The impact of the concept of modularity in reducing the cost of small, replicated, and mostly factory-built units is paramount. Proponents refer to this as the competition between the traditional economy of scale which has led to GWe-sized plants and the new economy of numbers which characterizes the construction of SMRs.

Further, the dramatically reduced power rating of SMRs provides significant potential for passive safety systems which simplify or eliminate active safety systems compared to those of current-generation reactors. As well, the SMRs can eliminate their reliance upon support systems as compared to the current LWRs’ need for such systems. The American Nuclear Society’s report on SMR generic licensing issues (see Tables 1.3 and 1.4 in American Nuclear Society, 2010) identifies specific candidate safety and support systems for such simplifications and eliminations. However, projections among analysts vary as to whether SMRs can achieve lower LUECs than traditional large plants. For example the OECD has reported (OECD, 2011) that the investment component of LUEC from an SMR would probably be higher than that of a large plant, even taking into account the SMR reduced construction schedule, shop fabrication, and learning curve. Further, the OECD concluded that SMRs, including twin-unit and multi-module plants, generally have higher values of LUEC than nuclear power plants with larger reactors. Thus achievement of a competitive SMR LUEC will be very difficult to accomplish: reference to independently validated projections is essential for developing realistic cost estimates.

Liquid-metal-cooled reactors are third behind water-cooled and gas-cooled reactors in terms of global commercial reactor experience. Test and demonstration reactors using

sodium, lead, or lead-bismuth coolant have been constructed in the United States, the Russian Federation, France, Japan, the United Kingdom, and most recently in China and India. Interest in liquid-metal coolants is driven by the desire to develop fast-spectrum reactors, i. e. reactors that generate most of the power from fissions resulting from high-energy neutrons unlike water-cooled reactors, which generate power from thermal neutron-induced fissions. The key advantage of fast-spectrum reactors is that there are more neutrons produced per fission and these ‘excess’ neutrons can be used for purposes other than sustaining the basic chain reaction. Initially, the extra neutrons were intended to be used to ‘breed’ fuel, i. e. produce new fissile fuel faster than it is consumed. As more and more uranium reserves were discovered worldwide, interest in fast-spectrum reactors turned away from the breeding function to a resource recovery, i. e. producing energy from the unburned fuel discharged from water-cooled reactors, and waste management, i. e. consuming the associated long-lived waste products from that fuel.

Another advantage of liquid-metal-cooled reactors is that the metals have high boiling temperatures, which allow the reactors to operate without pressurization of the primary coolant. Also, the coolant can be heated to a moderately high temperature, typically around 500 °C. Although higher than the 300-325 °C outlet temperature in a water-cooled reactor, it is still lower than the 750-850 °C outlet temperature in gas-cooled reactors. The higher temperature increases the power conversion efficiency relative to water-cooled reactors and can allow for more compact power conversion systems using supercritical Rankine or Brayton cycles.

Listed in this section and summarized in Table 2.4 are the three liquid-metal — cooled reactor designs that have near-term deployment potential by virtue of commercial support and some level of engagement by a licensing authority. Many other designs and projects are underway, but either are limited to research studies or are experimental/test reactors intended to be precursors for large commercial plants, such as the Traveling Wave Reactor being developed by TerraPower.

D. T. Ingersoll

NuScale Power LLC, Oak Ridge, TN, USA

2.1 Introduction

The pursuit of small modular reactors (SMRs) is both a persistent and global phenomenon with widespread interest from developers and customers alike. Some of the earliest concepts emerged in the 1970s for merchant ship propulsion and industrial process heat applications. Today, more than 50 concepts have been reported by the International Atomic Energy Agency (IAEA) ranging from minor evolutions of operational reactors to exotic liquid-fueled and fission-fusion hybrid designs. Chapter 1 describes many of the motivations that drive customer interests, including lower upfront capital investment, better match to projected demand, better compatibility with electrical grid infrastructure, and greater flexibility in site locations. After an extreme earthquake and induced tsunami in Japan destroyed the Fukushima Daiichi nuclear plant in 2011, the promise of enhanced safety and plant resilience of SMRs has become a central consideration in countries with established nuclear programs as well as countries seeking to initiate nuclear power programs.

