The International Atomic Energy Agency defines small reactors as those with an electricity output of less than 300 MWe. There are more than 45 SMR concepts under development for various applications. Many American SMR models are among the latest technology innovations being supported by the U.S. Department of Energy (DOE). Several of these concepts have advanced to the licensing stages and, according to the IAEA, the projected timelines of readiness for deployment of SMRs generally range from the present to 2025-2030. As of 2014, four SMR reactors were under construction: CAREM-25 in Argentina; the floating KLT-40S and RITM-200 in Russia; and the HTR-PM in China.
Three categories of SMR designs are currently receiving the most attention: light water, high-temperature gas-cooled and fast neutron reactors. Each design has its own benefits and the designs are developing with varying degrees of success.
Light Water Reactors (LWR)
Small LWRs are based on the technology employed by all existing U.S. power reactors and therefore offer the lowest technological risk and are the most compatible with the existing federal regulatory framework. Small LWRs are moderated and cooled by ordinary water, typically use fuel enriched to below 5% U-235, and have refueling cycles of up to 6 years. Small LWRs could be used to replace aging fossil-fired power plants because they are compatible with the infrastructure, cooling water, rail and transmission facilities that already exist at such facilities.
Babcock & Wilcox’s 180-MW Generation mPower SMR
The mPower advanced light water reactor is designed for below-ground containment and incorporates passive safety features from gravity, convection and conduction to cool the reactor in an emergency. The reactor uses fuel enriched to almost 5% with a 4-year refueling cycle. mPower was the first SMR to win funding from the DOE. Despite recent scale backs in development, B&W has not abandoned SMR development and the project is scheduled for completion in 2021.
Westinghouse’s 200-MW reactor
The Westinghouse SMR is a pressurized water reactor based on their AP1000 reactor design. It houses all primary components in a below-ground containment vessel and uses fuel enriched to below 5% with a 2-year refueling cycle. Westinghouse has recently scaled back development of the SMR, but maintains a working business and engineering staff on the project for when market conditions improve.
NuScale Power 45-MW reactor
The NuScale SMR is a smaller version of PWR technology. This reactor offers the passive safety features of natural circulation, convection, conduction and gravity to control the movement of coolant through the core and eliminate the need for coolant pumps. The reactor will shut down and self-cool without power, external water, or human action. The NuScale reactor is installed in a below-ground, water-filled pool and uses standard PWR fuel enriched 4.95% with a 2-year refueling cycle.
Holtec’s 160-MW Inherently Safe Modular Underground Reactor (HI-SMUR)
The HI-SMUR reactor is also installed underground, uses fuel similar to that of larger PWRs and has a 42-month refueling cycle.
High-Temperature Gas-Cooled Reactors (HTR)
High-temperature gas-cooled reactors can be used for electricity generation, but may be best suited for providing process heat for industrial and transport applications and hydrogen production. These reactors use helium, carbon dioxide or nitrogen as a coolant and graphite as a moderator. HTRs use tristructural-isotropic (TRISO) particles with uranium enriched up to 20% U-235 for fuel.
General Atomics Gas Turbine Modular Helium Reactor (GT-MHR)
The GT-MHR is a high-temperature reactor with advanced gas turbine technology. The GT-MHR uses TRISO fuel enriched up to 15.5% with half the core replaced every 18 months. The GT-MHR is being developed in partnership with Russia’s OKBM Afrikantov and Fuji Industries of Japan. The main goal of the project is hydrogen generation.
Pebble Bed Modular Reactor Ltd. (PBMR)
Development of the 165 MWe PBMR in South Africa was shut down in 2010 due to lack of funding. This reactor would have used helium as a coolant and repurposed its waste heat for industrial processes.
Fast Neutron Reactors (FNR)
FNRs are smaller and simpler than light-water reactors, with better fuel efficiency and longer refueling cycles (up to 30 years). FNRs have the potential for diverse applications including electricity, desalinization and heating in remote locations. FNRs can also breed their own fuel and can operate on the recycled waste from LWRs or the material from old nuclear weapons, thus limiting threats of proliferation. FNRs have no moderator, operate at near atmospheric pressure and have passive safety features such as automatic power regulation due to reactivity-feedback (higher core temperature naturally slows the reaction).
GE Hitachi Nuclear Energy Power Reactor Innovative Small Module (PRISM)
The PRISM is installed underground with seismic isolators to mitigate the effects of an earthquake. The PRISM uses a liquid sodium coolant and can be configured to use a variety of fuels such as plutonium stockpiles, used nuclear fuel and depleted uranium. The initial concept for the reactor was developed by GE in 1981 and, according to GE, this reactor has been fully tested by the NRC and is ready to go to market.
Integral Fast Reactor: ARC-100
This 100 MWe reactor is a fast-breeder reactor operating at atmospheric pressure, designed to be installed in a 20-foot-diameter silo underground. It is sodium-cooled, with uranium metal-alloy fuel rods, and operates on a closed 20 year refueling cycle. The ARC-100’s containment vessel uses a double walled, stainless steel tank that can factory-fabricated, transported to a site and operational within 18-24 months. The ARC-100 recycles its own and LWR waste for fuel, and offers passive safety features that make its operation so effectively failsafe as to be described as “walk-away safe.”
General Atomics Energy Multiplier Module (EM2)
The 240 MWe EM2 design is a helium-cooled fast neutron reactor based on the GT-MHR. The EM2 can convert nuclear waste into electricity and industrial process heat and has a 30 year refueling cycle.
Gen4 Energy: The Gen4 Module (G4M)
The G4M is designed to be a safe, clean, sustainable and cost-efficient transportable power source. The reactor is designed to deliver 70 MW of heat (25 MW of electricity) for a 10-year lifetime, without refueling.
Toshiba 4S (Super-Safe, Small and Simple)
This 10 MW reactor uses a liquid sodium coolant and is designed for use in remote areas.
TerraPower is a Bill Gates-funded start-up that is developing a larger, 500-megawatt, “traveling wave” reactor. Company CEO John Gilleland says it’s on track to deploy its first reactor in the 2020s.
Obstacles to SMR Commercialization
While SMR technology is diverse and full of potential, there are many obstacles in its path to commercialization. As the potential financial benefits of SMRs come from economies of scales, so do the current obstacles to commercialization. Without government subsidies, it is not economical to produce just one SMR. According to Westinghouse (which in January 2014 suspended all work on SMRs due to inadequate demand), in order for SMRs to be cost-competitive, they must be built in quantities of 30-50 reactors.
Another problem in the debate over the future of SMRs is a lack of consensus over the future costs of SMR deployment. Capital cost estimates remain very preliminary, making it difficult to perform a comparative assessment of cost competitiveness. In addition, public opinion, the lack of a carbon tax in the United States and low natural gas prices offer little encouragement to the nuclear power movement.