Small Is the New Big: The B&W Small Modular Reactor

Small reactors are big news, particularly the 180-MWe Generation III++ Babcock & Wilcox mPower small modular reactor (SMR). This SMR has all the features of its larger cousins, but the entire reactor and nuclear steam supply system are incorporated into one reactor vessel, all about the size of single full-size pressurized water reactor steam generator. Expect the first mPower—and probably the first SMR—to enter service before 2022.

During the formative years of the nuclear power industry, prototype reactors (such as the 67-MWe Big Rock Point, 94-MWe Fermi 1, and 275-MWe Indian Point Unit 1) were typically less than 300 MWe in size. Commercial reactors quickly scaled up to 1,000 MWe as technology matured and energy demand soared. Economies of scale soon made that size-class reactor the norm for baseload capacity in the U.S. While advances in new construction continue, small appears to be back in style, as demonstrated by the Babcock & Wilcox (B&W) mPower small modular reactor (SMR).

The U.S. Department of Energy defines SMRs as reactor designs with a nominal output of <300 MWe. The term “modular,” in the context of an SMR, refers to a single reactor module that can be grouped with other reactor modules to form a larger nuclear power plant, sized according to demand. Although the utility-scale advanced reactors currently under construction incorporate factory-fabricated modular components into their designs, a substantial amount of field work is still required to assemble these modules into an operational nuclear power plant. SMRs, on the other hand, are envisioned to require limited on-site preparation, and—other than loading the nuclear fuel—be ready to operate when they arrive from the factory.

POWER recently interviewed Christofer M. Mowry, CEO of Generation mPower LLC and president of Babcock & Wilcox mPower Inc., the B&W business group responsible for design, licensing, manufacture, and construction activities for the B&W mPower modular reactor (see sidebar). Mowry began by describing the SMR as a 180-MWe Generation III++ integral pressurized water reactor (iPWR).

Mowry said the SMR evolved from B&W’s first integral reactor prototype that was designed and built for the NS Otto Hahn, one of the first nuclear merchant ships, which sailed under nuclear power from 1968 through 1979. The reactor design, known as consolidated nuclear steam generators, continued to evolve over the decades to its current iPWR design. A formal program launch in terms of board approval to pursue full-scale development was received in 2009. The next important milestone is the B&W mPower reactor design certification application, which is expected to be submitted to the Nuclear Regulatory Commission (NRC) in the fourth quarter of 2013.

The Generation III++ designation is appropriate because of the integral design and robust safety margins addressing “beyond design-basis” accident scenarios. Although the reactor design is significantly smaller than that of conventional PWRs, it incorporates existing light water reactor technology; for example, its fuel assemblies are just smaller versions of the standard commercial 17 x 17 fuel assembly (Table 1). Several modules can be combined into a larger-sized power station, based on demand. Other unique features include the plant’s safety design, its underground installation, and spent fuel handling.

Table 1. Key features of the integral reactor coolant system (RCS) compared with existing pressurized water reactors. Source: Babcock & Wilcox mPower Inc.

Integral Design Approach

Mowry said the B&W mPower reactor’s integral design means that the entire reactor and nuclear steam supply system (NSSS) are incorporated into one reactor vessel. It is approximately the size of a conventional PWR steam generator, is rail shippable, and does not require on-site NSSS construction. From bottom to top the nuclear core/fuel assemblies, control rod drive mechanisms, steam generator, reactor coolant pump impellers, and pressurizer all are inside the single vessel instead of multiple vessels connected by large hot leg/cold leg piping (Figures 1 and 2).

1. Familiar steam circuit. This drawing shows the integral reactor arrangement and primary loop (reactor coolant) flow through the pressure vessel. The once-through steam generator is a vertical shell counterflow straight-tube heat exchanger design, which directly generates superheated steam as the feedwater flows through the steam generator in a single pass. Source: Babcock & Wilcox mPower Inc.
2. The B&W mPower reactor electric power generation cycle. Source: Babcock & Wilcox mPower Inc.

Mowry said removing the reactor vessel’s primary cooling circuit penetrations below the core eliminates the possibility of a worst-case design-basis accident in which a large loss of reactor cooling water is caused by a break in the piping. In contrast to the larger PWRs, this reactor’s small core combined with low power density reduces fuel and clad temperatures during accidents.

