1. Advantages and potential uses

2. Operation
    Cooling  Thermal/electrical generation  Staffing   Load following  

3. Waste reduction

4. Safety

5. Economics

6. Licensing

7. Non-proliferation

8. Reactor designs

9. Proposed sites
    United Kingdom  United States 

10. References

11. Further reading

12. External links

Small modular reactors (SMRs) are a type of nuclear fission reactor which are smaller than conventional reactors, and manufactured at a plant and brought to a site to be assembled. Modular reactors allow for less on-site construction, increased containment efficiency, and heightened nuclear materials security. SMRs have been proposed as a way to bypass financial barriers that have plagued conventional nuclear reactors.

Several designs exist for SMR, ranging from scaled down versions of existing nuclear reactor designs, to entirely new generation IV designs.

Both thermal-neutron reactors and fast-neutron reactors have been proposed.

Advantages and potential uses

The main advantage of small modular reactors is that they could be manufactured and assembled at a central factory location.

They can then be sent to their new location where they can be installed with very little difficulty.

SMRs are particularly useful in remote locations where there is usually a deficiency of trained workers and a higher cost of shipping.

Containment is more efficient, and proliferation concerns could be lowered.^[1]

SMRs are also more flexible in that they do not necessarily need to be hooked into a large power grid, and can generally be attached to other modules to provide increased power supplies if necessary.

The electricity needs in remote locations are usually small and highly variable.^[2] Large nuclear power plants are generally rather inflexible in their power generation capabilities. SMRs have a load-following design so that when electricity demands are low they will produce a lower amount of electricity.

Many SMRs are designed to use new fuel ideas that allow for higher burnup and longer fuel cycles. Longer refueling intervals can decrease proliferation risks and lower chances of radiation escaping containment. For reactors in remote areas, accessibility can be troublesome, so longer fuel life can be very helpful.

SMRs could be used to power significant users of energy, such as large vessels or production facilities (e.g. water treatment/purification, or mines).

Remote locations often have difficulty finding economically efficient, reliable energy sources.

Small nuclear reactors have been considered as solutions to many energy problems in these hard-to-reach places.

Cogeneration options are also possible.^[3]

Because of the lack of trained personnel available in remote areas, SMRs have to be inherently safe.

Many larger plants have active safety features that require "intelligent input", or human controls.

Many of these SMRs are being made using passive or inherent safety features. Passive safety features are engineered, but do not require outside input to work.

A pressure release valve may have a spring that can be pushed back when the pressure gets too high.

Inherent safety features require no engineered moving parts to work. They only depend on physical laws.^[4]

Operation

There are a variety of different types of SMR. Some are simplified versions of current reactors, others involve entirely new technologies.^[5]

All current small modular reactors use nuclear fission.

When an unstable nucleus (such as {{chem|link=Uranium|235|U}}) absorbs an extra neutron, the atom will split, releasing large quantities of energy in the form of heat and radiation.

The split atom will also release neutrons, which can then be absorbed by other unstable nuclei, causing a chain reaction. A sustained fission chain is necessary to generate nuclear power.

SMR designs include thermal-neutron reactors and fast-neutron reactors.

Thermal-neutron reactors rely on a moderator to slow neutrons and generally use {{chem|link=Uranium|235|U}} as fissile material. Most currently operating nuclear reactors are of this type.

Fast reactors don’t use moderators to slow down the neutrons, therefore they rely on the nuclear fuel being able to absorb neutrons travelling at higher speeds.

This usually means changing the fuel arrangement within the core, or using different fuel types. {{chem|link=Plutonium|239|Pu}} is more likely to absorb a high-speed neutron than {{chem|235|U}}.

A benefit of fast reactors is that they can be designed to be breeder reactors.

As these reactors produce energy, they also let off enough neutrons to transmute non-fissionable elements into fissionable ones.

A very common use for a breeder reactor is to surround the core in a "blanket" of {{chem|238|U}}, which is the most easily found isotope of uranium.

Once the {{chem|238|U}} undergoes a neutron absorption reaction, it becomes {{chem|239|Pu}}, which can be removed from the reactor once it is time to refuel, and used as more fuel once it has been cleaned.^[6]

Cooling

Currently, most reactors use water as a coolant.

New reactor designs are experimenting with different coolant types.

Liquid metal cooled reactors have been used both in the United States and other countries for some time.

Thermal/electrical generation

Traditionally, nuclear reactors use a coolant loop to heat water into steam, and use that steam to run turbines to generate electricity.

Some new gas-cooled reactor designs are meant to drive a gas-powered turbine, rather than using a secondary water system.

