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General aspects[edit]

Flexibility of SMR[edit]

SMRs offer significant advantages over conventional style nuclear reactors due to the flexibility of their modular design. Flexibility in the capabilities of SMRs offers advantages, incremental load capacity, ability for adaptation to current nuclear powerplant sites, utilization for industrial applications, improved operating time, and finally the ability to be “grid independent”. [1]

Technologies[edit]

Cooling[edit]

Conventional reactors use water as a coolant.[2] SMRs may use water, liquid metal, gas and molten salt as coolants.[3][4] Coolant type is determined based on the reactor type, reactor design, and the chosen application. Large-rated reactors primarily use light water as coolant, allowing for this cooling method to be easily applied to SMRs. Helium is often elected as a gas coolant for SMRs because it yields a high plant thermal efficiency and supplies a sufficient amount of reactor heat. Sodium, lead, and lead-bismuth are common liquid metal coolants of choice for SMRs. There was a large focus on sodium during early work on large-rated reactors which has since carried over to SMRs to be a prominent choice as a liquid metal coolant.[5] SMRs have lower cooling water requirements, which expands the number of places a SMR could be built to include remote areas such as mining and desalination.[6]

Staffing[edit]

The NuScale reactor plant uses a human-system interface which features an all-digital control room that allows for the monitoring and control of multiple reactor units from a single main control room. Passive safety systems, automation, and fail-safe design features allow control room staff to be optimized while maintaining a safe operating environment for employees. The interface uses high levels of automation to decrease human error and permit operators to perform more advanced operations. Additionally, NuScale has created a control room simulator to mimic the process of operating a NuScale SMR power plant. The simulator provides employees with the interactive experience of operating in the control room during the staffing certification process.[7]

Reactors such as the Toshiba 4S are designed to run with little supervision.[8]

Load following[edit]

SMR designs can provide base load power or can adjust their output based on demand.[citation needed] Another approach is to adopt cogeneration, maintaining consistent output, while diverting otherwise unneeded power to an auxiliary use.[which?]

District heating, desalination and hydrogen production have been proposed as cogeneration options.[9] Overnight desalination requires sufficient freshwater storage to enable water to be delivered at times other than when it is produced.[10] Membrane and thermal are the two principal categories of desalination technology. The membrane desalination process uses only electricity and is employed the most out of the two technologies. In the thermal process, the feed water stream is evaporated in different stages with continuous decreases in pressure between the stages. The thermal process primarily uses thermal energy and does not include the intermediate conversion of thermal power to electricity. Thermal desalination technology is further divided into two principal technologies: the Multi Stage Flash distillation (MSF) and the Multi Effect Desalination (MED).[11]

Safety[edit]

Coolant systems can use natural circulation – convection – to eliminate pumps that could break down. Convection can keep removing decay heat after reactor shutdown.

Negative temperature coefficients in the moderators and the fuels keep the fission reactions under control, causing the reaction to slow as temperature increases.[12]

Some SMRs may need an active cooling system to back up the passive system, increasing cost.[13] Additionally, SMR designs have less need for containment structures.[14]

Some SMR designs bury the reactor and spent-fuel storage pools underground.

Smaller reactors would be easier to upgrade.[15]

SMRs maintain core cooling with a passive safety system which eliminates the need for pressure injection systems. With a passive safety system, emergency AC power sourced from a diesel generator is not required for core cooling. A passive safety system is simpler, requires less testing, and does not lead to inadvertent initiation. SMRs do not require an active containment heat system due to passive heat rejection out of containment and a containment spray system is not required. An emergency feedwater system in not necessary for SMRs, allowing for core heat removal and enhancing safety.[16]

Some SMRs may need an active cooling system to back up the passive system, increasing cost.[17] Additionally, SMR designs have less need for containment structures.[18]

SMRs featuring water and sodium coolants increase reactor safety through their ability to withhold byproducts of the fissile fuel introduced into the coolants during a sever accident. This characteristic of a SMR allows for the ability of a SMR to mitigate the release of fissile material, contaminating the environment, in the event of a failure to maintain containment of nuclear material occurred.[5]

