Technology readiness assessment of Small Modular Reactor (SMR) designs

Progress in Nuclear Energy 70 (2014) 20e28

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Review

Technology readiness assessment of Small Modular Reactor (SMR) designs Zhitao Liu*, Jihong Fan State Nuclear Power Technology Corporation, Beijing 100029, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 22 January 2013 Received in revised form 25 June 2013 Accepted 12 July 2013

Small Modular Reactor (SMR) is an emerging energy technology that meets the demand of safety, efficiency and sustainability. This paper reviews the representative SMR designs. Comparisons are made between each current SMR and its originating design, obtaining a summarization of the development course and the innovation features of each SMR version. To get a comprehensive understanding of SMR, this paper suggests a bidirectional assessment method. In the longitudinal direction, assessments focus on technology evolution, especially SMR’s approaches to safety and its responses to Fukushima accident. In the transversal direction, a breakdown of SMR leads to the detailed assessment of its systems and equipments, thus identifying the maturity and inadequacy of a certain SMR design. Based on empirical analysis, this paper suggests an approximate ten year effort be needed for the leading light water reactor (LWR) based SMR to solidify the final form and operate under the full range of commissioning conditions. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Small Modular Reactors (SMR) Technology Readiness Level (TRL) Nuclear power Clean energy

1. Introduction In recent years, Small Modular Reactors (SMR) have been attracting considerable attention around the world. SMR provides an energy option with low carbon emission, enhanced safety conviction, convenient construction and operation, which is becoming more and more evident especially in the Post-Fukushima era (OECD/IEA, 2012; Kessides and Kuznetsov, 2012). The term “small” generally refers to the reactors with an equivalent electric power less than 300 MW, while “modular” means a single reactor that can be grouped with other modules to form a larger nuclear power plant (Status of Small Reactor D, 2007; The U.S. Department of Energy’s Office of Nuclear Energy and February 15, 2011). SMR can repower aging fossil plants, be coupled with other energy sources, including renewable and fossil energy, to produce multiple energy end-products (Ingersoll, 2009; Aumeier et al., September 2011). Concept development and design activities of SMR are progressing by universities, institutes and reactor vendors (Vujic et al., 2012). Tens of SMRs at various development stages can be

* Corresponding author. Building No. 1, Compound No. 29, North 3rd Ring, Xicheng District, Beijing, China. Tel.: þ86 10 5819 7732; fax: þ86 10 581970667732. E-mail addresses: [email protected], [email protected] (Z. Liu). 0149-1970/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pnucene.2013.07.005

categorized as light water reactors (LWR), gas cooled reactors, liquid metal cooled reactors, and molten salt reactors (Lommers et al., 2012; Baxi et al., 2008; The Gen4 Module (G4M). ht; Triplett et al., 2012; Holcomb et al., 2012; Singh et al., 2011; Kim et al., September 2012; Park, 10e14 October 2011a,b). In the category of LWR-based designs, main examples include mPower from Babcock and Wilcox (Halfinger Jeff and Haggerty Michael, 2012), W-SMR from Westinghouse (Fetterman et al., 2012), NuScale from Oregon State University (Reyes and Lorenzini, 2012), and institutions from China have reported the up-to-date SMR progress such as HTR-PM and ACP-100 (M. Hadid Subki, 5e9 December 2011). The diversity of the SMR concepts provides a good flexibility of selection, but the investors may be dazzled to decide which SMR design has the best maturity and prospect for commercial deployment. The regulatory authorities may be engaged in identifying the safety risks to response to public perceptions and establish licensing process. The technicians may be dedicated to resolve the problems SMRs suffered from decreased thermodynamic efficiencies and neutron economy compared to large reactors. The published papers and reports endeavored to introduce the details of a specific SMR design or analyze the economic competitiveness compared to large reactors. This abundant information is very beneficial to form a macroscopic view on SMR. But to address the concerns and problems above, it is more necessary and important