Thermal energy from nuclear reactors can also be used directly, without conversion to electricity.

Nuclear reactor heat can be used in hydrogen production and other commercial operations,^[7] such as water desalination, production of petroleum products (extracting oil from tar sands, creating synthetic oil from coal, etc.), and the production of hydrogen.^[9]

Staffing

Several SMR developers are claiming that their designs will require fewer staff members to run the reactors because of the increased inherent and passive safety systems. Fewer staff members is also a safety risk if plant owners decide to cut corners by assigning even fewer support staff to each reactor.^[10]Some of the reactors, like the Toshiba 4S, are reportedly designed to run with little supervision.^[11]

Load following

Nuclear power plants have been historically deployed to cover the base load of the electricity demand.^[12]

Some nuclear power plants might perform daily load cycling operation (i.e. load following) between 50% and 100% of their rated power.

With respect to the insertion of control rods or comparable action to reduce the nuclear power generation, a more efficient alternative might be the “Load Following by Cogeneration”, i.e. diverting the excess of power, respect to the electricity demand, to an auxiliary system. A suitable cogeneration system needs:

1. to have a demand of electricity and/or heat in the region of 500 MWe–1.5 GWt;
2. to meet a significant market demand;
3. to have access to adequate input to process;
4. to be flexible: cogeneration might operate at full load during the night when the request of electricity is low, and be turned off during the daytime.

From the economic standpoint, it is essential that the investment in the auxiliary system is profitable.

District heating, desalination and hydrogen have been proposed as technically and economically feasible options.^[13]

SMR can be ideal to do load following being used for desalination over the night. ^[14]

Waste reduction

Many SMRs are fast reactors that are designed to have higher fuel burnup rates, reducing the amount of waste produced. At higher neutron energy more fission products can be usually tolerated. As mentioned before, some SMRs are also breeder reactors, which not only "burn" fuels like {{chem|235|U}}, but will also convert fissionable materials like {{chem|238|U}} (which occurs naturally at a much higher concentration than {{chem|235|U}}) into usable fuels.^[6]

Some reactors are designed to run on alternative thorium fuel cycle, which offers significantly reduced long-term waste radiotoxicity compared to uranium cycle.^[15]

There has been some interest in the concept of a traveling wave reactor, a new type of breeder reactor that uses the fuel it breeds. The idea would eliminate the need to remove the spent fuel and "clean" it before reusing any newly bred fuel.^[16]

Safety

Since there are several different ideas for SMRs, there are many different safety features that can be involved.

Coolant systems can use natural circulation – convection – so there are no pumps, no moving parts that could break down, and they keep removing decay heat after the reactor shuts down, so that the core doesn’t overheat and melt.

Negative temperature coefficients in the moderators and the fuels keep the fission reactions under control, causing the fission reactions to slow down as temperature increases.^[17] While passive control is a key selling point, a functioning reactor may also need an active cooling system in case the passive system fails. This addition is expected to increase the cost of implementation.^[10] Additionally, SMR designs call for weaker containment structures.^[18]

Some SMR designs have underground placement of the reactors and spent-fuel storage pools, which provides more security.

Smaller reactors would be easier to upgrade quickly, require a permanent workforce, and have better passive quality controls.^[19]

Economics

A key driver of SMRs are the alleged improved economies of scale, compared to larger reactors, that stem from the ability to prefabricate them in a manufacturing plant/factory. Yet, according to some studies, the capital cost between SMRs and larger reactors are practically equivalent^[20]. A key disadvantage is that the improved affordability can only be realised if the factory is built in the first place, and this is likely to require initial orders for 40–70 units, which some experts think unlikely.^[21]

Another economic advantage of SMR is that the initial cost of building a power plant using SMR is much less than that of constructing a much more complex, non-modular, large nuclear plant.

This makes SMR a smaller-risk venture for power companies than other nuclear power plants.^[22]

Licensing

A major barrier is the licensing process, historically developed for large reactors, preventing the simple deployment of several identical units in different countries.^[23] In particular the US Nuclear Regulatory Commission process for licensing has focused mainly on large commercial reactors. The design and safety specifications, staffing requirements and licensing fees have all been geared toward reactors with an electrical output of more than 700MWe.^[24]

Licensing for SMRs has been an ongoing discussion. There was a workshop in October 2009 about licensing difficulties and another in June 2010, with a US congressional hearing in May 2010. With growing worries about climate change and greenhouse gas emissions, added to problems with hydrocarbon supplies from foreign countries and accidents like the BP oil rig explosion in the Gulf of Mexico, many US government agencies are working to push the development of different licensing for SMRs.^[25] However, some argue that weakening safety regulations to push the development of SMRs may cancel out their enhanced safety characteristics.^[26][18]

Non-proliferation

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 small, they are intended to be deployed in many more locations than existing nuclear plants.

