|Small Modular Reactors (SMRs)
Excerpts from the World Nuclear Association Website
As nuclear power generation has become established since the 1950s, the size of reactor units has grown from 60 MWe to more than 1600 MWe, with corresponding economies of scale in operation. At the same time there have been many hundreds of smaller power reactors built both for naval use (up to 190 MW thermal) and as neutron sources, yielding enormous expertise in the engineering of small units. The International Atomic Energy Agency (IAEA) defines 'small' as under 300 MWe, and up to 700 MWe as 'medium' – including many operational units from 20th century. Together they are now referred to as small and medium reactors (SMRs). This paper focuses on advanced designs in the small category, i.e. those now being built for the first time or still on the drawing board.
Today, due partly to the high capital cost of large power reactors generating electricity via the steam cycle and partly to the need to service small electricity grids under about 4 GWe, there is a move to develop smaller units. These may be built independently or as modules in a larger complex, with capacity added incrementally as required (see section below on Modular construction using small reactor units). Economies of scale are provided by the numbers produced. There are also moves to develop small units for remote sites.
Generally, modern small reactors for power generation are expected to have greater simplicity of design, economy of mass production, and reduced siting costs. Most are also designed for a high level of passive or inherent safety in the event of malfunctionc. A 2010 report by a special committee convened by the American Nuclear Society showed that many safety provisions necessary, or at least prudent, in large reactors are not necessary in the small designs forthcoming.
A 2009 assessment by the IAEA under its Innovative Nuclear Power Reactors & Fuel Cycle (INPRO) program concluded that there could be 96 small modular reactors (SMRs) in operation around the world by 2030 in its 'high' case, and 43 units in the 'low' case, none of them in the USA SMR Research and Deployment Objectives SMRs have great potential but still have some hurdles that need to be addressed. The LWR designs, though simply are smaller versions of the larger Gen III designs still have components that have not been tested fully. Non-LWR designed under review by the United States Nuclear Regulatory Commission (NRC) will be furnished designed certification on a case-by-case basis. Other issues the SMRs face include the process for licensing prototypes, manufacturing requirements, and many financial issues which do not involve capital cost.
Presently, there are 7 SMR designs that are planned to be reviewed by the NRC. The LWR designs that plan to be submitted for review by 2012 include Westinghouse IRIS, Babcock & Wilcox’s (B&W) mPower and NuScale Power Inc.’s NuScale. The non-LWR designs that are planned to be submitted for review by 2012 are General Electric’s (GE) PRISM, Toshiba’s 4S, and PBMR Ltd PBMR by 2013. HPG’s Hyperion reactor is under review for a Combined License (COL) and design verification. These reactors, which have been referred to as Advanced Reactors by the U.S. NRC, are facing new policies by the U.S. NRC. The policies for nuclear reactors presently are to deal with traditional nuclear reactors, and need to be amended or changed for these SMR designs. Topics that need to be addressed are: Licensing Process; Design Requirement Issues; Operational and Financial issues.
Current SMR Designs
Light Water Reactors
Liquid Metal Reactors
- CAREM. A joint venture between the Argentinean National Atomic Energy Commission (CNEA) and INVAP, this PWR reactor is a 24 MWe (100 MWt) integral PWR. In its design, it contains helical steam generators, and no coolant pumps. It uses UO2 fuel enriched to 3.4 %. The refueling cycle appears to be about once a year.
- IRIS. IRIS has primary coolant pumps, through it still uses helical coil steam generators. It uses UO2 fuel enriched to 4.95% and it is estimated that refueling would need to take place every 3-3.5 years.
- KTL-40S. A Russian reactor design, originally intended for use in Russian icebreaker vessels, this version has been modified for use as a mobile floating power station. The reactor is a 35 MWe (100 MWt) PWR, with nuclear fuel enriched to 15.7% and an operational capability of 3-4 years between refueling. This reactor also has the capability to divert 35 MWt for process heat for other applications, such as desalination.
- mPower. Babcock & Wilcox (B&W) have designed an integral PWR rated for 125 MWe (400MWt). While much of the design remains proprietary, such as fuel type and enrichment, B&W claims that the reactor may function for up to 4.5 years between refueling. One interesting innovation is that the spent fuel remains in the spent fuel pool for the life of the reactor, which looks to be approximately 60 years.
- NuScale. The NuScale integral PWR is a 45 MWE (150 MWt) reactor, that contains helical coil steam generators, and is cooled through natural circulation. This reactor contains UO2 enriched to 4.95% with a refueling cycle of about 2 years. This reactor was developed in conjunction with Oregon State Unversity.