Furthermore, the low power density combined with a large coolant water inventory results in operating and safety margins that are significantly more robust than those required by the NRC—two to three orders of magnitude safer or about 10-8 compared to the current NRC or EPRI Utility Requirements Document core damage frequency (CDF) benchmarks (10-5 to 10-6). The CDF expresses the likelihood that, given the way a reactor is designed and operated, an accident could cause the fuel in the reactor to be damaged.

The reactor module is located inside its own underground steel containment and nuclear island, effectively isolating the reactor along with dedicated safety systems (there is no sharing of safety systems) from external man-made threats such as aircraft and projectiles, and from Fukushima-type natural disasters. Small penetrations reduce the magnitude of a design-basis loss of coolant accident and, therefore, the rate of energy release to the containment. The watertight underground nuclear island contains all emergency cooling water sources needed to protect the reactor core for an extended period of time, including the refueling water storage tank.

The primary function of reactor safety systems is to prevent core damage due to overheating and to maintain reactor coolant pressure boundary integrity. In a conventional PWR, this requires that the core remains subcritical and covered with water. Current PWRs perform these functions through design features such as multiple high- and low-pressure injection pumps; low-pressure, closed-loop decay heat removal systems; and active cooling systems to maintain containment pressure within design-basis limits.

Mowry said that because the B&W mPower reactor employs conventional balance-of-plant systems, including the NSSS arrangement and system components within a single pressure vessel, most transients and accidents described in NUREG-0800, Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants: LWR Edition are either identical or very similar to operating PWRs or the advanced PWRs employing passive safety systems. He said no unique thermal-hydraulic or neutronics phenomena have been identified for this design.

The B&W mPower reactor’s inherent safety features use gravity-driven or natural convection systems rather than engineered pump-driven systems and thus do not require AC power (either onsite or offsite) to power any safety systems. For example, pumps are not required to inject cooling water to the core. Instead, the decay heat removal system serves as an emergency core cooling system, is powered by gravity, and maintains a minimum volume of water on top of the core after a transient. Natural circulation removes decay heat, and a gravity-drained storage tank supplies makeup water to cool the reactor core. This ultimate heat sink provides at least 14 days of cooling without the need for external intervention or AC power to maintain reactor core cooling and safe shutdown. This is attributable to the SMR having a combination of a much lower level of decay heat than larger plants and the unique ultimate heat sink design. This allows operators to focus on long-term event mitigation rather than immediate emergency actions.

Unlike at Fukushima, no diesel generators are required to provide power for any of these safety systems to perform their intended functions. However, Mowry said that in keeping with mPower’s defense-in-depth philosophy, two back-up diesel generators are provided in seismically qualified structures for added protection. A three-day battery supports all plant monitoring and control without reliance on AC power. Finally, passive hydrogen recombiners prevent the buildup of hydrogen either from the reactor core or the spent fuel pool. All of the inherent safety systems, including the ultimate reactor cooling water source (the ultimate heat sink), batteries, battery recharging system, and hydrogen recombiners are housed inside the protected underground nuclear island.

The B&W mPower design includes a fully protected spent fuel pool located within the underground nuclear island. As observed at Fukushima, protection of spent fuel is most critical in the first few years after it is removed from the reactor core. Therefore, the spent fuel pool is designed with a large heat sink to ensure that more than 30 days of fuel cooling is available without the need for external intervention and before sufficient pool water is lost through boiling to uncover the spent fuel. At Fukushima, sufficient water loss may have been experienced within one week of the March 2011 accident.

Instead of rotating the fuel for up to three refueling cycles, as in conventional PWRs, the mPower reactor fuel has a single four-year run, and then the entire core is replaced in one load. A gantry crane lifts off the top half of the vessel, exposing the reactor core for replacement. The spent fuel pool would store enough spent fuel for a 20-year lifetime. It has been suggested that potential customers consider buying an extra steam generator that could be swapped in during a refueling outage. The previous steam generator then could be inspected once the reactor is back online and off the critical path, saving time and money.

