In the wake of three mile island, Chernobyl and Fukushima disaster, the future of nuclear energy looks bleak as many companies and even nations are declaring themselves nuclear free. However, as concerns over rising energy cost and carbon dioxide production rises, nuclear energy is poised for a comeback. This year is a historic one for nuclear energy in the US as a new nuclear reactor is approved for construction by US regulators. The last one approved was in 1978. US Energy Secretary Steven Chu has positive views about nuclear energy and he believes that small scale nuclear energy can provide answers to the energy challenge. [1,2] International Atomic Energy Agency defines small scale nuclear reactors as those with an electricity output of less than 300 MWe.  In comparison, traditional nuclear reactors have electrical output in the range from 500MWe to 1500MWe. Practical and safety considerations limit the use of for small passenger vehicles and therefore the most feasible option is to use them for small cities or large vessels. Small scale nuclear reactors are not new; in fact they have been around for more than 60 years. Hundreds of them can be found in the hull of nuclear submarines, warships merchant ships, icebreakers and as research and medical isotope reactors at universities. They are now being considered for domestic use to power towns or even buildings. Although economies of scale might suggest that large scale reactor produce cheaper power compared to small scale, studies have been done that prove that it is not always the case. [4,5] Cost of energy production can be reduced by integral and modular design strategies of small modular reactors (SMR).
There are different designs of SMR. The most common reactor is based on the current light water reactor. Other designs include fast neutron reactors, high temperature gas cooled reactors, molten salt reactors and aqueous homogenous reactors.
One example of a light water reactor is Nuscale power modular reactor. Each module generates 45 MWe and it can be combined with multiple modules. It is a pressurized water reactor type which uses uranium of less than 5% U235 enrichment as the fuel with 24 month fuel refilling cycle. The 450 tonnes vessel containing the reactor and steam generator is installed in a water filled pool below ground level. The only moving part of the module is the control rod drives as the design takes advantage of convection for cooling. The company estimates in 2010 that overnight capital cost for a 12-module, 540 MWe NuScale plant is about $4000 per kilowatt. In March 2012 the US DOE signed an agreement with NuScale regarding constructing a demonstration unit at its Savannah River site in South Carolina. 
Another company that plans to deploy a reactor at the DOE Savannah River Site is Hyperion Power Generation. The company's fast neutron reactor module is a 25 MWe lead-bismuth cooled reactor concept using 20% enriched uranium nitride fuel. It is sealed, portable, has no moving parts and is designed to operate continuously for up top 10 years without refuelling. A secondary cooling circuit transfers heat to an external steam generator. The reactor is placed in an underground containment vaults for protection against intrusions or natural disasters.
The advantages of SMR are that they are highly flexible to different applications. Since they are designed to be modular, they can be scaled up according to the power requirement. Modular reactors are perfect power generation at remote, isolated, severe climate locations that lack transportation infrastructure. Mining operations and remote communities can benefit from the local and reliable power generated. If more power is needed, several modules can be used to increase the power output. Unlike conventional power plants, SMR can be assembled fully at the factory and shipped to the location whereas for traditional power plants the facility has to be built and parts have to be assembled at the location. The module also can be refuelled onsite or shipped back to the manufacturer. SMRs also makes more efficient use of space as the reactor itself is buried underground. Cooling stacks, containment buildings are not needed.
Due to its smaller size, the initial cost is much lower than a conventional nuclear power plant. A conventional reactor has a cost of $5-$15 billion before any power can be produced while SMR cost $25 - $200 million per unit. Maintenance costs are also reduced for SMR. Since SMRs are passively cooled by convection and has no moving parts, it is safer, cheaper and more reliable to maintain and operate. Pumps, emergency sumps, back up generators, heat containment systems can be omitted. Since SMRs do not use external water sources, extreme weather and bio fouling is not a problem. An indirect advantage of SMRs is that it can stimulate US job economy. The module can be built entirely in the US whereas for current reactors, the parts have to be shipped from other countries. 
Since SMRs design allow them to be used nearer to highly populated areas, widespread adoption may increase the risk of accidental exposure to the population. Although the nuclear fuel is sealed and buried underground, terrorists may find ways to retrieve them. SMRs also suffer from decreased thermodynamic efficiency and neutron economy compared to large scale reactors due to size. SMRs are making a small trade-off on fuel costs therefore it is important to balance slight losses with major gains in lower capital cost. The biggest disadvantage of SMRs is however the licensing process and public perception. 
A properly designed SMR is crucial if its widespread use is intended. SMRs has lower capital costs and is portable and scalable. The widespread use of SMRs is hampered by lengthy licensing process and public acceptance. The declining cost of other renewable energies such as solar, wind and energy storage also threatens the feasibility of SMRs.
© Eric Adijanto. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 J. McMahon, "Chu Touts Small Module Reactors as Answer to Nuclear Hazards," Forbes, 23 Mar 11.
 K. Hennessey and D. Lee, "U.S. Stands by Nuclear Power, Energy Secretary Says," Los Angeles Times, 16 Mar 11.
 "Status of Small Reactor Designs Without On-Site Fueling," International Atomic Energy Agency, IAEA-TECDOC-1536, January 2007.
 ""Mini Nuclear Reactors: Thinking Small," The Economist, 9 Dec 10.
 M. D. Carelli et al., "Economic Features of Integral, Modular, Small-to-Medium Size Reactors,'' Prog. Nucl. Energy 52, 403 (2010).
 J. Collins, "Savannah River Site Debuts New Green Power Plant," Bloomberg Businessweek, 12 Mar 12.
 P. Hise. "Mini Reactors Show Promise for Clean Nuclear Power's Future," Popular Mechanics, 18 Dec 10.
 ""Interim Report of the American Nuclear Society President's Special Committee on Small and Medium Sized Reactor (SMR) Licensing Issues," American Nuclear Society, July 2010.