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to make an assessment on the technology maturity, reliability and complexity. Based on these considerations, this paper presents the analysis and recognition of technological readiness level (TRL) of SMR development. 2. Main SMR design options A comprehensive summarization of the SMR concepts could be found in a series of international organization reports (International Atomic Energy Agency, 2009; Small Nuclear Power React, 2012), while this paper reviews only currently active designs that have some level of industry involvement which imply a potential of near-term commercialization. It should be noted that LWR-based SMRs are considered to be the most mature and have the lowest technical risk. Today there are 356 LWR nuclear power units in operation with a net electricity capacity of 328 072 MWe, 88% of the total nuclear power plant installed capacity (Operational & Long-Term S et al., 2012). The LWR-based SMRs incorporate these existing LWR experiences, with innovative technologies and some novel components. The design options below are all LWR-based SMRs. Some LWR-based SMRs are not mentioned in this paper either due to the lack of technology maturity or the limited pubic information for intellectual rights protection. 2.1. mPower The mPower reactor was unveiled by Babcock & Wilcox (B&W) in June 2009 (Nuclear Engineering International, 11 June 2009). The reactor had a planned capacity of 125 MWe when originally announced, and the reactor’s rated capacity was raised to 530 MWt of thermal power and 180 MWe of electrical power after its preapplication design certification interaction to the U.S. Nuclear Regulatory Commission (NRC) (US.NRC, July 10, 2012). The mPower reactor is a direct descendent of the B&W maritime reactor program, which was used in the nuclear powered merchant ship Otto Hahn that had been successfully launched in 1964 (Simpson, 1995). Key features of the Otto Hahn reactor design are incorporated in the B&W mPower reactor design, including: 1) The placement of nuclear steam supply system (NSSS) components within a single pressure vessel. Control rod drives do not penetrate the integral reactor vessel (IRV), but are instead wholly enclosed within the IRV. 2) The integral once-through steam generator is an advanced derivative of the steam generators used in older B&W designs. The mPower is designed to produce superheated steam and does not require steam separators and dryers prior to admitting steam into the high pressure turbine. The safety feature profits from the integral design of the reactor vessel. As it contains the entire primary coolant loop within the reactor pressure vessel with automatic primary loop depressurization, the integral reactor vessel does not have large cold or hot leg piping thus the potential of large break loss of coolant accidents is eliminated. As no electrically driven pumps are required, heat removal can be used in the event that these systems are exhausted by flooding the containment and establishing natural circulation. Passive safety concept is adequately utilized since the heat power is much smaller and the relative cooling ability could be enhanced. There are water supplies located above and within the containment that can cool the vessel with gravity driven-cooling if secondary cooling is lost (Fig. 1). 2.2. Westinghouse SMR The Westinghouse SMR (WSMR) was declared to be launched in February 2011 (POWERnews and February 23, 2011). This reactor

Fig. 1. Cutaway of B&W mPower reactor (Ferrara, August 10, 2012).

power is described as 800 MWt and the electrical output could be more than 225 MWe (Robert et al., 2011). The design uses many of the key features from the Westinghouse AP1000 plant. Passive safety systems are conceptually similar to the AP1000 plant, such as high head injection from core makeup tanks, ultimate transition to sump recirculation and decay heat removal by closed cycle natural circulation of heat exchangers. And the nuclear design is based on the proven product. The core of WSMR is made up of 89 Robust Fuel Assemblies (RFA) which have operated in 252 fuel cycles at 50 plants worldwide since 1997. The WSMR is compact, only 32 feet in containment vessel diameter, approximately 25 of these vessels will fit within the containment vessel of AP1000. But WSMR is not simply a smaller version of AP1000. As a modular reactor, all of the components associated with the reactor coolant system are contained within a single pressure vessel, thus eliminating large piping to connect the system components. The maximum diameter of the reactor is held to less than 12 feet to ensure that it can be shipped via standard rail package. The primary equipments are innovative designs to meet the demands of system integration. The steam generator is a straight tube configuration with the primary reactor coolant passing through the inside of the tubes and the secondary coolant on the outside. In this design, primary reactor coolant flow is directed vertically downward through the inside of the tubes where heat is transferred to the secondary fluid. Secondary flow enters the steam

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generator shell as a sub-cooled liquid and exits as a saturated steam mixture, where it is directed to the steam drum for moisture separation. The moisture separation equipment in the drum was selected and sized to produce dry steam conditions for input to the turbine for power generation. A bolted flange near the centre of the integral reactor assembly allows for the steam generatorepressurizer assembly to be removed, allowing access to the fuel during a refueling outage. The plant structures are arranged to provide strong defense under various conditions. The containment is designed to be submerged in water during normal operation, and it will also withstand relatively high internal and external pressure loadings. Above-grade floors will house equipment that is not subject to radionuclide releases. The spent fuel pool is located below grade to protect against external threats (Fig. 2). 2.3. NuScale NuScale is an integral pressurized-water reactor (iPWR), designed by NuScale Power, LLC. The design is based on MASLWR (Multi-Application Small Light Water Reactor) developed at Oregon State University during 2000e2003. Each NuScale SMR has a rated thermal output of 160 MWt and electrical output of 45 MWe (U.S. NRC, April 19, 2012). The maturity of design could be expressed as followed: 1) Nuclear fuel is a. 2) The NuScale design relies on well-established LWR technology, including a standard LWR fuel in 17  17 configuration, proven codes and methods, and existing regulatory standards. It is supported by a one-third scale, electrically heated integral test facility which operates at full pressure and temperature, which will

