The Future of Nuclear Power, Part III: Modular Nuclear Reactors

July 31, 2022

V: Modular Nuclear Reactors

It has become extremely difficult to construct new nuclear power plants in the U.S.  The costs and regulations have increased greatly; and in addition, a group proposing to construct a new nuclear power plant will face opposition from anti-nuclear groups.  As a result, new nuclear facilities in the U.S. have become quite rare.  On the other hand, it is difficult to imagine how we are to make the transition from fossil fuels to renewable energy without nuclear power providing some of the energy.  As a result, there has been considerable interest in modular nuclear reactors.  These are designed to remove some of the costs and dangers associated with large nuclear facilities. 

Here, we will discuss proposals for new modular nuclear reactors.  We will discuss the advantages claimed for this technology and we will review three of the proposals for new modular nuclear reactors in the U.S.  We will then mention some potential drawbacks for modular nuclear reactors. 

Advantages of Modular Nuclear Reactors:

Conventional nuclear power plants were designed to produce all the power needed by a particular area.  They typically generated at least 700 MW(e) of electrical power.  If a fuel source delivers a certain power input to a system, that is listed as MW(t), the total thermal power input in Mega-Watts (1 million Watts). A certain amount of the power input will be lost as waste heat.  The usable power output is defined as MW(e).  This is shown schematically in Fig. V.1.  The difference between different types of nuclear reactors is shown in Fig. V.2.  A conventional nuclear reactor generates at least 700MW(e) of power; a small modular reactor (SMR) is one that generates up to 300 MW(e), and a microreactor is one that generates 1-10 MW(e). 

Figure V.1:  Definitions of power generation.  A system produces a certain total amount of thermal power in MegaWatts (MW), defined as MW(t).  Some of that power is lost as waste heat; the remainder is available as the electrical power output to the system, defined as MW(e). 

Figure V.2: Definition of various nuclear power generating facilities.  A conventional nuclear reactor is one that produces 700 MW(e) or more of output power.  A small modular reactor is one that produces up to 300 MW(e), while a microreactor is one that generates up to 10 MW(e). 

Small modular nuclear reactors are designed to provide the flexibility lacking in current large nuclear reactor complexes.  The idea is to manufacture small nuclear reactors at a dedicated plant.  A modular nuclear reactor would look roughly like the schematic diagram in Fig. V.3, which shows a small pressurized-water modular nuclear reactor.  It would have several features in common with larger facilities.  A steel-lined reactor vessel would contain a core with fuel rods surrounded by a water coolant.  The water under high pressure would be heated by fission in the reactor core.  That water would then be run through a heat-exchanger to transfer heat to a secondary water source used to produce steam that would spin a turbine connected to an electric generator.  However, a single module would have far fewer fuel rods than a conventional large nuclear reactor. 

Figure V.3:  A schematic diagram of a small modular pressurized water nuclear reactor.  It has many of the same features as a large nuclear plant: a reactor vessel containing a core of fuel rods with enriched uranium fuel; a coolant that uses water under high pressure to absorb heat from the fission process and then transfer that heat to a second water vessel; a generating area where the latter water produces steam, which is sent to a turbine that powers an electrical generator.  However, the number of fuel rods in a single module is far smaller than in a conventional large reactor. 

A plant would be built to construct these modules; all modules would be produced at this facility and shipped to the location of the reactor complex.  Individual modules could be linked together to provide electrical power, as shown in Fig. V.4.  During times when a smaller amount of electrical energy was needed, individual modules could be shut down for days or even months. 

Figure V.4: A schematic diagram of a nuclear power plant (in this case a design by NuScale) constructed by linking together several identical small modular nuclear reactors.  Each of the modules can generate energy that will be combined with other modules to provide the desired amount. The total number of modules would be determined by the peak power needed from this reactor. If less than peak energy is needed, one or more individual modules could be turned off for a period of time.  The modules would be produced at a single location and shipped to the reactor complex.

Small modular nuclear reactors are designed to provide flexibility that is lacking in conventional large nuclear reactor complexes.  The Office of Nuclear Energy of the U.S. Department of Energy (DOE) has listed the advantages of small modular reactors (SMRs) compared with conventional facilities. 