Some SMR designs feature an integral design of which the primary reactor core, steam generator and the pressurizer are integrated within the sealed reactor vessel. This integrated design allows for the reduction of a possible accident as radiation leaks can easily be contained. In comparison to larger reactors having numerous components outside the reactor vessel, this feature drastically increases the safety by decreasing the chance of an uncontained accident. Furthermore, this feature allows many SMR designs bury the reactor and spent-fuel storage pools underground at the end of their service life therefore increasing the safety of waste disposal.[1]

Smaller reactors would be easier to upgrade.[19]

Flexibility of SMR[edit]

Small Nuclear Reactors in comparison to conventional nuclear power generation plants offer many notable technological advancements due to the flexibility of their modular construction.[1] This flexibility in the modularity of a SMR system allows for additional units to be incrementally added in the event load on the grid increases. Additionally, this flexibility in a standardized SMRs design revolving around modularity allows for rapid production at a decreasing cost following the completion of the first reactor on site.[1][20]

The flexibility and modularity of SMR allows this form of power generation to be installed at existing powerplants; therefore, allowing for SMRs to supply additional energy to the aging grid of fossil fuel power plants with an easy adaptation to the existing grid structure. Modularity of a SMR plant allows for “a single site can have three or four SMRs, allowing one to go off-line for refueling while the other reactors stay online”.[1]

The flexibility of SMRs provides additional opportunities for industrial usage through saving energy lost through the transfer of energy from thermal to electrical. Applications for a SMR under these conditions of direct energy transfer include “desalination, industrial processes, hydrogen production, oil shale recovery, and district heating” of which a conventional large reactor is not capable.[1][21]

Economics[edit]

A key driver of interest in SMRs is the claimed economies of scale in production as they can be manufactured in an offsite factory. Some studies instead find the capital cost of SMRs to be equivalent to larger reactors.[22] Substantial capital is needed to construct the factory. Amortizing that cost requires significant volume, estimated to be 40–70 units.[23]

Compared to the total cost of offshore wind, solar thermal, biomass, and solar photovoltaic electricity generation plants, the total cost of using SMRs for electricity generation is significantly lower.[16]

When comparing SMRs with Large Reactors, however, the unique characteristics of SMRs that should compensate for the lack of the economy of scale should also be considered, although no SMR design presents all of them. Given the lower capacity, these characteristics will increase the demand for construction sites to obtain the same power of a Large Reactor, but will in itself not increase the demand for nuclear power plants.[24] Financial and economic issues can hinder SMR construction.[25]

Construction costs per SMR reactor are claimed to be less than that for a conventional nuclear plant, while exploitation costs may be higher for SMRs due to low scale economics and the higher number of reactors. Staffing costs per unit output increase as reactor size decreases, due to fixed costs. SMR staff costs per unit output can be as much as 190% higher than the fixed operating cost of large reactors.[26] Modular building is a very complex process and there is "extremely limited information about SMR modules transportation", according to a 2019 report.[27]

A production cost calculation done by the German Federal Office for the Safety of Nuclear Waste Management (BASE), taking into account economies of scale and learning effects from the nuclear industry, suggests that an average of 3,000 SMR would have to be produced before SMR production would be worthwhile. This is because the construction costs of SMRs are relatively higher than those of large nuclear power plants due to the low electrical output.[28]

In 2017 an Energy Innovation Reform Project study of eight companies looked at reactor designs with capacity between 47.5 MWe and 1,648 MWe.[29] The study reported average capital cost of $3,782/kW, average operating cost total of $21/MWh and levelized cost of electricity of $60/MWh.

Energy Impact Center founder Bret Kugelmass claimed that thousands of SMRs could be built in parallel, "thus reducing costs associated with long borrowing times for prolonged construction schedules and reducing risk premiums currently linked to large projects."[30] GE Hitachi Nuclear Energy Executive Vice President Jon Ball agreed, saying the modular elements of SMRs would also help reduce costs associated with extended construction times.[30]

Licensing[edit]