support the licensing within the existing LWR regulatory framework. A high degree integration of the NuScale design is reflected in the simplification of equipment, even the cancellation of some traditional key equipment. Reactor coolant pumps are eliminated in Nuscale design (Reyes, 5e9 December 2011). The nuclear core is cooled entirely by natural circulation during normal operation as well as transient or accident conditions. Water is heated in the nuclear core to produce a low density fluid that travels upward through the hot leg riser. The density difference acting over an elevation difference results in a buoyancy force that drives the fluid flow around the loop. A once-through steam generator contributes to the intensification of NSSS in mPower, WSMR as well as in NuScale design. The steam generator of NuScale is a helical-coil, once-through heat exchanger located in the annular space between the hot leg riser and the reactor vessel’s inside wall. Feedwater enters the tubes at the bottom and superheated steam exits at the top. Two redundant, independent sets of steam generator tube banks occupy the steam generator region. Each NuScale module includes two redundant passive safety systems to provide pathways for decay heat to reach the containment pool, the decay heat removal system (DHRS) and the containment heat removal system (CHRS). These systems do not require external power for actuation. To practically eliminate radiation release, NuScale has seven layers of barriers between fuel and environment. Besides fuel pallet and cladding, reactor vessel, and containment in conventional nuclear plants, it adds water in reactor pool, stainless steel lined concrete reactor pool, biological shield covers each reactor, and reactor building as release defense (Fig. 3). The design parameters of several SMRs mentioned above are summarized in Table 1. 3. Technology evolution The SMR design options listed above are some of the diversified designs worldwide in recent years. The assessment of the SMR

Fig. 2. Containment of WSMR (Blinn and June 12, 2012).

Fig. 3. NuScale Reactor Module (Ingersoll and June 12, 2012).

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Table 1 Key design parameters of several SMRs. mPower

Westinghouse SMR

NuScale

Thermal/Electric output, MW Thermal dynamic type/Efficiency

400/125 Indirect rankine cycle/31%

800/225 Indirect rankine cycle/28%

Primary coolant/Circulation mode Primary pressure, MPa Core inlet/Outlet temperatures,  C Core diameter  height, mm Fuel type/initial enrichment, % Burn-up cycle duration, equivalent full power days Average bun-up of discharge fuel, MWday/kg Mode of reactivity control in operation

Light water/Forced 13.1 297/321 2000  2030 UO2/5% 235U 1644

Light water/Forced N/A N/A (N/A)  2400 UO2/<5% 235U 700

160/48 Indirect rankine cycle on superheated steam/30% Light water/Natural 10.7 247.9/288.9 N/A, reduced height core UO2/3e4% 235U 732

40

70

62

- Mechanical control rods with internal drives; - No liquid boron 3600  22 000

- Mechanical control rods with internal drives; - Liquid boron 3500  24 700

- Mechanical control rods with external drives; - Liquid boron 2740  13 176

5.7 163/300

N/A N/A

N/A N/A

Not specified; Air cooled condenser Based on the state of the art for PWR and marine reactors Cylindrical containment with spherical dome; Secondary containment provided by underground reactor building.

N/A OVATION-based Digital Control System Compact containment submerged in water, 9.8  27.1

Standard 45 MW turbine State-of-the-art PWR digital systems

Reactor vessel diameter  height, mm Secondary pressure, MPa SG secondary side inlet/outlet temperatures,  C Turbine type I&C system Containment type and dimensions, m

technologies will be important to make a full view of the status of the design, the gaps to the targets, the time and finance and other resources still be needed, and the risk of developing the technology. A scientific assessment of the SMR technology should be at least from two perspectives: 1) a longitudinal retrospection of the technology evolution, to identify the origin of a specific SMR design, the history of its development, and the main approaches to the enhanced safety, economy and operability. 2) a transverse analysis of the systems and components of the SMR design from a top-down method, to identify the maturity of the key systems and components which are based on proven technology or on entirely innovative design. A longitudinal retrospection of the technology evolution could be carried out as followed. 3.1. Balance of tradition and innovation The concept of SMR dates back up to 1960s (International Atomic Energy Agency, 1961). The current SMR designs mainly rooted from two origins: 1) marine-based power reactors, such as the mPower derived from the Otto Hahn marine reactor, 2) landbased electricity generation reactor, such as WSMR from AP1000. The design objectives of SMR are smaller power grade, smaller configuration size, smaller generation cost, and smaller operation risk. Consequently the SMR designs absorb the advantages of the existing marine-based and land-base reactors, meeting the design objectives through progressive or significant innovation. 3.2. Approaches to safety In SMR designs, the defense in depth strategy is used as in larger reactor designs to protect the public and environment from accidental releases of radiation. The main goal is to prevent or eliminate as many accident initiators and accident consequences as possible. Certain common measures of SMR lead themselves to safety are, relatively smaller core sizes enabling integral coolant system layouts, larger reactor surface-to-volume ratios, lower core power densities et al. The intended outcome is greater plant simplicity

