The Advent of the Small Modular Nuclear Reactor
Seventy years ago, a team of 49 scientists, led by Italian-born Enrico Fermi, succeeded in
creating the world's first self-sustaining nuclear chain reaction with the Chicago Pile 1 reactor
(Fermi, 1961). At home in Idaho, on December 20, 1951 the Experimental Breeder Reactor I
(EBR-I) became the world's first electricity-generating nuclear power plant when it produced
sufficient electricity to illuminate four 200-watt light bulbs. A re-creation of this historic event is
shown in Figure 1. In the summer of 2006, I toured the EBR-I research reactor with my
grandparents (see Figure 2). That visit instilled in me a fascination for all things nuclear and a
hunger to learn more.
Figure 1. Re-creation of the original four
light bulbs lit by nuclear power at EBR-I.
Figure 2. Photo of EBR-I (me at age 8 with
my grandparents).
These landmark achievements ushered in an era that spawned the peaceful uses of nuclear
energy. Today, there are 437 operating nuclear power reactors in the world, which are located in
over 31 countries and supply about 13.5% of the world’s electricity (World Nuclear News,
2012). Improvements and advances in reactor design over the last several decades have produced
new nuclear reactor designs – both large and small. These advanced designs offer even higher
levels of safety than legacy reactors. This is partly because the advanced reactor designs are
simpler than the current generation of operating plants. They rely less on engineered safety
features (“active” safety systems like pumps and motors), and rely instead on natural safety
features (“passive” safety systems like gravity feed of cooling water in the event of loss of
electrical power and natural convection to carry away heat).
The U.S. Department of Energy (DOE) is pursuing research, development and deployment
(RD&D) activities for advanced reactor concepts pertaining to both thermal and fast spectrum
nuclear reactors. These RD&D activities will lead to improved safety, technical, economical, and
environmental advancements of innovative nuclear energy technologies. DOE’s Office of
Nuclear Energy (NE) is pursuing these advancements through partnerships with national
laboratories, universities, nuclear industry and international partners with a focus on advancing
scientific understanding of these technologies, establishing an international network of user
facilities for nuclear RD&D, improving economic competitiveness, and reducing the technical
and regulatory uncertainties for deploying new nuclear reactor technologies.
Nuclear reactors are not limited to producing electricity. Other applications include:
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Desalination plants to produce large quantities of fresh water from seawater
Production of hydrogen for use in fuel-cell vehicles
Production of synthetic fuels and other chemicals for the petrochemical industry
In a combined heat and power mode, production of process heat and steam
Extraction of oil from oil sands in the Alberta region of Canada
Several concepts for new fission reactor technologies have been investigated. These systems
include the gas-cooled fast reactor (GFR), the lead-cooled fast reactor (LFR), molten salt reactor
(MSR), sodium-cooled fast reactor (SFR), supercritical water-cooled (SCWR), and very high
temperature reactor (VHTR) (DOE Generation IV Roadmap, 2002), briefly described as follows:
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The GFR system features a fast neutron spectrum, helium-cooled reactor with a closed
fuel cycle. Like thermal spectrum helium-cooled reactors, the high outlet temperature of
the helium coolant makes it possible to deliver electricity, hydrogen, or process heat with
high efficiency. Through the combination of a fast spectrum and full recycle of actinides,
the GFR minimizes the production of long-lived radioactive waste.
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The LFR system features a fast spectrum lead or lead/bismuth eutectic liquid metalcooled reactor and a closed fuel cycle. The LFR is cooled by natural convection with a
reactor outlet coolant temperature of 550°C, possibly as high as 800°C with advanced
materials. The higher temperature enables the production of hydrogen by thermochemical
processes.
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The MSR system produces fission power in a circulating molten salt fuel mixture with an
epithermal spectrum reactor and a closed fuel cycle. In the MSR system, the fuel consists
of a circulating liquid mixture of sodium, zirconium and uranium fluorides. Molten
fluoride salts have excellent heat transfer characteristics and a very low vapor pressure,
which reduce stresses on the vessel and piping.
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The SFR system features a fast spectrum, sodium-cooled reactor and a closed fuel cycle.
Important safety features of the system include a long thermal response time, a large
margin to coolant boiling, a primary system that operates near atmospheric pressure, and
an intermediate sodium system between the radioactive sodium in the primary system and
the water and steam in the power plant.
