By James L. Buelt, Pacific Northwest National Laboratory
With the pending nuclear renaissance, a series of alternate reactor designs have been emerging based on smaller, modular designs. Although varied in type and state of development, these reactors offer potential advantages to numerous domestic and international needs for clean, carbon-free, baseload electrical energy.
In the 1980s, the Air Force evaluated the use of small nuclear power reactors at bases to provide reliable electrical power to its facilities to counter threats to the electric grid from sabotage and terrorism. In such an event, the Air Force would only be able to rely on small diesel generators and batteries with limited capacity.
Recently, the Air Force has been considering the deployment of privately owned and operated reactors to be sited on one or more of its bases under a long-term power purchase agreement. Ideally, the reactor would be large enough to sustain base operations, yet transportable enough to be deployed in field operations abroad.
Today, the concept of using small nuclear power reactors is re-emergingnot only at Air Force bases but for international and domestic electricity production.
In 2004, the city of Galena, Alaska, passed a resolution calling for the deployment of a 10 MWe modular reactor of the design proposed by Toshiba1.
In 2006, the Department of Energy launched the Global Nuclear Energy Partnership, which included a small modular reactor component in the program for development of reactors that were sized for the electrical grids of developing countries, generally at less than 500 MWe.
This year, Energy Northwest, the utility that operates the Pacific Northwest’s only commercial power reactor, announced its intent to deploy a small modular reactor adjacent to its existing 1,100 MWe plant.
And recently, four U.S. companies have announced new designs that they intend to develop and deploy for domestic and international purposes.
Worldwide, development of small reactors has produced as many as 60 innovative reactor designs in various stages of development2. They incorporate a range of different coolants, fuels, materials and engineering features.
Small Modular Reactors
The global nuclear renaissance will be driven by the need for clean, carbon-free baseload electrical power.
A study conducted by Pacific Northwest National Laboratory’s Joint Global Change Research Institute has projected the required growth of nuclear power, along with advanced fossil, renewables and conservation/efficiency to stabilize carbon concentrations in the atmosphere3. Under carbon-constrained scenarios, nuclear power is expected to more than double by 2035, nearly triple by mid-century and increase more than tenfold by the end of the century.
Development of small reactors has produced as many as 60 innovative reactor designs in various stages of development.
Currently, there are 430 nuclear power reactors in operation worldwide producing 372 GWe, according to the World Nuclear Association4. The majority of these reactors (nearly 60 percent) are Generation II- and III-type light water reactors operating in the 800+ MWe range. In the U.S., that number exceeds 80 percent.
Generation III+ reactors being planned in the U.S. and abroad range from 1,100 MWe for the Westinghouse AP-1000 to 1,700 MWe for Mitsubishi’s US-APWR, but it is not yet clear if the demand will be met by larger scale Generation III+ reactors. Given the advantages that small modular reactors provide, they are expected to meet a significant portion of this anticipated demand.
Here is a brief overview of some of the modular reactors that may contribute to the nuclear renaissance domestically and internationally.
B&W mPower Reactor, Babcock and Wilcox
One of the newest reactor concepts to be announced, this integral light water reactor is designed to provide 125 MWe5. The integral design houses the reactor core and the steam generator in a 5-meter diameter module that is designed to be delivered to the reactor site on a rail car. (See Figure 1)
International Reactor Innovative and Secure (IRIS)6, Westinghouse Electric Co.
A light water reactor-based design with relatively mature development and licensing status, current design efforts are focused on 335 MWe. Principal design features include an integrated reactor vessel and steam generator system with internal control rod drive mechanisms to save overall containment building space. The IRIS reactor is expected to receive a design certification from the Nuclear Regulatory Commission (NRC) prior to the construction of a commercial prototype and is consequently projected to be ready for deployment by 2015. The reactor vessel for the 335 MWe unit measures 22.2 meters in height. (See Fig. 2.)
GasTurbine Modular Helium Reactor (GT-MHR)7, General Atomics
A 286 MWe unit boasting thermal efficiencies approaching 50 percent, the GT-MHR is helium-cooled, eliminating steam generators and secondary cooling loops. This reactor has the unique feature of directly generating electricity in the adjacent turbine using the primary reactor coolant, which in this case is helium gas, thereby eliminating the need for a steam generator. The system uses graphite as the moderator that encases small (~1 mm diameter) fuel particles known as TRISO fuel, which are triply encased in pyrolytic carbon and silicon carbide (SiC).
