in water with a moderator (control rods) that are raised to start the reactor and lowered to stop the reaction. Water is used to carry the heat to a secondary loop to turn a steam turbine.
Fast neutron reactors (FNR) are a newer design, that both make efficient use of fuel, but also consume the long-lived isotopes that can be made in the reactor. While they are a minority of commercial reactors, they do have a track record of operations. The fuel can be of a lower fissionable grade than light water reactors. The type of reactor can be designed as fast breeder reactors (FBR) the design can also set up to consume bomb grade material and create medical isotopes and other short-lived products.
Graphite-moderated high temperature reactors (GHTR) use a gas like nitrogen or helium instead of water to moderate the reactor and collect the heat to make electricity. One of the features of these reactors is that the reactors need to be pressurized to start the reaction. The best-known reactor type of this class is a pebble bed reactor, which uses small bits of fuel wrapped in graphite to form tennis ball sized fuel balls. (This is one I got to work on in South Africa). The reactors are also known as High Temperature gas-cooled reactors (HTGR) with operating temperatures over 750 degrees C.
Molten Salt Reactors (MSR) use a molten salt core with the fuel melted into the salts. They operate at close to atmospheric pressure but at a high temperature. The fuel can be drained in a cooling chamber and solidified to prevent a meltdown, no pumps required, just open the drain. The MSR can be refueled in operation by draining a portion of the fuel out of the core and adding more fuel to the core, no shutdown required.
There are many variants of these 4 major types and other single designs that differ from these 4, but these are the 4 classes of reactors that make up the majority of SMR designs.
SMR I - Announcement of Small Modular Reactor Series
NEXT: SMR III-Â Why is 300 megawatts the dividing line for SMR?