Deep vacuum compact containment submerged in water pool, 4.570  18.290

with high safety levels and possibly reduced emergency response requirements. 3.2.1. Main methods for safety The basic consideration is to prevent accident imitators. 1) Integral pressurized water reactor design. The NuScale, mPower, and W-SMR designs use an integral pressurized water reactor (iPWR) design in which most of the primary system components are contained within a single vessel. The integral design reduces the number and size of penetrations and welding links through the reactor pressure vessel, eliminating the high-consequence accident scenario of a large pipe-break LOCA. In an iPWR the maximum size pipe penetrating the reactor vessel is 5e7 cm in diameter, while in a large PWR pipes that connect the reactor vessel to the external steam generator vessels are 80e90 cm diameter. SMR focus it safety functions on proper cooling of reactor core in case of accidents through following methods. 2) Increased relative coolant inventory. An enlarged vessel yields a larger inventory of water per unit of power than in the looptype plant, which increases the relative thermal inertia within the reactor vessel. This result in a reduction in the rate at which the system temperature increases during a loss of forced flow transient, providing the operators with more time to respond to an upset condition. 3) Increased relative heat transfer area. A simple calculation could reveal that relative surface area of the iPWR vessel per unit power is increased. Roughly speaking, if a diameter of a SMR reactor core is 1/n of a large reactor, then the relative surface area of reactor vessel per unit power could be n times of a large reactor. 4) Increased passive cooling capability. The vessel height-todiameter ratio of a SMR is 2e3 times larger than that of a large reactor since more equipments are incorporated vertically inside the vessel. This increases gravity-driven natural convection circulation capability. In the NuScale design, the natural circulation driving force is designed to be sufficiently strong to be used as a core cooling mechanism for full power operation, thus eliminating the need for pumps entirely.

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On prevention of radiation release, SMRs have following measures. 5) Smaller radionuclide inventory. The radionuclide inventory in a reactor core is roughly proportional to power level. In addition to the intrinsically smaller radionuclide inventory of an SMR, some SMR designs add additional barriers to fission-product release to achieve a dramatically smaller accident source term. 6) Under-ground construction. The smaller plant footprint of an SMR makes it more economically viable to construct the primary reactor system fully below ground level, which significantly hardens it against external impacts such as aircraft or natural disasters. As an example, the WSMR design has a containment vessel volume that is more than 23 times smaller than the Westinghouse AP-1000 containment. Below-grade construction of the reactor and containment vessels also provides the potential for additional seismic resistance and helps reduce the number of paths for fission-product release in the event of an accident.

3.2.2. Measures responded to Fukushima accident The Fukushima disaster revealed deficiencies in the design of nuclear power plants. The SMR designs have their protection measures against Fukushima accident (IAEA, 5e9 December 2011). The measures are summarized as Table 2. 4. Systems and equipments SMR power plant is a set of systems and components, including reactor coolant system along with connected primary systems, fuel and core, engineered safety features, instrumentation and controls,

auxiliary systems, steam and power conversion systems. To get a full view of the technology readiness of a certain SMR design, it is necessary to make a transverse analysis of the constituent systems and equipments. In this respect, it will be helpful to refer to the Technology Readiness Assessment Guide (U.S. Department of Energy and Sep 15, 2011) released by U.S. DOE to indicate the maturity level of a given technology, as defined in Table 3. 4.1. Reactor coolant system and connected primary systems The Reactor Coolant System consists of major components other than the reactor, such as steam generator, reactor coolant pump, pressurizer, and related pipings linking the equipments. For SMRs, the structure and functions of pressurizer and pipings are relatively simple and they are mostly settled into the integrated pressure vessel. So they will not be discussed specially in this paper. 4.1.1. Integral PWR One of SMR’s basic design considerations is to minimize the size of the plant configuration. The reactor coolant system of SMR listed in this paper all use integral PWR system. In fact, during the development of PWR in the past decades, three kinds of coolant systems could be summarized, that is, the disperse layout, the compact layout, and the integral layout. The disperse layout is a conventional layout. Different equipments of the reactor coolant system are connected by long piping. This not only occupies a large volume, but also increases the flow resistance. The compact layout use two-layer stub pipe instead of long pipe to connect the equipments. So they are closer to the reactor pressure vessel, with a space of only about 0.5 m. The integral design, the reactor core and steam generator are all installed in the RPV, and the pump and the