  • Modularity: Components of a small modular nuclear reactor would be fabricated and assembled at a dedicated plant.  Since the SMRs from a given company are all identical, they could be manufactured using assembly-line techniques, provided that a demand existed for several modular reactors.  While conventional nuclear plants do most of the assembly on-site, SMRs would be completely constructed at a single off-site facility.  This is expected to simplify the construction of a reactor, incorporate enhanced safety features, and provide more flexibility as compared to conventional reactors. 
  • Lower capital investment:  An SMR should reduce the capital investment needed due to the smaller size of the SMR.  Also, the construction cost of an SMR should be smaller, and they can be fabricated much faster than on the site of a conventional nuclear plant.
  • Siting flexibility: SMRs could be built in areas where large nuclear plants are not needed, or in places where the infrastructure can’t support a large plant.  SMRs could be built in smaller electrical markets or in isolated areas.  One could also imagine industrial applications for SMRs.  They would also be attractive options to replace aging fossil-fuel plants with one that emits no greenhouse gases.  For example, coal-fired fossil fuel plants that were retired during 2010 – 2012 averaged 97 MW(e), and those expected to retire between 2021 and 2025 will average 145 MW(e). 
  • Greater Efficiency: SMRs could be coupled to other energy sources, either fossil-fuel plants or renewable-energy sources.  This should increase the stability and security of the grid, and provide higher efficiency for power generation. 
  • Safeguards and Security:  SMRs can be built with modern safeguards and security requirements.  This can minimize the dangers of theft or sabotage at a nuclear plant.  For example, siting SMRs underground would greatly decrease the risk of someone crashing a plane into the reactor.  SMRs can also be constructed to minimize the threat of theft or sabotage of plutonium-enriched materials.  Individual SMR units could be shipped back to the factory for refueling.  This could help minimize the dangers involved in transporting nuclear materials. 
  • Industry and Job Growth: The manufacture and assembly of SMRs would spur the development of high-paying jobs.  There is a world-wide market for SMRs, and the U.S. economy would benefit greatly if it was a major player on the international market for SMRs. 
  • Economic Development: A 2010 study by the Energy Policy Institute estimated that a hypothetical 100 MW(e) SMR that cost $500 million to manufacture and install would create nearly 7,000 jobs and generate $1.3 billion in sales, $404 billion in payroll, and $35 million in business taxes.  The report considered 4 different scenarios for SMR deployment rates: low (1-2 units/year); moderate (30 units/year); high (40 units/year); and disruptive (85 units/year).  The authors found that the economic impact would be significant, even at moderate levels of deployment of SMRs. 

Conventional nuclear reactors have traditionally been constructed to deliver maximum peak loads of energy, and they were designed to produce electricity at their full capacity (as “base load”) nearly 24 hours for every day of the year.  In several areas of the country, nuclear power plants provide a significant fraction of the power generation in their region.  However, in many communities a much smaller reactor would suffice for energy generation.  Also, in most communities the energy delivered from a nuclear reactor now needs to supplement energy from conventional power plants, plus renewables such as wind, solar and hydroelectric power.  New nuclear power plants need to compete with conventional plants and renewables in price.  And they also need to reckon with the fact that renewable energy sources – solar, wind and hydroelectric – tend to vary greatly in their energy production at different times of day or seasons of the year.

It is claimed that small modular nuclear reactors could solve many of the financial issues facing nuclear plants.  The cost of a module would be far smaller than the investment needed for a large conventional nuclear plant (even though the cost per unit of energy produced would be higher at a modular facility). 

In the U.S., conventional nuclear plants are each designed separately.  This is unlike the situation in France, where the government has standardized the design of all its nuclear power plants.  France generates 75% of its electrical energy from nuclear power.  The standardization of facilities ensures that the costs are minimized.  For example, electricity prices in France are 17 cents per kilowatt-hour (kw-hr), compared to 37 cents per kw-hr in Germany and 34 cents per kw-hr in Poland. 

In 2016 the Tennessee Valley Authority (TVA) stated that it was developing a site at Oak Ridge for a SMR at Clinch River.  In Feb. 2021, TVA announced its intent to produce an environmental impact statement for the construction, operation and decommissioning of an advanced nuclear reactor at Clinch River.  The total electrical output was stated as up to 800 MW(e). 