Small, sealed transportable autonomous reactor (SSTAR), Lawrence Livermore National Laboratory
A lead-cooled reactor designed to be compact and portable, the SSTAR is capable of producing 10 to 100 MWe and expected to be ready for testing by 20158. Being a fast reactor, the SSTAR is capable of generating fissile material from fertile material such as U-238, allowing 30 years of operation without refueling. The reactor is designed to be transported via ship or heavy transport, weighing approximately 500 tonnes and measuring 3 meters diameter by 15 meters tall in a shipping cask. The reactor vessel is equipped with an internal steam generator, thereby promoting its self-contained features.
Super-Safe Small and Simple Reactor 4S, Toshiba and CRIEPI, Japan
A sodium-cooled small reactor without on-site refueling, versions of this reactor range from 10 to 50 MWe. (See Fig. 3.) The reactor vessel is intended to be operated underground with a turbine building on the surface. This reactor has been proposed for deployment in Galena, Alaska, in 2012 or 2013. The 4S reactor being considered for the Galena facility is a pool-type fast neutron reactor that, when coupled to power generation equipment, has an electrical output of 10 MWe (30 MWt)9.
The primary heat transport system consists of the containment guard vessel, reactor vessel, intermediate heat exchanger, electromagnetic (EM) pumps, internal structures, core and shielding, all of which are located below grade. Heat from the intermediate heat transport system is exchanged in a steam generator (also located below grade) to produce steam, which drives conventional steam turbine generator equipment. In the standard plant, the ultimate heat sink is designed to be air-cooled heat exchangers, although district heating may be possible as a plant modification.
Water-cooled reactors with small coated particle fuel without on-site refueling, AFPR, Pacific Northwest National Laboratory, U.S.
A reactor concept that continually feeds fuel material through the reactor to eliminate the need for refueling, it was developed to demonstrate the potential for long-term operation without the need for refueling from external sources. No significant development program is currently underway with this reactor.
NuScale Power System, being developed by NuScale Power and Oregon State University
A 45 MWe light water reactor (see Fig. 4) that incorporates integral steam generation capacity, control systems and safety systems10, the reactor is designed to be modular so that multiple units can be economically deployed to meet expanding electricity requirements.
This below-grade reactor is self-contained in an air-evacuated containment vessel that is immersed in a pool of water. The integrated reactor and containment vessel is prefabricated in a vessel less than 20 meters by 5 meters diameter. It is cooled by natural convection with integral steam generators. It relies on vacuum insulation between the reactor vessel and inner containment wall thereby eliminating the potential for pump screen blockage from degraded insulation from a loss-of-cooling event.
The reactor operates on conventional light water reactor fuel and is refueled by removing the containment vessel temporarily to a refueling pool. Reactor developers are actively pursuing design certification with the NRC.
Hyperion Power Generation, Hyperion
The Hyperion consists of a novel uranium hydride fuel and reactor concept that operates at higher temperatures and is thus capable of producing process heat as well as electricity11. Its design offers 25 MWe or 70 MWt and is touted to be “battery”-like in its operational simplicity. It is designed to operate seven to 10 years in an underground environment without the need for refueling12. Dan Ingersoll, one of the leading national authorities on small modular reactors, reports that the developers initiated discussions with the NRC in 2008 on the Hyperion design and are targeting 2013 for initial deployment.
TerraPower, a company spun off by Intellectual Ventures in 2009
Arguably one of the more innovative concepts, the TerraPower reactor is designed to run on depleted uranium without the benefit of uranium enrichment. It does this through a traveling wave concept that supports a sustainable reaction through a matrix of depleted uranium fuel. Developers envision 50 to 100 years of operation without refueling in a once-through fuel cycle, thereby extracting most of the available energy from the reactor fuel in a single pass. This concept is not restricted to small modular designs; 100 MWe through 1,000+ MWe are envisioned with this concept13.