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The SCWR system is a high-temperature, high-pressure water-cooled reactor that
operates above the thermodynamic critical point of water (374°C, 22.1 MPa), which
enables a thermal efficiency about one-third higher than current light water reactors. The
balance of plant is considerably simplified because the coolant does not change phase
within the reactor and is directly coupled to the energy conversion equipment. The
system can either be open cycle with a thermal spectrum reactor or closed cycle with a
fast spectrum reactor.
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The VHTR is a graphite-moderated, helium-cooled reactor with a once-through uranium
fuel cycle. The reactor core can be a prismatic block or a pebble-bed design. It supplies
heat with core outlet temperatures of up to 1000°C, which enables production of
hydrogen or process heat. High-temperature processes such as high temperature
steam electrolysis and the sulfur–iodine and calcium-bromine cycles require higher
temperatures (>750°C) than can be achieved by water-cooled reactors.
These systems incorporate advanced technologies that offer ways of reducing the quantity of
nuclear waste. New types of nuclear power plants that have a closed fuel cycle provide a
promising alternative to the problematic dilemma of nuclear waste disposal. New fuel forms are
being developed and tested by the Idaho National Laboratory (INL) and other research
institutions that promise benefits of accident tolerance, better retention of fission products and
operation at very high temperatures. Some of these new fuels and materials are being tested at
INL’s Advanced Test Reactor.
Currently, the hot topic is small modular reactors (SMRs). SMRs are not a new concept and
represent a more near-term option for deployment than the six advanced technologies described
above. The U.S. Navy has had small nuclear reactors for decades. Also, small reactors were used
to prototype commercial designs for very large plants with capacities exceeding 1000 megawatts
electric (MWe). SMRs are typically about one-third the size of existing nuclear power plants.
The nuclear industry is in a race to develop several types of SMRs. Babcock & Wilcox (B&W),
NuScale Power, Westinghouse and Holtec are designing light-water reactors. Gen4 Energy,
Toshiba and General Electric Hitachi are concentrating on liquid metal-cooled systems, while
General Atomics and Pebble Bed Modular Reactor are focusing on high temperature gas-cooled
reactors. TerraPower, a start-up company that has attracted investment by software giant Bill
Gates, is developing a 500 MWe "traveling wave" reactor that is more aptly termed a small-tomedium sized reactor.
SMRs have the ability to satisfy requirements of increased sustainability, passive safety,
proliferation resistance, simplicity of construction and operation, and affordability. SMRs could
be used by smaller utilities, by smaller countries with financial or infrastructural constraints, in
isolated regions or for distributed power needs, and for various non-electrical applications
(process heat, desalination, oil recovery for tar sands and oil shale, and district heating). SMRs
can be coupled with other energy sources, including renewable (such as, wind, biomass,
geothermal, hydro and solar) and fossil energy. INL has just built a new Energy Systems
Laboratory to conduct research in hybrid energy systems. This hybrid energy approach leverages
resources, achieves higher efficiencies, produces multiple energy end-products, and increases
electrical grid stability and security (MIT, 2009). SMRs offer increased safety by eliminating
most of accident initiators (for example, large pipes in primary circuit), by improving decay heat
removal and including more efficient passive heat removal from reactor vessel, more in-factory
fabrications, transportability and site selection flexibility, smaller plant footprint and use of
seismic isolators for increased earthquake safety, as well as reduced investment risk (Vujic et al.,
2012). SMR design incorporates many features to minimize the effects of, and in some cases
eliminates the possibility of certain postulated accidents. The potential consequences of an
accident relative to that of a large plant are reduced due to the small size of the reactor and the
low power density limits. Elimination of large loop piping significantly reduces the flow area of
postulated loss of coolant accidents (Wright and Arnold, 2012).
Dr. Steven Chu (Secretary of Energy) and Dr. Arun Majumdar (U.S. DOE) declare, "It is
possible that safe nuclear power can be made more accessible through the economy of
constructing dozens of reactors in a factory rather than one at a time at each site. Also, with the
risk of licensing and construction delays reduced, small-modular reactors may represent a new
paradigm in nuclear construction." (Chu and Majumdar, 2012). This new paradigm could offer
lower upfront costs, improved safety and greater flexibility. To develop a new generation of
domestically-produced nuclear power, the Obama administration announced on November 20,
2012 that it will fund up to half the cost of a five-year project to design and commercialize SMRs
in the United States. This public-private partnership represents a significant investment in firstof-a-kind engineering, design certification and licensing for SMRs. DOE’s goal is to have these
reactors operational by 2022. B&W, an energy technology company based in Charlotte, N.C. is
leading the project in partnership with the Tennessee Valley Authority (TVA) and Bechtel
International. According to B&W, TVA is preparing an application to the Nuclear Regulatory
Commission to license up to four SMRs at its Clinch River Site in Oak Ridge, Tennessee.