In Sept. 2018 Congress passed the Nuclear Energy Innovation Capabilities Act and the Department of Energy Research and Innovation Act.  The first Act established a National Reactor Innovation Center to set up partnerships between the DOE and private industry, with the goal of creating privately-funded advanced reactor prototypes at DOE sites.  The second Act combined seven previously-passed science bills to provide policy direction to the DOE on nuclear energy research and development. 

The Office of Nuclear Energy of the U.S. Dept. of Energy is currently working with several private companies to develop modular reactor technology.  Next, we will review several of these modular nuclear reactor designs, each of which incorporates safety features to avoid some of the serious risk elements exposed by the reactor accidents summarized in Part II of this post. 

NuScale:

The DOE began by funding the original development for what has become the NuScale Power Company.  After initial DOE support, research was continued at Oregon State University in 2007.  In Oct. 2011, Fluor Corporation provided NuScale with $30 million in return for a 50% share in NuScale.  In May 2022, NuScale Power merged with Spring Valley Acquisition Company.  The NuScale Power Company is now listed on the New York Stock Exchange. 

The DOE has provided several research grants to NuScale for development and testing of its SMR, called the NuScale Power Module and illustrated in Fig. V.5.  The Power Module proposal was for a pressurized-water modular reactor that would produce 250 MW(t) and 77 MW(e).  The NuScale reactor vessel would be 3 meters in diameter and would use fuel enriched to 4.95% U-235.  The module would be installed underground in a pool of water, and the containment vessel would be 4.6 meters in diameter and 23 m high.  A “complete” power plant would contain 12 modules linked together, as illustrated in Fig. V.6, and is projected to produce 924 MW(e). 

The Power Module is officially designated as an integrated pressurized water reactor (iPWR), in the sense that operations in the separated reactor and steam generation vessels in a conventional PWR are here combined within a single steel vessel. This integration facilitates a passive water cooling system that eliminates the needs for pumps which could fail in a power outage, as at Fukushima. The natural water flow in this system is illustrated in Fig. V.5. It relies on the fact that the density of liquid water decreases with increasing temperature, so that water heated by fuel rods rises within the central “riser” region of the reactor vessel. When that hot water reaches the top of the vessel, it flows toward the outer regions of the vessel and then falls back down via gravity, alongside heat exchanger pipes. On its downward path, the hot water cools by transferring heat to cooler water fed from an external source into the heat exchanger pipes, and it is that secondary water which produces the steam.

Figure V.5. Schematic illustration of primary and secondary water flow within the NuScale Power Module. The arrows indicate that the primary cooling water, heated by the fuel rods, rises through the central region of the reactor vessel and then falls past heat exchanger pipes, in which initially cooler water at low pressure boils to produce steam.

Figure V.6. Top view of a proposed 12-module NuScale SMR power plant, with all 12 modules situated within a common water pool.

The Power Module is designed to have a 60-year lifetime.  In July 2017, the NuScale highly integrated protection system (HIPS) was approved by the Nuclear Regulatory Commission (NRC), making NuScale’s Power Module the first SMR to receive (in August 2020) NRC design licensing.  In particular, NRC confirmed that NuScale’s passive water cooling eliminates the need for DC battery backup. The NRC approval also validates NuScale’s claim that the external water pool in which the Power Modules are submerged provides sufficient passive cooling to prevent thermal runaway of the reactor when the control rods are inserted to stop the fission process. Heat generated by the decay of radioactive products within the reactor core would be naturally removed by the water pool over a period of 30 days, as illustrated in Fig. V.7.

Figure V.7. Illustration of passive cooling provided by the external water pool to prevent thermal runaway of a Power Module once the control rods are inserted to stop the fission process.

In 2013 NuScale launched a Western Initiative for Nuclear program with the states of Washington, Oregon, Wyoming, Utah and Idaho.  NuScale plans to complete a demonstration SMR that is projected to be operational by 2024, at the Idaho National Laboratory (INL) run by the U.S. DOE.  Subsequently, they plan to construct a full-scale plant at INL that would be owned by Utah Associated Municipal Power Systems and run by Energy Northwest. 

