Small Modular Reactors (SMRs) have power levels of less than 300 MW, a fraction of the typical power level of reactors that have been constructed in the last two decades. These have been receiving increasing attention in recent years and many consider SMRs the most serious candidate technology in the nuclear area in the aftermath of the Fukushima accidents.
Several countries are developing SMRs, including the United States, Russia, China, France, Japan, South Korea, India, and Argentina. A wide variety of SMR designs with distinct characteristics are under development. These could be categorized on the basis of the coolant they use into light water reactors, heavy water reactors, high temperature gas cooled reactors, and fast reactors cooled with molten metals. SMRs could also be categorized on the basis of how they might be deployed (for example, on barges or under water, remotely or near urban centers), and on their purposes (for example, power generation, heat production, or desalination). We focus here on reactors used for power generation.
Many SMRs use water as coolant and moderator. The relatively large number of SMR designs based on LWR technology is primarily because of the predominance of this technology in operating power and submarine reactors. Many of these LWR based SMRs have an integral primary system. In such a reactor, the reactor core, the steam generator(s), and the pressurizer are all inside one common pressure vessel. This is unlike currently common large LWRs where the steam generator(s) and pressurizer are outside the pressure vessel. Some of the SMR based on LWR technology are described here.
Some proposed SMRs exploit the fast neutron spectrum and do not use a moderator. These reactors typically use liquid metals as coolant, though there is at least one SMR design that uses helium as coolant. Because of the high boiling points of liquid metals, these reactors typically operate at or near atmospheric pressure. Some of the SMRs based on fast neutrons are described here.
Some proposed SMRs are being designed to operate at high temperatures, close to 1000°C. These reactors typically use helium as coolant and graphite as moderator. Some of the gas cooled SMRs under development are described here.