Small Modular Reactor
What is a small modular reactor?
Small Modular Reactors (SMRs) are nuclear power plants that are smaller in size (300 MWe or less) than most of the current generation base load plants (1,000 MWe or higher). These smaller, compact designs have the potential to be factory-fabricated and can be transported by truck or rail to a nuclear power site, giving them short construction periods. 
An SMR has many potential technical and economic benefits  –
Safety – An SMR would have a smaller core, and thus a smaller source term, to account for in an accident scenario. This could reduce operations costs from the perspective of both emergency planning and required maintenance and inspection of active safety-related systems. However, the benefit of smaller source terms must be compared to the implications of deploying a larger number of reactors. The risk and economic effects of increasing the number of potential sources, but decreasing the potential consequences for each source, are not yet quantified. Also, many SMRs are designed to run for extended periods without refueling and are factory sealed in small units, so this reduces the material mishandling probability.
Another beneficial safety aspect to SMRs is their use of passive safety features. This means that the safety systems do not rely on external power to protect the reactor from damage, but will naturally occur during an adverse event. Although this is not a benefit exclusive to small reactors, the smaller design makes these systems even more reliable and much simpler to implement.
Better ability to match existing electric grid infrastructure – Since the SMR rated power range is on the order of 10s to 100s of megawatts electric (MWe), as opposed to 1000+ MWe, they can be constructed in area where the existing grid could not support a large power station without requiring upgrades. This increases the opportunities for SMR deployment, making them competitors in small, grid-constrained markets.
Modular – A huge benefit is the ability to be built in stages to achieve a total power output or respond to market conditions. As an aside, the economics for reactors in the 10s of MWe would likely be different from reactors in the 100s of MWe. The smaller ones are more readily suited to dedicated-purpose applications, while the larger ones are more closely related to the current power plants deployed. However, in order to justify the capital expense of the factories to produce the components—up to and including the reactor—there must be a sufficient planned and guaranteed order book since the cost of the factory is amortized over the number of components produced.
Low cost – SMRs are cheaper and quicker to deploy relative to larger central-station reactors. While SMRs may have a larger specific cost (given in $/kWe) to build, the total amount of capital required to build an SMR unit will be less simply because of the smaller size of the plant.
What are the different types of SMRs deployed or being planned worldwide?
Any reactor type can be toned down to build a small modular reactor. Most of the research around the world has focused on light water reactors, heavy water reactors, fast neutron reactors, graphite-moderated high temperature reactors and various kinds of molten salt reactors (MSRs). Few examples are –
Light Water Reactor – CNP-300 (China & Pakistan), SMART 100 (Korea), NuScale 50 (USA), CAREM 27 (Argentina; Saudi Arabia is interested in using this design for desalination)
Heavy Water Reactor – PHWR-220 (India), EC6 (Canada).
Liquid Metal Cooled Reactor – PRISM 311 (USA)
High Temperature Reactor – PBMR-165 (South Africa), SC-HTGR 250 (France), HTR-PM,HTR-200 (China)
Molten Salt Reactor – Terrestrial IMSR (Canada), Moltex SSR -60 (UK)