NuScale has also collaborated with other governments in development projects for small modular reactors.  It signed an agreement with the United Kingdom in Sept. 2017, with the claim that the first of its UK units would be operational by the mid-2020s.  NuScale also signed an agreement with Ukrainian energy companies (although this may not materialize due to Ukraine’s current war with Russia).  In May 2021, the Canadian Nuclear Safety Commission announced that it would conduct pre-licensing design reviews of ten small reactors, including NuScale’s Power Module design.  NuScale is also developing a microreactor in the range 1 – 10 MWe, called Heatpipe. 

NuScale is also researching the possibility of using SMRs for other development projects.  Some of those include desalination plants, oil recovery from tar sands, hydrogen production through high-temperature steam electrolysis, and backup energy for wind farms.  Together with Prodigy Clean Energy, NuScale is working on a prototype for a floating version of Power Module that could be deployed in the sea close to a shoreline. 

GE-Hitachi Boiling-Water Reactor (BWRX-300):  

The company GE Hitachi Nuclear Energy has developed a modular boiling-water nuclear reactor, the BWRX-300.  A schematic of the BWRX-300 is shown in Fig. V.8.  In such a reactor, the coolant is pure water maintained at substantially lower pressure than in a PWR. At that lower pressure, the primary cooling water can boil at the temperatures reached via contact with the fuel rods. Inside the reactor vessel, the water is heated by fission in the core, and becomes steam.  The steam exits the containment vessel where it spins turbines that power generators.  The steam is then cooled and condenses in an Isolation Condenser System, and the water is returned to the containment vessel. 

Figure V.8: A schematic picture of the GE-Hitachi boiling-water modular reactor the BWRX-300.  The reactor vessel is the long tube in the center of the photo.  The core is at the bottom of the tube, which is suspended inside a containment vessel.  The reactor is also sited underground.  Water heated by fission in the core turns into steam, which exits the reactor vessel at top right and spins turbines that power generators.  As the steam cools, it condenses and is returned to the containment vessel.  This design uses “passive controls;” instead of utilizing pumps to circulate the water/steam, this device uses gravity to circulate the steam as it condenses. 

The GEH BWRX-300 reactor has no pumps to circulate the water and steam.  As in the NuScale Power Module, the BWRX-300 uses “natural circulation,” where the steam rises and after it passes through the turbines, it passes through the Isolation Condenser System where it condenses into water.  The process is then repeated. 

GEH claim that, by doing away with pumps, the reactor will run safely with no danger of the “loss of coolant accident,” one of the most serious dangers of a conventional reactor.  If a reactor is shut down suddenly, because of loss of coolant or other accident, the reactor continues to generate heat, from the many radioactive isotopes that accumulate in the fuel rods.  The claim is that with the GEH BWRX-300 boiling-water modular reactor, the water will still maintain its natural circulation.  The company claims that the natural circulation of water and steam through the system can carry off heat from the reactor vessel for seven days after a sudden shutdown.  This should provide sufficient time for the reactor staff to take other measures to stabilize the system.

In comparison with the NuScale design, the BWRX-300 passive cooling system seems to us capable of carrying heat away from the reactor core at higher rates, simply because steam has still much lower density than the heated liquid water in the NuScale SMR. The lower pressure in a BWR than in a PWR also is a safety advantage, reducing the odds of an explosion. The disadvantage of boiling water reactors, however, is that the primary cooling water which, under meltdown conditions, may itself contain significant radioactivity, escapes the reactor vessel as steam, requiring special care outside the reactor vessel to prevent radioactive leaks to the atmosphere.

GE-Hitachi claim that their modular boiling-water reactor can be constructed with initial capital costs of $2,250/kilowatt.  One of the more recent estimates of nuclear power plant capital costs was a 2007 proposal by NRG to build two conventional boiling-water reactors in Texas.  Their cost estimate was $2,700/kW.  Those NRG reactors were never built. 

The GEH BWRX-300 reactor has had much preliminary success in forging collaborations with other countries.  In May 2019, GE Hitachi Nuclear Energy initiated a vendor review of its boiling-water modular reactor with the Canadian Nuclear Safety Commission (CNSC).  In Oct. 2019, GEH signed an agreement for a collaboration to study possible deployment of the GEH modular nuclear reactors in Poland.  That same month, GEH announced that it was carrying out exploratory work for modular nuclear reactors in Estonia. 