Table 2 Protection against Fukushima-type Events. Events and threats

mPower

WSMR

NuScale

Earthquakes and floods

 Deeply embedded reactor building dissipates energy, limits motion  Separated, waterproof reactor compartments address unexpected events  Safety-related DC power supports all accident mitigation for 72 h.  Auxiliary Power Units inside reactor building recharge battery system.  Space allocated for 7 days battery supply for plant monitoring/control.

 Seismic isolation is being considered to limit seismic effects on the containment vessel and equipment in the containment.  Features such as watertight doors and below-grade building are being implemented in the design.

 System is submerged in a pool of water below ground in an earthquake resistant building. Reactor pool attenuates ground motion and dissipates energy.  Deeply embedded reactor building provides flood protection. Passive cooling of reactor for 30 days with water followed by an unlimited period of air cooling

Station blackout

Containment integrity

Passive hydrogen recombiners are used to prevent explosions without need for power supply

Spent fuel pool integrity and cooling

 The structure is underground, inside reactor service building, located on base mat.  Large heat sink provides cooling for 30 more days before boiling and uncovering of fuel with 20 years of spent fuel.

 Redundant safety-related DC batteries support safe shutdown for 72 h.  Seven days of natural circulation core cooling is provided with no need for any AC power  Two redundant, small ancillary diesels are provided to supply essential plant indication to the operator. Ancillary diesels can run for four days with no operator support.  The 13/4 inch steel containment is a high integrity steel pressure vessel surrounded by a shield building and imbedded below grade for protection from projectiles, including aircraft impact.  Battery powered hydrogen igniters & passive hydrogen recombiners to prevent explosions.  Spent fuel pool is located below grade and contains heavily reinforced concrete structures lined with steel.  Spent fuel will remain covered with gravity flow of water from safety related Decay Heat Removal Tanks for a period of seven days.

Insulating vacuum to improve steam condensation rates during a LOCA by eliminating air, to prevent combustible hydrogen mixture in the unlikely event of a severe accident (i.e., little or no oxygen), to eliminate corrosion and humidity problems inside containment.  Below ground spent fuel pool is housed in a seismically robust reactor building and stainless steel refueling pool liners are independent from concrete structure to retain integrity.  More water volume for cooling per fuel assembly than current designs  Auxiliary external water supply connections are easily accessible to plant.

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Table 3 Technology Readiness Levels (TRL 6 to TRL 9). Technology readiness level

TRL definition

Description

TRL 9

Actual system operated over the full range of expected mission conditions.

TRL 8

Actual system completed and qualified through test and demonstration. Full-scale, similar (prototypical) system demonstrated in relevant environment Engineering/pilot-scale, similar (prototypical) system validation in relevant environment

The technology is in its final form and operated under the full range of operating mission conditions. Examples include using the actual system with the full range of wastes in hot operations. Technology is proven to work - Actual technology completed and qualified through test and demonstration. Represents a major step up from TRL 6, requiring demonstration of an actual system prototype in an operational environment. Representative model or prototype system, which is well beyond that of TRL 5, is tested in a relevant environment. Represents a major step up in a technology’s demonstrated readiness. Examples include testing a prototype in a high-fidelity laboratory environment or in simulated operational environment.

TRL 7 TRL 6

pressurizer are mounted on the RPV. The primary piping is eliminated. This design decreases the volume of the system and related shielding. It will be favorable for the construction of SMR in large scale. But meanwhile it should be noted that the higher integration of the primary system also poses the biggest challenge to SMR development. Since all the primary system equipments are integrated into one single vessel, the structure inside the RPV is more complicated. A component inside the RPV will be more prone to affect other components compressed in the same small RPV. The radiation from reactor core will be more intensive and therefore the SMR will need a high reliability of the quality of the welding, the tube material, and the water of the secondary system. The difficulty in equipment manufacture will turn to the assembling and

commissioning from forgings processing. And maintenance of such a compact structure could be more difficult. In fact, the concept of SMR starts at 1950’s. In the U.S, B&W improved the marine reactor into IBR, and designed the CNSG series (1\2\3\4\4A). CE designed UNIMOD at the same time. In Russia, they designed the compact reactor, such as KLT-3, KLT-4, and the integral reactor, such as ABV-6, ABV-6Y, and ABV-6M. In France, they developed independently the CAP series integral reactor, such as K-48, K-150. The output power of these reactors is relatively small. In the newly designed SMR for electricity generation purpose, the output power is promoted and the parameters such as inventory, pressure, temperature will change. Adaptive design will be needed based on the previous experience. The technology readiness level could be 8.