Reactor developers worldwide are seeking to develop SMR designs to meet the large anticipated market demand. Designs are emerging from both traditional reactor vendors and new start-up companies, and also are being developed in both traditional and new nuclear supplier countries. Many SMR concepts are being developed by research organizations, typically characterized by advanced fuels, materials and coolants, and often with exotic design features that will require decades to develop and qualify for commercial application. All of the SMRs discussed in this chapter are actively being developed by commercial companies and most have some level of engagement by a licensing authority, either in the country of development or country of potential deployment. As such, these designs are considered to have the potential to be deployed within the next 10-15 years, depending on developer commitment and customer interest. An effort was made to select designs that span the gamut from traditional technology and engineering to novel technology and highly innovative engineering. Keeping with the widely accepted definition of ‘small,’ the designs presented in this chapter all have an electrical output of less than approximately 300 MWe, although some designs exceed this limit by a modest amount.

Handbook of Small Modular Nuclear Reactors. http://dx. doi. Org/10.1533/9780857098535.1.27

Copyright © 2015 Elsevier Ltd. All rights reserved.

As discussed in Chapter 1, SMRs do not represent a unique reactor technology, but rather reflect the same spectrum of technologies considered for large plants. Brief summaries of 22 commercially developed SMRs designs are provided in the following sections, organized by technology type. This is a natural approach for organizing the designs since the different technology classes generally target different energy applications. Within a technology class, the designs are presented by country in alphabetical order.

The information presented in this chapter was gleaned from publicly available information gathered in reports, papers, presentations, websites and personal communications. Many of these sources are listed in Section 2.7. Because all of the designs have commercial interests, detailed information regarding design features is generally treated as proprietary and is not publicly available. Also, most of the designs continue to evolve at a rapid pace and design parameters change equally fast. In some cases, multiple sources for a single design have conflicting information. Every attempt has been made to present the latest and most accurate information available. Preference was given to sources of information provided directly by the designer rather than third party informants. Even with these sources, it was often difficult to glean factual information from bold marketing claims. Therefore, no claims are made regarding the accuracy or currency of the information. The information presented in the remainder of this chapter represents a best effort, objective portrayal of the leading commercial SMRs under development worldwide.

The mPower design was announced by Babcock and Wilcox (B&W) in 2009. Responsibility for the design was transferred in July 2010 to Generation mPower, LLC, which is a partnership of Babcock & Wilcox and Bechtel. The design is a 180 MWe LWR-based integral system. The core consists of 69 traditional 17 X 17 pin array fuel assemblies with an active fuel length of 2.4 m. The fuel is conventional UO2 with an enrichment of less than 5% 235U.

An individual mPower module is intended to be completely factory fabricated using currently available components. Each module is placed within a steel containment dome and has a dedicated turbine-generator power conversion unit. An option is being developed to use air-cooled condensers, which reduces the output to 155 MWe, for use where water resources are restricted. It is also being designed to enhance load-following capabilities for connection to more dynamic grids.

A scaled plant simulator has been constructed and is being used to demonstrate the safety performance of the mPower design. This facility was completed in September 2011 and underwent start-up commissioning in early 2012.

Originally, the reference mPower plant contained four 125 MWe reactor modules to yield a plant size of 500 MWe. In 2011, a number of design changes were made to increase the power rating of each module to 180 MWe and the reference plant became a twin unit with total capacity of 360 MWe. The Tennessee Valley Authority has announced its intent to pursue the construction of up to four mPower modules at the Clinch River site in Oak Ridge, Tennessee. Key parameters and a representative graphic for the mPower design are given in Figure 2.10. [8]

Figure 2.10 mPower (United States) — Generation mPower (B&W/Bechtel).

The SMRs of different coolant types employ very different fuel types. The water — cooled as well as the lead-bismuth-cooled SMRs use uranium dioxide (UO2) ceramic fuel; the gas-cooled SMRs use graphite and silicon carbide coated UO2 particles in graphite compacts or pebbles; the sodium-cooled reactor uses metallic UZr with minor actinides; and the lead-cooled SMR uses mononitride mixed fuel (UN-PuN). The water-cooled SMR fuel is the same as that of the operating plants and of the GEN Ш+ plants currently being deployed. All the liquid-metal-cooled reactor fuels will have an enrichment significantly more than the 5% of current water-cooled fuel.

Although a US national repository is not yet identified, this water-cooled SMR fuel will be handled consistent with the anticipated US policy yet to be finalized. The gas-cooled SMR fuel, the same as that used in the Fort St. Vrain reactor, has significantly more volume per unit energy generation but lower heat load per unit volume than LWR UO2 fuel. The characteristic of this fuel will require a different overall disposal strategy, although it would likely be compatible with the the strategy of the national repository for ceramic UO2-zircaloy clad fuel since the tristructural isotropic-type (TRISO) fuel particles form good barriers that provide excellent fission product retention.