The Fukushima Effect

Critics of multiple-reactor sites are quick to reference the events at Fukushima, where the plant operator was forced to shift resources from one unit to another, thus making these sites less attractive from a safety perspective. However, the events at Fukushima were, more than anything else, the result of plant and site configuration. This viewpoint also ignores the inherent differences between SMRs and the Fukushima plant, Mowry said. Table 2 summarizes the multi-layer design features that mitigate extreme beyond-design-basis challenges and “Fukushima-type” events.

Table 2. Multi-layer design features mitigate extreme beyond-design-basis and “Fukushima-type” events. Source: Babcock & Wilcox mPower Inc.

The Babcock & Wilcox Co. manufactures naval nuclear reactors for submarines and aircraft carriers. For security purposes, U.S. military technology will not be transferred to the mPower reactor project; however, the factories already exist and the additional investments for the initial stages of market adoption are minimal. Another advantage is that the reactor is small enough for the reactor vessel head and bottom to be forged in North America. The B&W Nuclear Operations Group’s Barberton, Ohio, and Mount Vernon, Ind., locations specialize in the design and manufacture of large, heavy components. These two locations are ASME N-Stamp accredited, making them two of only a few North American suppliers of large, heavy-walled nuclear components and vessels.

The construction process is equivalent to that for a combined cycle gas turbine plant rather than a standard large commercial nuclear reactor and is estimated to take about three years. The plan is to build the power plant first and then bring in and bolt together the integrated modules in parallel with field activities to shorten construction time (Figure 3).

3. Clean plant layout. The B&W mPower reactor “twin pack” site layout lacks a prominent containment dome because the reactors are underground. Courtesy: Babcock & Wilcox mPower Inc.

Mowry said the B&W mPower reactor is “plowing new ground as it relates to the robustness of our seismic design.” For example, the current reactor design addresses conditions covering the vast majority of the U.S. He added, “We do not think that SMRs are going to be successful in the long term unless you have a stable platform and stable design that can be replicated for some period of time before you evolve into the next level of design. This is a big departure in philosophy from where people design to current regulation. Then, when the regulations change, you have to change the design. We do not think that is the path to success for mPower.”

Mowry said that by taking this bounding, robust approach to the reactor design—for instance, achieving reactor safety levels several orders of magnitude beyond existing CDF requirements—he expects they will not have to change the design as regulations evolve over the next 10 years.

Integrated System Test Facility

In September 2011, B&W opened the mPower Integrated System Test (IST) facility, which is located at the Center for Advanced Engineering and Research (CAER) at the New London Business & Technology Center in Bedford County, Va. (Figure 4).

4. B&W testing facility. The Integrated System Test (IST) facility is located at the Center for Advanced Engineering & Research (CAER). The CAER is an initiative of Virginia’s Region 2000 Partnership to develop an industry-focused regional research and development center that drives the creation of innovative products and processes. Opened in August 2011, it provides access to university and federal research and targets the growth industries specific to the region, particularly nuclear energy and wireless technologies. The IST loop and emergency core cooling systems are located in a 110-foot-tall tower visible in the photo. Courtesy: Babcock & Wilcox mPower Inc. and CAER

The IST facility contains a scaled prototype of the B&W mPower reactor and all important nuclear island systems, which were installed in July 2011. All of the technical features of the B&W mPower reactor are included in the IST, although the source of reactor core power is electric rather than nuclear. The reactor will undergo testing and data will be collected to demonstrate the reactor’s thermo-hydraulic characteristics that support the safety analysis methodology, the protection systems, and the passive engineered safety features used in the reactor’s design certification application. The facility is being used for developing test procedures and simulations for operator training.

TVA Leads the Way

A memorandum of understanding has been signed by B&W, Tennessee Valley Authority (TVA), and a consortium of regional municipal and cooperative utilities to explore the future construction of a fleet of B&W mPower reactors with operation of the first unit before 2022. In November 2010, TVA informed the NRC of its plans to evaluate the feasibility for one or more SMR modules at its Clinch River site in Roane County, Tenn.