Fig. 4. Schematic diagram comparison of steam generator (IAEA-TECDOC-1668, 2011).

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4.1.2. Steam generator The steam generator is a heat exchanger transferring energy from reactor core to the turbines as well as a barrier isolating contaminations from the primary system to the secondary system. For SMRs, steam generator is an important equipment to guarantee that the plant is in reliable service. There are two kinds of steam generators as shown in Fig.4. Large reactors mostly use recirculating steam generators (RSG). While for SMRs, once-through steam generators (OTSG) are preferentially adopted. In an OTSG, the flow in the secondary system is forced circulated by feed-water pumps. Superheated steam is generated which both enhances the heat efficiency and simplifies the system since equipments such as moisture separators will not be needed. Straight-tube OTSG is an OTSG with vertical straight tube as heat exchangers. B&W incorporated its first straight-tube OTSG into NSSS at Oconee Unit1 in 1973. Straight-tube OTSGs have now been installed in Arkansas 1, Oconee 2, Oconee 3. The problem is that the absence of blow-down system makes the salinity deposit on the surface of heat transfer tubes, thus the requirements on tube materials and water quality will be more stringent. Helicoil-tube OTSG is another type of OTSG. More heat exchange surface could be achieved in certain space volume. And such a geometry caused a secondary flow in pipes, prompting heat exchange. Helicoil-tube OTSG was first equipped on nuclear icebreaker Lenin in 1958. The important issue of a helicoil-tube OTSG is the non-uniformity of the convective heat transfer. The excessive temperature deviation inside the steam generator will cause damage to heat transfer tubes. Two advanced gas cooled. reactors (AGR) at Heysham U.K. installed helicoil-tube OTSGs had to operate under low power conditions to protect the tubes. In general, the TRL of steam generator could be 7. 4.1.3. Primary coolant pump Reactor coolant pump is a piece of equipment to drive the coolant to circulate through the primary loop so that the heat generated in the core can be transferred to the steam generator. For large reactors, shaft seal pumps have been widely used. A shaft seal pump is composed of a routine squirrel cage induction motor routine with low cost and high pump efficiency. A heavy flywheel is fitted on the electric motor to provide enough coast-down inertia to maintain sufficient fluid flow to the reactor in the case of a loss of power. And the maintenance of the shaft seal pumps is convenient with only about 10 h to replace the shaft seal structure. For SMRs, canned motor pumps are used instead of shaft seal pumps (primary coolant pumps are even canceled in Nuscale reactor). A canned motor pump contains the motor and all rotating components inside a pressure vessel. The pressure vessel consists of the pump casing, thermal barrier, stator shell, and stator cap, which are designed for full reactor coolant system pressure. The stator and rotor are encased in corrosion-resistant cans that prevent contact of the rotor bars and stator windings by the reactor coolant. Because the shaft for the impeller and rotor is contained within the pressure boundary, seals are not required to restrict leakage out of the pump into containment. A gasket and canopy seal type connection between the pump casing, the stator flange, and the thermal barrier is provided. This design provides definitive leak protection for the pump closure. To access the internals of the pump and motor, the canopy seal weld is severed. When the pump is reassembled a canopy seal is rewelded. At least two advantages are obvious for canned motor pump’s application in SMR. Firstly, because it is lack of seal shaft and the supporting system, the structure of a canned motor pump could be more compact. Secondly, enhanced safety could be realized since seal shaft is a potential source of coolant leakage.