“Low-carbon nuclear energy has an important role to play in America's energy future," Dr.
Steven Chu stated in announcing the award, citing President Obama's thrust for an energy
strategy that reduces greenhouse gases (GHG) emissions and provides affordable energy. The
growing concern about global warming and climate change has instilled a sense of urgency to
develop and deploy electricity generating technologies that do not emit CO2 or other GHGs.
Although some of the technology has been used for naval propulsion applications, these systems
have not yet been commercialized in the United States. The components of the SMRs will be
“Made in the USA, “ which will create new jobs and increase exports. SMRs can be
manufactured in U.S. factories and then be transported to construction sites that include remote
locations or military bases where large reactors are not practical. The modular approach enables
a "plug and play" implementation that requires minimal on-site preparation (U.S. DOE, SMR
Fact Sheet). "You can put them together like Legos on a job site," says Christopher Mowry of
B&W (USA Today, 2012). Additional modules can be incorporated incrementally as the demand
for energy increases and financing becomes available.
Each reactor is designed to provide 180 MWe, in contrast to about 1,000 MWe for a typical large
reactor, and is envisioned to operate in pairs sharing a common turbine-generator unit. The
integral design integrates the nuclear core and steam generators within a single vessel (shown in
Figure 3). Based on mature pressurized water reactor technology, the system can be contained
entirely underground to enhance safety and security. The light-water-cooled SMRs are designed
to have separate, independent underground containments for each small reactor, as well as
separate, independent safety systems protected within those underground containments (Figure
4). Combined with the smaller size of each unit, this configuration offers significant safety
advantages. In addition, since SMRs are currently still in the design phase, lessons learned from
the Fukushima disaster can be incorporated into their designs.
In the future, I see myself performing computer analyses and conducting experiments to improve
the next generation of reactor designs. In the future, as the demand for energy throughout the
world increases, the supply must also increase. My life’s goal is to have a positive impact on the
world and for my generation to leave the world in a better condition than we found it. For this to
be a reality, we need to invest in energy technologies that are sustainable, such as nuclear energy.
Perhaps by the time I graduate from college, nuclear fusion will be a possibility and I could be
designing fusion reactors.
In summary, nuclear power is here to stay. It is vital to our nation’s energy security and
economic prosperity, two factors that are inextricably linked to the foundations upon which our
great country was built – freedom.
Figure 3. The B&W mPower SMR integrates the
nuclear core and steam generators in a single vessel
(source: greencarcongress.com).
Figure 4. Underground containment
structure housing two B&W mPower SMR
modules (source: generationmpower.com).
References
Chu, S. and Majumdar, A., Opportunities and challenges for a sustainable energy future, Nature,
Vol. 488, August 16, 2012, p. 294-303.
Fermi, L., The Story of Atomic Energy, Random House, 1961.
Ingersoll, D.T., Foreword - Special Issue on Small Modular Reactors, Nuclear Technology, Vol.
178, May 2012.
Massachusetts Institute of Technology. Update of the MIT 2003 Future of Nuclear Power Study
http://web.mit.edu/nuclearpower/pdf/nuclearpower-update2009.pdf, 2009.
U.S. Department of Energy, Generation IV Roadmap, September 18, 2002.
U.S. Department of Energy, Small Modular Reactor Fact Sheet
http://www.ne.doe.gov/pdfFiles/factSheets/2012_SMR_Factsheet_final.pdf, 2012
USA Today, http://www.usatoday.com/story/news/nation/2012/11/26/nuclear-small-modularreactors/1727001/, November 27, 2012
Vujic, J., Bergmann, R.M. Skoda, R., and Miletic, M., Small modular reactors: Simpler, safer,
cheaper?, Energy, 45 (2012), p. 288-295.
World Nuclear News, http://www.world-nuclear-news.org, December 3, 2012.
Wright, R.F. and Arnold, R.P., Westinghouse Small Modular Reactor Small Break LOCA
Phenomena Identification & Ranking Table, Advances in Thermal Hydraulics (ATH’12),
November 11-15, 2012, San Diego, CA.