In Dec. 2021, the GEH boiling-water modular reactor BWRX-300 was selected by the Ontario Power Generation as the technology partner for the province’s Darlington New Nuclear Project.  The goal is to construct a small modular reactor at the Darlington site, that could be completed as early as 2028.  General Electric has been providing support to Canada’s nuclear reactor projects for nearly 70 years.  They helped construct the fleet of Canadian “CANDU” reactors that used heavy water (deuterium) as the coolant.    

In June 2022, the Saskatchewan power utility SaskPower selected the GE-Hitachi modular boiling-water nuclear reactor BWRX-300 as their candidate for potential deployment in the province.  The decision took place after a Canadian assessment of energy needs, and a review by an independent engineering firm.  Earlier in 2022, Saskatchewan had collaborated with the governments of several other Canadian provinces in developing a strategic plan for utilizing small modular reactors (SMRs).  However, the final decision as to whether to build an SMR will not be made until 2029.  It is interesting that although Saskatchewan has nearly all of Canada’s supply of uranium, at present there are no nuclear reactors operating in that province. 

GE – HItachi Modular Fast Breeder Reactor:

GE-Hitachi has also carried out research on a modular type of breeder reactor.  These reactors use a very high flux of unmoderated fast neutrons to initiate fission.  Interest and research on breeder reactors was very high in the late 1950s, for several reasons. 

  • In the 1950s, it was believed that nuclear reactors would soon become a dominant form of energy production.  At that time, it was believed that uranium was rare and in danger of being depleted.  However, uranium turned out to be both more abundant and cheaper than had been assumed.
  • It was believed that breeder reactors would be competitive in price with light-water reactors (LWRs). However, the cost of breeder reactors has remained at least 25% higher than LWRs. 
  • Early estimates suggested that breeder reactors would be as safe as light-water reactors.  However, the coolant for breeder reactors is often liquid sodium, which will ignite if exposed to air. 
  • It was believed that proliferation risks would be manageable (breeder reactors using U-238 fuels produce a lot of plutonium – Pu – this is what they “breed”).  In reprocessing, the Pu recovered could be diverted to produce nuclear weapons, or it could be stolen. 

Given all these negatives, why should we be interested in breeder reactors?  The first reason is that a breeder reactor is capable of producing additional fissile material.  For example, burning naturally abundant Uranium-238 produces plutonium, which is itself fissile.  Thus, in principle a breeder reactor could produce its own fuel supply.  Indeed, a breeder reactor can produce more fissile material than it consumes; however, the fuel rods need to be re-processed at regular intervals, since some of the isotopes produced in a breeder reactor absorb neutrons, and such isotopes could slow down or even halt the fission reactions.  And since plutonium is one of the elements that increase in a breeder reactor, the issues with re-processing plutonium again arise. 

One big advantage is that breeder reactors can mitigate some of the problems with nuclear waste, as we explain further below.  In a light-water reactor, slow neutrons cause U-235 to fission.  Fission of uranium produces lighter nuclei, which generally do not themselves fission; however, many of these isotopes are radioactive.  When they decay, they generate a great deal of heat and produce harmful radiation.  In addition, some of the neutrons are captured by uranium atoms.  These produce “transuranic” isotopes, which are also radioactive.  One of these is plutonium, but several other transuranic atoms are also produced in the fuel rods.  When the transuranic nuclei decay, they produce heat and radiation.  Many of the transuranic isotopes can also fission. 

The amount of time that the radioactivity in fuel rods persists depends on the half-life of the radioactive atoms.  There is a major difference between the typical half-lives of the fission product isotopes and the transuranic nuclei.  The vast majority of the fission isotopes have half-lives of 100 years or less.  But several of the transuranic isotopes have extremely long half-lives, some of these as long as millions of years.  Spent fuel rods from LWRs contain a mixture of fission fragments and transuranic elements. 