This means both at more sites in existing nuclear power states, and in more countries that previously did not have nuclear plants.

It is also intended that SMR sites have much lower staffing levels than current nuclear plants.

Because of the increased number of sites, with fewer staff, physical protection and security becomes an increased challenge which could increase proliferation risks.^[27][28]

Many SMRs are designed to lessen the danger of materials being stolen or misplaced.

Nuclear reactor fuel can be low-enriched uranium, with a concentration of less than 20% of fissile {{chem|235|U}}.

This low quantity, non-weapons-grade uranium makes the fuel less desirable for weapons production.

Once the fuel has been irradiated, the fission products mixed with the fissile materials are highly radioactive and require special handling to remove safely, another non-proliferation feature.

Some SMR designs are intended to have lifetime cores so the SMRs do not need refuelling. This improves proliferation resistance by not requiring any on-site nuclear fuel handling. But it also means that there will be large inventories of fissile material within the SMRs to sustain a long lifetime, which could make it a more attractive proliferation target. A 200 MWe 30-year core life light water SMR could contain about 2.5 tonnes of plutonium toward the end of its working life.^[28]

The modular construction of SMRs is another useful feature. Because the reactor core is often constructed completely inside a central manufacturing facility, fewer people have access to the fuel before and after irradiation.{{citation needed|reason=highly dependant on how many people handle the fuel in the production phase of SMRs relative to with full-scale reactors. Peer-reviewed proof required to justify.|date=April 2016}}

Reactor designs

There are numerous new reactor designs being generated all over the world. A small selection of the current SMR designs is listed below.

Proposed sites

United Kingdom

In 2016 it was reported that the UK Government was assessing sites for deploying SMRs in Wales - including the former Trawsfynydd nuclear power station - and on the site of former nuclear or coal-fired power stations in Northern England. Existing nuclear sites including Bradwell, Hartlepool, Heysham, Oldbury, Sizewell, Sellafield and Wylfa are thought to be possibilities.^[31]

United States

The Tennessee Valley Authority announced it will be submitting an Early Site Permit Application (ESPA) to the Nuclear Regulatory Commission in May 2016 for potentially siting an SMR at its Clinch River Site in Tennessee. This ESPA would be valid for up to 20 years, and addresses site safety, environmental protection and emergency preparedness associated. TVA has not made a technology selection so this ESPA would be applicable for any of the light-water reactor SMR designs under development in the United States.^[32]

The Utah Associated Municipal Power Systems (UAMPS) announced a teaming partnership with Energy Northwest to explore siting a NuScale Power reactor in Idaho, possibly on the Department of Energy's Idaho National Laboratory.^[33]

The Galena Nuclear Power Plant in Galena, Alaska was a proposed micro nuclear reactor installation intended to reduce the costs and environmental pollution required to power the town. It was a potential deployment for the Toshiba 4S reactor.

References

Further reading

• {{cite web |author=Office of Nuclear Energy, Science and Technology |title=DOE Fundamentals Handbook: Nuclear Physics and Reactor Theory |volume=2 |date=January 1993 |publisher=U.S. Department of Energy |url=http://www.hss.doe.gov/nuclearsafety/techstds/docs/handbook/h1019v2.pdf |id=DOE-HDBK-1019, DE93012223 |ref={{harvid|DOE-HDBK-1019|1993}} |deadurl=yes |archiveurl=https://web.archive.org/web/20121109194948/http://www.hss.doe.gov/nuclearsafety/techstds/docs/handbook/h1019v2.pdf |archivedate=2012-11-09 |df= }}
• {{cite web |author=Office of Nuclear Energy, Science and Technology |title=Report to Congress on Small Modular Nuclear Reactors |date=May 2001 |publisher=U.S. Department of Energy |url=http://www.ne.doe.gov/pdfFiles/Cong-Rpt-may01.pdf |ref={{harvid|Report to Congress|2001}} |deadurl=yes |archiveurl=https://web.archive.org/web/20110716055134/http://www.ne.doe.gov/pdfFiles/Cong-Rpt-may01.pdf |archivedate=2011-07-16 |df= }}

External links

• [https://web.archive.org/web/20100624035928/http://www.ne.doe.gov/ DOE Office of Nuclear Energy]
• American Nuclear Regulatory Commission
• World Nuclear Association
• American Nuclear Society
• International Atomic Energy Agency
• Overview and Status of SMRs Being Developed in the United States

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