Mowry said that the first B&W mPower construction permit application at Clinch River would be prepared in accordance with the two-part licensing process under 10 CFR Part 50. “This is something that we have had a lot of dialogue [about] with both the NRC commissioners and the staff, and everyone is comfortable with this approach.” From a best practice/risk management commercial perspective, the two-stage Part 50 process—a construction permit (CP) followed by an operating license (OL)—provides the flexibility necessary to enable a quicker construction start and support design modifications, because the requirements for a CP are relatively limited. The OL process would benefit from the experience gained during design and construction of the initial SMR deployments. Experience with the initial CP/OL framework could then be transferred to a combined operating license (COL) structure under the Part 52 process.

In accordance with the Part 50 licensing process, TVA would develop a Preliminary Safety Analysis Report (PSAR). The PSAR would be prepared utilizing the guidance of Regulatory Guide 1.70, Revision 3, Standard Format and Content of Safety Analysis Reports for Nuclear Power Plants (LWR Edition) and the organizational structure of the standard review plan. In addition, the application would include an environmental report addressing the environmental standard review plan guidance contained in NUREG 1555, Standard Review Plans for Environmental Reviews for Nuclear Power Plants: Environmental Standard Review Plan.

TVA plans to use Generation mPower as its vendor for developing the B&W mPower reactors. Fabrication of major plant components may begin before issuance of the construction permits and may require NRC inspection resources in advance of issuing the construction permits. This will necessitate close coordination and timely communication of manufacturing plans and schedules to facilitate NRC inspection activities.

The SMR initial test program would be developed using Regulatory Guide 1.68, Revision 3, Initial Test Programs for Water-Cooled Nuclear Power Plants to ensure that all systems, structures, and components important to safety are tested and demonstrate that the facility can be operated in accordance with design requirements and in a manner that will not endanger the health and safety of the public. The scope of the inspection and enforcement program—along with the initial test program that encompasses site preparation inspections, construction inspections, manufacturing inspections, and system tests through hot functional testing—will inform and demonstrate successful execution of future inspections, tests, analysis, and acceptance criteria that may be specified in design certification or COL applications.

SMR Cost-Sharing Program

In March 2012, the Department of Energy’s Office of Nuclear Energy’s Small Modular Reactor Licensing Technical Support program issued a funding opportunity announcement (FOA) on first-of-a-kind engineering projects that promote the accelerated commercialization of SMR technologies that can be expeditiously licensed to achieve a commercial operation date on a domestic site by 2022. The application submittal closing date was May 21, 2012.

A minimum 50% industry cost sharing (for example, by SMR vendors and utility partnerships) on an annual basis is required over the life of the cooperative agreements. Also, the FOA states that proposed contributions greater than 50% is a selection factor in evaluating program proposals. The total government funding available for two awards under this FOA is a maximum of $452 million over five years. The actual level of funding will depend on congressional appropriations.

The cost-sharing approach is used to offset the risks associated with developing and deploying this first-of-a-kind nuclear technology. Also, a public-private partnership to develop SMRs is necessary to share these risks and make the long-term investment justifiable to shareholders and investors by showing the government’s commitment to the future of nuclear power and SMRs. In addition, broad market adoption of SMR technology depends on a successful first-of-a-kind project.

Mowry said he believes there is a critical need for the DOE’s cost-sharing program to spur market development and the viability of SMRs in order to improve energy security supported by deployment of SMR technology.

A growing SMR industry creates the potential to establish a large domestic manufacturing base building upon existing U.S. manufacturing infrastructure, including underutilized domestic nuclear component and equipment plants. Therefore, a viable U.S.-centric SMR industry would enable the U.S. to recapture leadership in commercial nuclear technology, which has been lost to suppliers in France, Japan, Korea, Russia, and China.

Much Work Remains

Mowry said the B&W mPower NSSS design has progressed well beyond the preliminary stages. “The design teams have been very disciplined about how much new technology to inject into this effort. This is definitely not a fourth-generation technology. This is all about capturing best-in-class ideas. There are some new technologies in the design, but they are very focused and very limited.” For example, the prototype internal control rod drives are already being tested and validated.

Mowry added: “We are in a transition phase with mPower, moving from a conceptual preliminary design into final design and testing activities. The mPower development program, as it relates to safety and economic performance goals, is making very successful progress.”

James M. Hylko is a POWER contributing editor.

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