A major barrier to SMR adoption is the licensing process. It was developed for conventional, custom-built reactors, preventing the simple deployment of identical units at different sites.[31] In particular the US Nuclear Regulatory Commission process for licensing has focused mainly on conventional reactors. Design and safety specifications, staffing requirements and licensing fees have all been geared toward reactors with electrical output of more than 700MWe.[32] With a sizable focus on large reactors, it is probable that many countries will have to adapt their policies to coincide with SMRs, which can be a costly and time-consuming process. The International Atomic Energy Agency has placed emphasis on creating a central licensing system for SMRs to ensure proper guidelines in the interest of overall public safety.[33]

SMRs caused a reevalution of the licensing process. One workshop in October 2009 and another in June 2010 considered the topic, followed by a Congressional hearing in May 2010. Multiple US agencies are working to define SMR licensing. However, some argue that weakening safety regulations to push the development of SMRs may offset their enhanced safety characteristics.[34][35]

The U.S. Advanced Reactor Demonstration Program was expected to help license and build two prototype SMRs during the 2020s, with up to $4 billion of government funding.[36]

Nuclear Proliferation[edit]

Nuclear proliferation, or the use of nuclear materials to create weapons, is a concern for small modular reactors. As SMRs have lower generation capacity and are physically smaller, they are intended to be deployed in many more locations than conventional plants.[37] SMRs are expected to substantially reduce staffing levels. The combination creates physical protection and security concerns.[38][39]

Many SMRs are designed to address these concerns. Fuel can be low-enriched uranium, with less than 20% fissile 235
U. This low quantity, sub-weapons-grade uranium is less desirable for weapons production. Once the fuel has been irradiated, the mixture of fission products and fissile materials is highly radioactive and requires special handling, preventing casual theft.

Contrasting to conventional large reactors SMRs can without difficulty be adapted to be installed in a sealed underground chamber; therefore, “reducing the vulnerability of the reactor to a terrorist attack or a natural disaster”.[1] New SMR designs enhance the proliferation resistance, such as those from the reactor design company Gen4.These models of SMR offer a solution capable of operating sealed underground for the life of the reactor following installation.[1][40]

Some SMR designs are designed for one-time fueling. This improves proliferation resistance by eliminating on-site nuclear fuel handling and means that the fuel can be sealed within the reactor. However, this design requires large amounts of fuel, which could make it a more attractive target. A 200 MWe 30-year core life light water SMR could contain about 2.5 tonnes of plutonium at end of life.[39]

Furthermore many SMRs offer the ability to go periods of greater than 10 years without requiring any form of refueling therefore improving the proliferation resistance as compared to conventional large reactors of which entail refueling every 18-24 months[1]

Light-water reactors designed to run on thorium offer increased proliferation resistance compared to the conventional uranium cycle, though molten salt reactors have a substantial risk.[41][42]

SMR factories reduce access, because the reactor is fueled before transport, instead of on the ultimate site.[citation needed]

Proposed sites[edit]

Canada[edit]

In 2018, the Canadian province of New Brunswick announced it would invest $10 million for a demonstration project at the Point Lepreau Nuclear Generating Station.[43] It was later announced that SMR proponents Advanced Reactor Concepts[44] and Moltex[45] would open offices there.

On 1 December 2019, the Premiers of Ontario, New Brunswick and Saskatchewan signed a memorandum of understanding [46] "committing to collaborate on the development and deployment of innovative, versatile and scalable nuclear reactors, known as Small Modular Reactors (SMRs)."[47] They were joined by Alberta in August 2020.[48] With continued support from citizens and government officials have led to the execution of a selected SMR at the Canadian National Nuclear Laboratory.[5]

In 2021 Ontario Power Generation announced they plan to build a BWRX-300 SMR at their Darlington site to be completed by 2028. A licence for construction still had to be applied for.[49]

References[edit]

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  40. ^ Ingersoll, D.T. (2009). "Deliberately small reactors and the second nuclear era". Progress in Nuclear Energy. 51 (4–5): 589–603. doi:10.1016/j.pnucene.2009.01.003. ISSN 0149-1970.
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  46. ^ "COLLABORATION MEMORANDUM OF UNDERSTANDING" (PDF). Government of Ontario. Retrieved 2 December 2019.
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  49. ^ OPG chooses BWRX-300 SMR for Darlington new build. WNN, 2 Dec 2021
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