Power Reactor Innovative Small Module (PRISM), GE-Hitachi
A liquid sodium-cooled fast reactor in a modular framework14, it features a below-grade pool-type reactor module that houses its secondary hardware and steam generator in a separate building. Multiple modules share common support facilities, including the electrical turbine generator, such that power production may remain uninterrupted during a single module refueling or maintenance operation. Each module is capable of delivering 311 MWe for a paired power block of 622 MWe.
Modular Reactor Safety
According to the World Nuclear Association, there have been more than 12,700 reactor-years of commercial operation worldwide since the advent of the peaceful use of the atom more than 50 years ago. During that time, there has only been one serious accident in the U.S. with commercial reactors, Three Mile Island in 1979, and one abroad, Chernobyl in 1986. Since that time, the industry has been focusing on including inherent safety features within its designs and small reactors are no exception.
One small reactor manufacturer utilizes a vacuum for thermal insulation (similar to a coffee thermos) instead of insulation material to alleviate concerns of sump-screen plugging during an accident condition. Most of the systems are designed to be cooled by natural circulation during an accident condition, eliminating the need for active emergency cooling systems. Most importantly, these systems will be regulated by the NRC throughout the design, construction and operational phases to ensure their safety and security to workers and the general public.
The safety performance of nuclear power in general has contributed to today’s 63 percent favorable rating by the public for nuclear power, as reported by the Nuclear Energy Institute.
Selecting Modular Reactors
In addition to those described above, many other reactor types are being developed, each with their own advantages and disadvantages.
In order to select the appropriate reactor design, a structured methodology must be followed to ensure that the objectives of deploying a licensable, economic and reliable system within the desired timeframe and power levels are achieved. Some of the attributes to be evaluated include:
• State of Development
Some small reactors have active development activities while others are not actively supported. It is important that an active development program be leveraged when selecting the appropriate reactor.
• Size and Weight
Depending on power ratings, coolant design and other factors, reactor vessels can weigh several hundred tons and be more than 50 feet long. The field footprint for the reactor and associated infrastructure for small reactor systems is expected to be in the one- to five-acre range. Size, transportability and field footprint will be important considerations for field deployment.
• Licensing Status and Approach
Although no design certifications for small modular reactors have yet been submitted in the U.S. according to the NRC website15, some reactor vendors are actively pursuing an aggressive licensing strategy. Time for design certification and combined Construction and Operating License will be a critical path element in the deployment process.
One of the features of a small modular reactor is enhanced manufacturing options. Many of the Gen III+ designs are restricted by forging capability, currently only available in Japan. Small modular reactors by the virtue of their size enjoy the advantage of having multiple domestic and international manufacturing and forging options.
• Capital Cost
Small reactor systems must be in the competitive range of Generation III+ full-scale reactors on a $/installed kW capacity basis. In-shop fabrication of integral modular reactors and domestic fabrication of components are expected to offset economies of scale associated with construction of more conventional, larger systems.
• Operational Cost
There are many factors that go into determining operational cost. Operational cost will be an important consideration in the selection and must be determined on a site-specific basis.
Some reactors are designed to operate 10 to 30 years without refueling, while others operate on a 1.5-to-3-year refueling cycle. Multiple modular systems requiring refueling on a 1.5-to-3-year cycle afford the opportunity to refuel one module at a time, thereby avoiding complete interruption of power to the grid.
• Reliability and Operating Efficiency
One of the features small modular reactors offer operating utilities is the elimination of a single shaft needed to generate power. Because modular reactors can be bundled to operate with multiple generators, a single module does not prevent continued operation of the other modules. Hence, modular reactors offer enhanced reliability of maintaining power to the grid during single unit refueling, maintenance, and unplanned events.
In summary, small modular reactors offer an important alternative to more conventional, large-scale power reactors for supporting the nuclear renaissance. Their smaller size and modular nature have the potential for generating electricity and revenue at lower capital investment while enhancing the utilization of domestic manufacturing capabilities.
Author: James L. Buelt is the manager of the nuclear energy sector at Pacific Northwest National Laboratory (PNNL), responsible for developing the relationship and business with the DOE Office of Nuclear Energy and the nuclear industry, which currently totals approximately $15 million/year at PNNL. Buelt serves as the face to the client for all of PNNL’s nuclear energy science and engineering activities and is responsible for developing and executing the Laboratory’s nuclear energy strategy to best deploy its scientific and facility resources for the safe, secure, and economic expansion of nuclear power.
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