The fuel of sodium — and lead-cooled SMR reactors exploits the inherent incentive of these fast neutron spectrum reactors to undergo reprocessing and recycling. This fuel cycle will entail construction and operation of reprocessing and fuel fabrication facilities while most likely it would also be integrated with reprocessing of some light water fleet fuels as feedstock for the plutonium needed for initial loading of a growing fleet of fast reactors. The spent fuel constituents ultimately requiring disposal will be predominantly fission products of much less volume than the spent fuel bundles of thermal spectrum water reactors per equivalent unit of energy generated. However, the deployment of fast spectrum SFRs based on the closed fuel cycle would require significant expansion of reprocessing and fuel fabrication facilities compared to the needs for the existing LWR fleet and LWR SMRs operating on the once-through fuel cycle.

The Toshiba Super Safe Small and Simple (4S) reactor design is a sodium-cooled fast-spectrum reactor with an output of either 10 or 50 MWe. The reactor has a compact core design with steel-clad metal-alloy uranium fuel. A unique feature of
the core design is that it does not require refueling over the 30-year lifetime of the plant (10 MWe version). This is accomplished by designing for a high conversion of the fertile material in the core and by using a slowly moving reflector to compensate for fuel burn-up over the core lifetime. A 50 MWe design option is available with a 10-year refueling cycle. The U-10%Zr metal alloy fuel has an enrichment of less than 20% 235U and is clad in HT-9 alloy. The basic layout of 4S is a pool-type configuration, with the electromagnetic (EM) pumps and a single intermediate heat exchanger contained inside the primary vessel. An intermediate sodium loop delivers heat from the primary system to the external steam generator used to generate steam for the Rankine power-conversion system. The reactor containment consists of a lower nitrogen-filled steel guard vessel and an upper steel dome.

The 4S design is supported by extensive testing facilities in Japan, including the Toshiba Sodium Loop Test facility. The design is targeting the diverse and remote energy market where alternative energy sources are very expensive or difficult to sustain. A consortium including the local government in Galena, Alaska, initiated pre-application meetings with the US Nuclear Regulatory Commission in 2007. This project has not yet proceeded into formal licensing and currently Toshiba is exploring alternative first customers. Key parameters and a representative graphic for the 4S design are given in Figure 2.20. [18, 19]

Figure 2.20 4S (Japan) — Toshiba/Westinghouse © Toshiba Corporation.

The vast majority of operating reactors, commercial and military, use light (normal) water as the primary reactor coolant. Given the extensive operational experience with light-water reactors (LWRs), it is the technology of choice for reactor vendors who want to get their product to market quickly and for potential customers who are concerned about investment risk. Thirteen of the 22 SMR designs presented in this chapter are based on LWR technology. Although they all share a common coolant choice, other design features are quite varied. Overall configurations include traditional loop design, compact loop design in which the external vessels are flanged directly to the reactor pressure vessel, and integral system design in which most or all of the primary system components are located within the reactor pressure vessel. Individual module output capacities range from 8 to 300 MWe and plant-level strategies range from using a single reactor per plant to as many as 12 reactors per plant. Also, the plants may be sited on land, operated on floating barges or submerged below the ocean surface.

Table 2.1 provides a summary of the LWR-based SMR designs described in the following sections.

NuScale Power LLC was formed in 2007 to commercialize an SMR design that had been initially developed during 2000-2003 by a research team at Oregon State University and the Idaho National Laboratory. Fluor joined NuScale Power in 2011 as its major investor and strategic partner. Each NuScale Power Module produces 50 MWe (gross) from a simplified integral system configuration that uses natural circulation of the primary coolant. The core is composed of 37 half-height 17 X 17 pin array fuel assemblies and is controlled using 16 control rods and soluble boron within the primary coolant. Superheated steam is produced in two co-mingled helical coil steam generators that surround the central hot leg riser. A NuScale plant can be scaled to accommodate up to 12 modules, which can be installed incrementally to best match a desired investment or demand profile.