The Ge-Hitachi (GEH) Power Reactor Innovative Small Module (PRISM) uses liquid sodium as a coolant.  The PRISM reactor is a modular liquid-metal breeder reactor of the “pool” type.  A schematic diagram of such a reactor is shown in Fig. V.9.  The reactor vessel contains fuel rods (in the PRISM reactor the fuel is a mixture of plutonium and depleted uranium (DU)).  Since fast neutrons are used to produce fission, one does not use water as a coolant since it slows down and absorbs neutrons.  Instead, one uses a coolant that is transparent to neutrons, in this case liquid sodium.  Sodium surrounding the reactor core is heated by the fission reactions.  The sodium then passes to a secondary sodium vessel, where heated sodium produces steam in a steam generator.  The steam spins a turbine that drives an electric generator.  

Figure V.9: A schematic diagram of a liquid-metal breeder reactor of the “pool” type.  The reactor vessel contains a core with its fuel rods and control rods.  The coolant is liquid sodium (Na), which becomes heated by the energy from the fission reactions.  The reactor vessel is surrounded by a pool of cool liquid Na.  The hot sodium travels to a heat exchanger that heats up liquid sodium.  That secondary sodium then travels to a steam generator.  The steam produced spins a turbine which drives an electric generator. 

The density and speed of neutrons in a liquid-metal breeder reactor are much higher than in a conventional fission reactor, and the design of these reactors is based on the Experimental Breeder Reactor prototype at Idaho National Laboratory (INL).  The Experimental Breeder Reactor produced 19 MW(e), and the fuel was refined spent fuel from light water reactors.  The INL reactor operated from 1963 to 1994.   The PRISM reactor is envisioned to combine two modules of 311 MW(e) each; each module will have one steam generator, and collectively the two modules will drive a single turbine.  The sodium coolant is supposed to operate at 500oC, and an intermediate sodium loop will transport heat to steam generators.  The fuels in the breeder reactor contain high amounts of uranium – 15-20% uranium is a typical value for fast-neutron reactors.  These machines typically use boron carbide control rods. 

An interesting aspect of the GEH PRISM reactor is that it has some unique safety features.  The bath of sodium surrounding the reactor vessel should carry off excess heat rather efficiently.  The GEH designers claim that air circulating around the lower containment vessel is all that is needed to keep the reactor fuel cool.  But if the reactor should begin to overheat, the metallic core will expand, decreasing its density.  As the density decreases, the reaction will slow down; further temperature increases will shut down the fission reactions.  GEH claims that this “passive safety” feature means that one will not rely on “human, mechanical or electronic intervention to mitigate the risk of an accident.”  So, in this breeder reactor a loss of coolant accident (LOCA), one of the most dangerous issues in a conventional light-water reactor, should not be a problem.  A crucial feature of passive safety is that if the reactor generates too much heat, the fission reactions must shut down sufficiently fast that the temperatures do not harm the reactor. A Nuclear Regulatory Commission review must ensure that in an emergency, the core temperatures cool down sufficiently rapidly.  

There are various uses for fast-neutron breeder reactors.  One of these is to generate plutonium.  Operated in a breeder mode, the PRISM reactor is designed to produce 57 kg/year of weapons-grade plutonium.  A second use for a fast-neutron reactor is to burn transuranic elements that have been produced in spent fuel rods from a conventional light-water nuclear reactor.  In this case, one surrounds the breeder reactor fuel with reprocessed spent fuel rods.  The fast neutrons then initiate fission of the transuranic elements in the spent fuel rods.  Operated in this mode, the PRISM reactor can generate heat for electrical energy, while at the same time burning transuranic elements, particularly plutonium.  The PRISM reactor is claimed to consume 72 kg/year of fissile plutonium from LWR spent fuel rods. 

A breeder reactor burns a much higher fraction of the fissile material in the fuel rods than a light-water reactor.  In a pressurized-water reactor roughly 1% of the uranium fuel is converted to energy.  In principle, a breeder reactor could burn nearly 100% of the transuranic elements in spent fuel rods.  The remaining radioactive material in the rods would be the fission fragments.  Since their half-lives are generally shorter than 100 years, the radiation and heat in the fuel rods should dissipate rapidly.  This would mean that those fuel rods could be moved to dry-cask storage rather quickly, and would not retain most of the very long-lived radiative decay products characteristic of radioactive waste from LWRs.  At present, GEH is negotiating with the UK to license PRISM technology to dispose of the UK’s plutonium stockpile.  Supposedly, the PRISM reactor would irradiate fuel that consisted of 20% plutonium, zirconium and DU.  After 90 days of irradiation, the spent fuel rods could be stored in air-cooled silos. 