Canned motor pumps have a long history of safe, reliable performance in military and commercial nuclear plant service. CurtisseWright developed the canned motor pumps used in the first nuclear submarine, as well as in the first commercial nuclear power plant in 1957. And the world’s largest canned motor pump has been fixed in AP1000 nuclear power plant at Sanmen, China. Base on its application experience, the TRL of primary coolant pump could be 8 to 9. 4.2. Fuel and core design The common design goal of SMR fuel and core is higher burnup and longer lifecycles. A higher burnup value will lead to a more fully utilization of uranium resources. A longer refueling interval will decrease proliferation risks and lower chances of radiation escaping containment. In particular for reactors in remote areas, longer fuel life can be very helpful to make unattended operation a reality. The basic approach of current SMR fuel development is simplification of design and ease of licensing. Reactor fuel development is both time consuming and costly. Challenges come from clad oxidation, water chemistry, assembly strengthening, fission gas release, burnable poison, and manufacturing et al. So the shortcut is usually taken by refining on fuels with good operating experience. Fuel of WSMR is a derivative of the successful 17  17 Robust Fuel Assembly (RFA) design. Over 14 000 RFA assemlies have operated in 252 fuel cycles at 50 plants worldwide and lead test assemblies have operated to approximately 70 GWD/MTU. In February 2013, fabrication of prototype WSMR fuel assemblies for testing started at the Fuel Fabrication Facility in Columbia, SC. Results of the test will be submitted to the US NRC for design certification. Facilities to develop mPower fuel have been launched to develop and demonstrate key fabrication technologies, and serve for testing and licensing process. In April 2013, a test program for Nuscale fuel was successfully completed. A full-length, full-power, electrically-heated fuel assembly mock-up with spacer grids was tested for a wide range of natural circulation flow rates. The critical heat flux (CHF) data collected from the test will be used to define the limiting conditions for fuel performance. The design of SMR reactor core focuses on three issues: the core dimensions, the optimized core loading as well as effective fuel cycle, and the safety limits. These three issues are interactive, and a general equilibrium is pursued among them. The reactor core dimension is an essential parameter in SMR design. The diameter of the core is directly related to the radial size of the reactor pressure vessel, which itself is limited by the requirement to ship the vessel by rail car. The height of the core directly influences the length of the control rods and their drives and the reactor internals. Given the maximum core radial size, the active core height will then be selected to support the fuel cycle energy requirement with a reasonable number of feed assemblies each cycle, while also maintaining a relatively modest core height. The core loading design has to provide sufficient excess reactivity in the core to achieve the required cycle energy while the safety criteria are met. Most SMRs tend to use mechanical shim instead of chemical shim for reactivity control during normal operation. Such a method will probably lead to more localized parameters that in large reactors. For example, cold shutdown margin, the core axial offset evolution through the cycle showed strongly localized phenomenon in SMR reactors. One way to solve this problem is to increase the neutron poison composition in those adjacent locations. The fuel and core design is an iterative process between assembly design and the core management. It is relatively long process that may take years of efforts. So the TRL of fuel and core is close to 7.

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4.3. Engineered safety features Engineered safety features (ESF) are consisted in the nuclear plant to ensure a safe shutdown of the reactor, removal of core residual heat and preservation of containment integrity. In case of accidents, ESF will inject cooling water into the core for preventing core meltdown, depressurize the containment space for preventing radiation release, and keep the hydrogen from concentrating. The equipments of ESF are not so complicated. But whether the function of SMR ESF could efficiently behave is a vital issue. Many different safety features are involved in SMR design and related information could be found in Table 2 in this paper. These SMRs are being made using passive safety features and inherent safety features. Passive safety features are engineered, but do not require outside input to work. They only depend on physical laws. So SMRs in remote areas could operate safely. The most crucial system can be attributed to passive core cooling system. An effective cooling system will lead to a cold shutdown when accident happens. An SMR try to equip itself with passive safety features that draw heavily from the plant design that is certified by nuclear safety authorities. For example, the key components of the SMR passive core cooling systems are four core makeup tanks (CMTs), an in-containment pool (ICP) and associated ICP tanks, an automatic depressurization system (ADS), an outside containment pool (OCP) and two ultimate heat sink (UHS) tanks. Integrated into the CMTs are passive residual heat removal heat exchangers. During an event, the WSMR relies on the natural forces of gravity and convection to shutdown and maintains the plant in a safe condition. Both of these short-term and long-term reactivity and decay heat control strategies for the WSMR are similar to those of the AP1000 plant. But after all there are differences between them as shown in Table 4 (Matthew et al., 2012). And test will still be necessary to prove the validity of the new system. So the TRL of engineered safety features could be 8. 4.4. Instrumentation and controls SMR instrumentation and controls (I&C) technology encompasses many technical aspects that include measurement, diagnostics and prognostics, controls and plant operations, and operational infrastructures (Clayton et al., 2010). The integral nature of SMR brings about several measurement technology differences from that of large reactors. Measurement of certain key parameters of SMR can best be accomplished using invessel sensors that require development and demonstration. These in-vessel measurement capabilities include flux/power, primary flow, reactor coolant system temperatures, primary water inventory, steam generator water inventory, and steam generator stability, etc. Additionally, SMR will operate with extended operational cycles and longer maintenance intervals. Sensors developed for SMR plants must be capable of long-term operation without