The reactor pressure vessel is contained within a compact, high-pressure, steel containment vessel, which is immersed in a below-grade pool shared by all modules. The shared pool is the ultimate heat sink for residual heat removal. A fail-safe emergency core cooling system can provide an unlimited post-accident grace period with no operator action, no AC or DC power, and no make-up water. Each module has an independent skid-mounted turbine-generator set for power conversion and

Figure 2.11 NuScale (United States) — NuScale Power, LLC (Fluor) © NuScale Power.

can continue to operate while other modules are being refueled. The modules, including the reactor and containment vessels, can be shipped to the site by truck, rail or barge.

A scaled integral test facility was built in 2003 as part of the original design development project and has been used throughout the design refinement process to inform design modifications and validate its safety performance. A 12-module control room simulator was commissioned in 2012 to evaluate operator performance for multi-module control rooms. A first plant is being pursued by a consortium of utilities in the western United States to be potentially sited in Idaho. Key parameters and a representative graphic for the NuScale design are given in Figure 2.11. [9]

As with the current large-rated reactors, SMR coolants can be light water, gas, or liquid metal. Key SMR examples of these primary system coolant types with their principal design parameters are presented in Table 1.2. The coolant properties which dictate the different design characteristics of these SMRs are presented in Table 1.3. Principal among them are:

• the very high outlet temperature (750-950 °C) of the high-temperature gas reactor (HTGR) possible with the use of helium as coolant and graphite as the principal core material, yielding a high plant thermal efficiency and supply of reactor heat for processes requiring high temperature heat;

• the low primary operating pressure of the liquid metal reactors permitted by the low vapor pressure of their primary coolant at their high operating temperature; and

• the high power density of the sodium-cooled reactor possible because of its operation with a fast neutron spectrum coupled with a very high heat transfer coefficient that allows tight packing of its fuel pins.

The predominant use of light water in both pressurized and boiling water large-rated reactors currently in use can be readily replicated for SMR application. The smaller primary system components of pressurized water SMRs allows their arrangement within the pressure vessel as is already done even for large power rated BWRs. This PWR configuration, the integral reactor, was pioneered (as discussed in Section 1.1.3) in the commercial merchant vessel, the German Otto Hahn, and is a principal configuration of current PWR SMRs as elaborated in Chapter 3.

Helium has been the gas coolant of SMR choice, although carbon dioxide is used in advanced gas reactors (AGRs) operating in the UK which are currently slated for retirement. The liquid metal coolants of SMR choice are sodium, lead, and lead — bismuth. Sodium has been exploited significantly for large-rated reactors based on early work with sodium-potassium and sodium, while more exotic coolants such as lithium have been used for electricity-generating space reactors, e. g., the SNAP (Systems for Nuclear Auxiliary Power) series. For SMRs attention is focused on sodium and the variants of lead cooling — both pure lead and lead-bismuth eutectic.

Differentiation among reactor types and specific reactor designs within a coolant — type design is based on their satisfaction of a selected mission and then a set of criteria including operational reliability, protection of public health and safety, and finally economic competitiveness. The salient characteristics of the SMR reactors as they relate to these factors are presented next. Chapter 2 and the chapters in Part Four elaborate the detailed technical features of SMRs covering this range of primary coolants.

The SVBR-100 builds on the former Russian experience with lead-bismuth reactor technology used for several submarine propulsion units. It is a pool-type fast — spectrum reactor with forced circulation of the low-pressure primary coolant using two main circulation pumps. The initial core fuel is UO2 with a 235U enrichment below 20%, although subsequent fuel loads may contain U-Pu mixed oxide fuel or UN-PuN fuel. The core is composed of 61 fuel assemblies; however, a fresh core is loaded as a single cassette with whole-core replacement every eight years. Internal straight-tube steam generators are used to supply steam to external steam separators and a Rankine power-conversion unit. Although material corrosion issues plagued the early Russian submarine experience, corrosion problems were generally resolved by the conclusion of the earlier program.

The 100 MWe SVBR-100 is envisioned to supply both electricity and process heat for non-electrical applications. A single unit is expected to deliver 580 tons/h of process steam, 70 Gkal/h of district heat, or 200 000 tons/day of desalinated water if configured for these applications.

The SVBR-100 design effort was approved in 2006. A demonstration project was approved in 2011, at which time the reactor designer, OKB Gidropress, teamed with AKME Engineering to construct the first plant. Design and site licensing has been

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Figure 2.21 SVBR-100 (Russian Federation) — AKME Engineering/ОКБ Gildropress © AKME Engineering/OKB Gidropress.

ongoing since 2011 and approval was granted in 2013 with operations commencing in 2017. Key parameters and a representative graphic for the SVBR-100 design are given in Figure 2.21. [20]