- SMART. System-integrated Modular Advance Reactor is an integral PWR developed by the Korea Atomic Energy Research Institute (KAERI), rated for 100 MWe (330 MWt). While there is no public information on fuel type or enrichment, KAERI claims that the refueling cycle will be one every 3 years, with a reactor total life of about 60 years. The reactor can also be used for process heat applications, such as desalination
- VK-300. The VK-300 is a Russian designed LWR, however instead of an integral PWR design, it is a BWR developed specifically for both electric power and process heat. It is rated for 250 MWe (750 MWt) and cools by natural convection. While there is no information on the type of fuel used, it is enriched to 3.6%, with a period of about a year between refueling.
Gas Cooled Reactors
- 4S. Toshiba’s Super Save, Small and Simple reactor is sodium cooled and rated for 10 MWe (30 MWt). The fuel is a U-Zr Alloy enriched to 19.9%. It consisted of a primary and secondary sodium loop, each using an electromagnetic pump to circulate the sodium coolant. The steam generator consists of a helical coil, and operation between refueling cycles appears to be about 30 years.
- BREST. Based off of the NIKEIT reactor, this Russian design is a lead-cooled LMR, rated for 300 MWe (700 MWt). Its fuel is a mix of U-N and Pu-N, and needs to be refueled once a year. Although the parent company AtomEnergoProm does not refer to the fuel’s enrichment, it plans to release two models, consisting of the 300 MWe and a larger 1200 MWe version.
- Hyperion. The brainchild of Dr. Otis Peterson, the Hyperion Power Module is a lead-bismuth cooled reactor, capable of producing 25 MWe (70 MWt). It uses U-N with an enrichment of less than 20%, though no specific amount is given. Its operational time is 7-10 years, after which time it is removed and taken back to the factory. The module is replaced with a new one, and can continue operation.
- PRISM. A designed based off EBR-II, the Power Reactor Innovative Small Module, designed by GE-Hitachi, is a sodium cooled LMR rated for 311 MWe (840 MWt). While it is undisclosed, the each type of fuel or enrichment, it is metallic fuel type and can operate for 1-2 years in between refueling. It uses passive air cooling for the primary system.
- SVBR-100. Another Russian design, the SVBR-100 is a lead-bismuth cooled reactor rate for 75 MWe (286 MWt). It uses UO2, enriched to 15.6%, and has an operational period of about 8 years in between refueling. The design is based off experience from Russia’s submarine reactors.
- SFR. The Sodium-Cooled Fast Reactor (SFR) system features a fast-spectrum, sodium-cooled reactor and a closed fuel cycle for efficient management of actinides and conversion of fertile uranium. The SFR is designed for management of high-level wastes and, in particular, management of plutonium and other actinides. Important safety features of the system include a long thermal response time, a large margin to coolant boiling, a primary system that operates near atmospheric pressure, and intermediate sodium system between the radioactive sodium in the primary system and the power conversion system. Water/steam and carbon-dioxide are being considered as the working fluids for the power conversion system in order to achieve high-level performances in thermal efficiency, safety and reliability. With innovations to reduce capital cost, the SFR can serve markets for electricity. The small SFR is between 50 and 150MWe that is a modular-type sodium-cooled reactor employing uranium-plutonium-minor-actinide-zirconium metal alloy fuel, supported by a fuel cycle based on pyrometallurgical processing in facilities integrated with the reactor. The outlet temperature is approximately 550 degrees celsius.
- PBMR. South Africa designed and researched, the Pebble Bed Modular Reactor, and is a gas-cooled reactor that used UO2 particles coated with a TRISO process containing layers of silicon carbide and graphite matrix. The graphite spheres are then loaded into the reactor. The last design was rated for 80 MWe (200 MWt). Presently the enrichment of the UO2 is 9.6%, with operational tines of about 3 years in between refueling. Further development on the PBMR has slowed since investment capital has been lost for the project, however, the South African Utility Company Eskom has partnered with Mitsubishi Heavy Industries to continue development of the reactor.
- GT-MHR. General Atomics Gas Turbine-Modular Helium Reactor is a gas-cooled reactor aimed at increased thermodynamic efficiency, through a Brayton cycle turbine. The present SMR design is rated for 25 MWe (52 MWt) at the predicted efficiency of 47%. The fuel consists of hexagonal elements placed in graphite blocks (for the larger 285 MWe version). In the smaller 25 MWe version the fuel is enriched to 20% and would need to be refueled every 6-8 years.
- HTR-PM. Though based off of the smallest HTR-10, China’s HTR-PM is a gas-cooled 200 MWe (450 MWt) reactor. Though the fuel type is unknown, the enrichment is thought to be about 9%, with a total operational lifetime of about 60 years at an 85% load factor.