Table 4 WSMR and AP1000 passive core cooling system. Function

AP1000

Westinghouse SMR

Short term reactivity controls Long-term reactivity controls Decay heat removal

Control rods

Control rods

2 CMTs

4 CMTs

1 PRHR/PCS 1 IRWST/Sump

4 CMTs w/integral heat exchangers 2 ICP tanks/sump

PCS (72 h)

2 UHS tanks (72 h each)

Long-term makeup water supply Ultimate heat sink

27

requiring maintenance intervention to address drift or degradation due to harsh environmental stress. Advanced diagnostics and prognostic systems in SMR are closely related to extend the interval between maintenance outages through better equipment monitoring, reduce labor demands for equipment surveillance, and reduce risks through clearer understanding of equipment conditions and failure margins. It is necessary to validate prognostic capabilities to provide optimal plant operation with high reliability. SMR objectives for reduced staffing imply highly automated plant control with greatly reduced reliance upon on-site highly skilled staff for interactive operational control under normal conditions and immediate intervention for event management. Highly automated, intelligent control capabilities have not been demonstrated for nuclear power plant operations and there is limited experience in other application domains. Thus, an investigation of the state-of-the-art for control technology is necessary. Given the identification of the state-of-the-art and an understanding of the SMR requirements and application constraints, a definition of the needed degree of automation and intelligence can be developed in the context of near-term and long-term capabilities. Advanced I&C in SMR will bring benefits to the construction and operation of the plants. Reducing cable installations using wireless or shared wire communications will reduce construction cost of a SMR plant. Using remote handling techniques will make it feasibly to conduct some routine maintenance between scheduled shutdowns. But many components of this I&C system are not commercially available at present, all necessary components still needs engineering development efforts. So the TRL of engineered safety features could be 7. 4.5. Auxiliary systems, steam and power conversion system The auxiliary systems include primary and secondary water supply and treatment systems, chemical and volume control systems, component cooling systems, a refueling system, and spent fuel storage pools. The steam and power conversion system include the main steam system, the main turbine generator system, main condenser, condenser evacuation system, turbine gland seal system, turbine bypass system, extraction steam system, condensate cleanup system, and the condensate and feedwater pumping and heating system. There is little special requirement regarding to the SMRs. And the designs and equipments of these systems are very mature. 5. Conclusion SMR is a novel concept combining fresh ideas, varying inventiveness and extensive heritage from existing technology. LWR based SMR designs are leading technologies as a result of the deep understanding of the physical phenomena and abundant experience in equipment fabrication as well as plant operation. Since the power per unit is relatively smaller, the major equipment could be much smaller than that in large reactors. Consequently the fabrication, transport and installation of the equipment could be simplified. Long lead item procurement usually significantly influences the mainline duration of plant construction. In a SMR plant, long lead item procurement will save time, contributing to a shorter construction schedule. And at a certain site, to achieve an equal output power, more SMR modules will be constructed that large unit. It will make fuller use of sale scale effects, learning effects and localization effects. SMR will win a competitive prospect. Currently there is no SMR plant in construction or in commission. The biggest challenge in the development of SMR is the higher integration of the primary system. Efforts will be needed on the following aspects: 1) to develop and test materials working in

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harsh environment; 2) to study the integrated components manufacturing techniques; 3) to develop advanced instrumentation and controls; 4) to complete the verification and validation through single effect tests and integrated tests. These efforts require a lot of investment. And international exchanges and cooperation is beneficial to accelerate development process. Up till now, mPower’s full-fidelity integrated systems test (IST) facility is operational, WSMR is expected to file its design certification application to the NRC in the second quarter of 2014, NuScale in underway in Design Control Document submittal to U.S. NRC. On account of the TRL analysis in the preceding paragraphs, fundamental systems of SMR have reached TRL7 to 8. On account of experience in the development of large reactors, the integral effect test will take about 3 years to complete facility establishment and obtain the experimental verification results, the detailed design for a certain first-of-a-kind (FOAK) project will take about 2 years from solidify design input, the design certification will take 3 years for safety assessment. And it will take 2 years for the plant to work in full power conditions to test the system operation. These procedures could overlap, but some uncertainty factors could happen delaying the schedule. It is estimated that TEN years will be indispensable to lift SMR to TRL9, and by then a more confident option will be provided to meet the future energy demands.

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