In 2019, the U.S. DOE commenced its Versatile Test Reactor (VTR) program that would be run at Idaho National Laboratory.  It would utilize a specially-adapted PRISM reactor as a test facility. GEH then signed a collaborative agreement with TerraPower to design and construct the VTR for INL.  Figure V.10 shows a cutaway of the GE-Hitachi PRISM design. 

Figure V.10: A schematic drawing of the GE-Hitachi PRISM reactor.  The reactor vessel is shown at the bottom front of the drawing.

Terrestrial Energy Molten Salt Reactor:

We should mention that the “passive safety” feature claimed for PRISM is even more apparent in a separate class of SMRs being developed to deploy molten salts as both reactor fuel and coolant. The fuel is a salt containing thorium (used to breed fissile U-233), uranium or plutonium dissolved in lithium fluoride or lithium-beryllium fluoride, which remain in the liquid state without pressurization from 500°C to about 1400°C. Molten salt reactors can thus operate at high temperatures (typically about 600°C) in unpressurized vessels, removing many risks associated with the high pressures in a PWR. While molten salt reactors were first developed at Oak Ridge National Laboratory in the U.S., global research on molten salt reactors is currently led by China.  The company Terrestrial Energy is developing a small modular version (IMSR) illustrated in Fig. V.11. As moderators, the IMSR uses graphite rods, which are chemically compatible with the fluoride salt, but only last for about seven years of operation.

The density of the fissile material in the molten salt is sufficient to sustain a chain reaction in the reactor core. But the heat from fission raises the liquid temperature and lowers the fissile material density sufficiently that fission ceases as the molten salt rises toward the top of the reactor vessel. Near the top, it transfers heat to a secondary, non-radioactive, coolant salt system. The cooling primary salt falls to the bottom of the vessel and re-attains the density needed for fission to proceed, so that the cycle continues. Thermal runaway is naturally prevented because if the liquid gets too hot, it becomes unable to sustain a chain reaction. Furthermore, radioactive products remain bound within the molten salt, so they are not volatile and should not spread to the atmosphere. However, the somewhat exotic fuel may lead to more complex compounds containing radioactive isotopes.

The higher temperature of a molten salt reactor, compared to a water reactor, allows substantially more efficient eventual conversion of heat energy to electrical energy. That energy can be generated in the traditional way by running turbines, or it can be delivered directly as heat for some applications. Furthermore, the molten salts are efficient at storing heat for a considerable length of time, which makes a molten salt reactor especially convenient as an on-demand supplement to renewable wind or solar energy generators. Development of modular molten salt reactors currently lags behind the NuScale and GEH designs discussed above, so is unlikely to be ready for commercial installations before the 2030s. One particular drawback, however, is the need to replace reactor modules every seven years or so, as the graphite moderators are worn down.

Figure V.11. Schematic illustration of the operation of Terrestrial Energy’s proposed modular molten salt reactor. The liquid fuel containing fissile nuclei circulates through graphite moderators. When the fuel density is high, it yields efficient fission. But heat from the fission lowers the molten salt density sufficiently that it rises and becomes unable to support a chain reaction. Near the top of the reactor vessel, the molten salt falls past heat exchangers carrying a secondary, non-radioactive, coolant salt which, in turn, is used to heat a third liquid salt bath to 600°C. At that temperature, the liquid salt can produce energy efficiently in the several forms indicated in the diagram.

Drawbacks of Modular Nuclear Reactors:

We have listed some of the benefits of modular nuclear reactors.  What are the drawbacks of such reactors?  Corporations producing modular nuclear reactors claim that one benefit of modular reactors is that they will produce fewer radioactive by-products, and that transporting nuclear fuel rods to the companies that manufacture the modular reactors will reduce the amount of waste and environmental damage from nuclear reactors.  However, a recent Stanford-University of British Columbia study, published in the Proceedings of the National Academies of Science, came to exactly the opposite conclusion. 

That article began by noting that “remarkably few studies have analyzed the management and disposal of nuclear waste streams from small modular reactors.”  The authors reviewed the situation for modular reactors being developed by Toshiba, NuScale and Terrestrial Energy, since each company had proposed a different design for modular reactors.  The authors noted that none of the modular reactors are yet in operation, and that some reactors have proprietary designs, making it difficult to assess their properties.  However, the authors pointed to the following issues with modular reactors:

  • Neutron leakage: nuclear fission reactors use neutrons arising from fission of U-235 to initiate fission in another atom of uranium.  However, some of the neutrons released in fission do not interact until they have left the reactor core.  Those neutrons are absorbed in the steel or concrete surrounding the reactor vessel.  The investigators found that the smaller vessels found in modular reactors will experience comparably more neutron “leakage” than conventional reactors.  Rodney Ewing, one of the co-authors of the study, said “Small modular reactors will generate at least nine times more neutron-activated steel than conventional power plants.” 
  • Radiotoxicity:  the study found that small modular designs will have greater problems “with respect to radioactive waste generation, management requirements and disposal options.”  The researchers estimated that, over a 10,000-year period, the radiotoxicity of plutonium in spent fuels from the three modular designs would be at least 50% higher than Pu from conventional spent fuel.  Also, spent nuclear fuel from modular reactors could be far more complex than the spent fuel discharged from existing large power plants.  Co-author Allison Macfarlane stated. “Some small modular reactor designs call for chemically exotic fuels and coolants that can produce difficult-to-manage wastes for disposal.”  The high level of radiotoxicity of spent fuel from modular reactors should make it extremely important to carefully choose sites for storing nuclear waste. 

Prof. Ewing commented, “We shouldn’t be the ones doing this kind of study.  The vendors … should be concerned about the waste and conducting research that can be reviewed in the open literature.”  The article concluded “These findings stand in sharp contrast to the cost and waste reduction benefits that advocates have claimed for advanced nuclear technologies.”  

In summary, it appears that modular nuclear reactors can address several issues with conventional nuclear reactors.  The fact that modular nuclear reactors can be constructed using assembly-line techniques at a dedicated facility suggests that these reactors should be cheaper and faster to produce than conventional nuclear plants, in the long run.  Modular nuclear reactors are generally designed to be located underground, which would remove the possibility that an airplane or drone could crash into the reactor containment vessel and cause the release of radioactive materials. 

The overall size of the plant can be determined by the number of modules that are combined to produce the power plant.  This means that a small modular reactor could be used in areas where the total demand for energy is smaller, or at sites that would not accommodate a large conventional reactor.  A major advantage of the modular nuclear reactor is that they are designed to vary the energy output.  One or more of the modules could be turned off if less power was needed, or turned on at peak periods.  An entire module could be disassembled and shipped back to the construction facility for reprocessing; this should help minimize the dangers of transporting radioactive materials. 

It appears that there will be significant demand for modular nuclear reactors in other countries.  If the U.S. rapidly develops the capability to produce these reactors, there could be significant economic gains in providing high-paying jobs for Americans. 

One thing that modular reactors have not done is to solve the problem of radioactive waste.  Small modular pressurized-water reactors would still produce long-lived radioactive waste, and the problems with storing this waste, and cooling it until its temperature decreases substantially, will still persist with modular reactors.  The breeder reactor, operated in burn-up mode, can burn most of the transuranic elements from spent fuel rods.  However, commercial breeder reactors have operated only in France.  The French Superphenix sodium-cooled and plutonium-fueled reactor was operated until 1998, but then shut down because its costs were significantly higher than conventional nuclear reactors.  And the predecessor to Superphenix, the Phenix breeder reactor, began operating in 1973 and was shut down in 2009.  It “experienced numerous sodium leaks and fires and a series of potentially serious reactivity incidents.”  It appears that the safety issues with liquid sodium coolants have persisted.  GE-Hitachi would have to prove that their design alleviates these concerns, through a successful test program at a lab like the Idaho Nuclear Facility, before one could think of rolling out a commercial breeder reactor. 

So, it remains an open question whether the modular breeder reactor will ever make it past the experimental stage, whether it can be operated safely, and whether it can burn many of the transuranic elements that exist in spent fuel rods from conventional nuclear reactors.