Sodium Cooled Reactor

Advanced Nuclear Reactors

Advanced designs of nuclear reactors are being developed that will overcome many issues associated with current pressurized water reactors.

I have been interested in nuclear energy for electrical power for many years. In April 2023 I published a blog article that asked the question Why Nuclear Power? This showed that nuclear power is essential for green baseload power to supplement other green sources such as solar and wind, which are highly intermittent. For several months, I have been researching data on advanced nuclear rectors with emphasis on projects in the USA. I came up with the conclusion to look further into four promising advanced technologies:

  • Molten Salt reactors using technology developed at Oak Ridge in the 1950's and 60's. For this I found two promising contenders – The Kairos Power Hermes project now at Oak Ridge and the Thorcon project in Indonesia.
  • Sodium-cooled fast reactors developed in the 1950s by Walter Zinn and others.
  • Fast Reactors using molten lead as coolant such as the Dual Fluid reactor projects in Canada and Rwanda

While I have accumulated quite a lot of information, condensing it seemed somewhat burdensome, so I decided to turn to Google's ”AI browser bot” Bard to help me move forward. Following are several questions I posed to Bard.

Which are the best technologies for advanced nuclear reactors, those not using water as coolant?


The best technologies for advanced nuclear reactors that do not use water as coolant are:

  • Sodium-cooled fast reactors (SFRs):

SFRs use liquid sodium as a coolant, which allows them to operate at higher temperatures and lower pressures than current water-cooled reactors. This makes them more efficient and less likely to overheat. SFRs can also use a wider range of fuels, including spent fuel from current reactors.

  • Molten salt reactors

(MSRs): MSRs use a molten salt mixture as a coolant and moderator. MSRs can operate at very high temperatures, which makes them very efficient. They can also use a wide range of fuels, including thorium. MSRs are still in the early stages of development, but they have the potential to be very safe and efficient reactors.

  • High-temperature gas-cooled reactors (HTGRs):

HTGRs use helium gas as a coolant and graphite as a moderator. HTGRs can also operate at very high temperatures, which makes them very efficient. HTGRs are also very safe, and they can be used to produce electricity, heat, and hydrogen.

Other promising advanced reactor technologies that do not use water as coolant include:

  • Gas-cooled fast reactors (GFRs)
  • Lead-cooled fast reactors (LFRs)
  • Molten lead reactors (MLRs)
  • Very high temperature reactors (VHTRs)

These technologies are all in various stages of development, but they have the potential to offer significant advantages over current water-cooled reactors, including:

  • Higher efficiency
  • Improved safety
  • Greater flexibility in fuel use
  • Reduced waste production

Advanced nuclear reactors have the potential to play a major role in meeting the world's growing energy needs while reducing greenhouse gas emissions. The best technologies for advanced nuclear reactors will depend on the specific needs of each country or region. However, the technologies listed above are all promising candidates, and they are worth investing in research and development.

Which technology is the best?

It is difficult to say definitively which of the above technologies is the best for advanced nuclear reactors. Each technology has its own advantages and disadvantages. For example, SFRs are well-developed and have a proven track record, but they can be complex and expensive to build. MSRs are very efficient and have the potential to be very safe, but they are still in the early stages of development. HTGRs are also very efficient and safe, but they can be expensive to build.

The best technology for a particular country or region will depend on its specific needs and resources. For example, a country with a limited water supply may want to consider an HTGR or SFR, which can operate without water. A country with a lot of thorium resources may want to consider an MSR, which can use thorium as fuel. It is also important to note that the technologies listed above are not mutually exclusive. It is possible to combine different technologies to create new and even better reactor designs. For example, some companies are developing molten salt reactors that use

gas as a coolant. This hybrid design could combine the advantages of

both MSRs and HTGRs.

Overall, there is no single "best" technology for advanced nuclear reactors. The best technology will depend on the specific needs and resources of each country or region. However, all of the technologies listed above are promising candidates, and they are worth investing in research and development.


I regarded Bard's response as a very accurate yet concise answer to my imposing question. I noted that three of my choices were included in Bard's first group of “best technologies," and my fourth choice was in its second group of “promising technologies.” Moreover, its explanation for its findings included the same four advantages I had compiled, and its response to “which is best” gave the same type of incertitude that I was experiencing, as so many factors are involved in selecting a technology, none of which has yet to be finally realized.

I found one site that lists 3 Advanced Reactor Systems to Watch. This may have influenced Bard's response, as it lists the same three technologies Bard listed as best technologies. Another probable source of information is the Generation IV International Forum(GIF), which has a list of six promising Advanced technologies.

Based on this impressive Bard response, I decided to query Bard regarding some specific projects that are underway utilizing my four selected technologies.

What is the current status of the Kairos Power reactor project?


The Kairos Power reactor project is currently in the pre-application phase with the US Nuclear Regulatory Commission (NRC). This means that Kairos Power has submitted a preliminary design of its reactor and is working with the NRC to develop a licensing plan.

Kairos Power is developing a fluoride salt-cooled high temperature reactor (FHR) using TRISO fuel in pebble form. FHRs are a type of advanced nuclear reactor that is designed to be more efficient and safer than traditional nuclear reactors. TRISO fuel is a type of nuclear fuel that is coated in multiple layers of ceramic, making it very resistant to failure.

Kairos Power is also developing a low-power test reactor called Hermes. Hermes will be used to test components and systems for the KP-FHR. Construction on Hermes is expected to begin in 2024.

Overall, the Kairos Power reactor project is progressing well. The company is on track to submit a construction permit application for the KP-FHR in 2026. If the application is approved, construction on the reactor could begin in 2027.

Here is a summary of the key milestones for the Kairos Power

reactor project:

  • 2018: Kairos Power begins pre-application activities with the NRC.
  • 2022: Kairos Power submits a preliminary design report for the KP-FHR to the NRC.
  • 2023: Kairos Power begins construction on the Hermes low-power test reactor.
  • 2024: Kairos Power submits a construction permit application for the KP-FHR to the NRC.
  • 2026: Construction on the KP-FHR begins.
  • 2028: The KP-FHR is expected to begin commercial operation.

It is important to note that these dates are estimates and may change depending on a variety of factors, such as the regulatory review process and the availability of funding.


Now that was an up-to-date and accurate response! Moreover, it pointed out one of the reasons why these nuclear projects have a certain degree of uncertainty and can take so long - “the regulatory review process and the availability of funding.” To some degree, our Congress has recognized these issues and are seeking to abrogate them.

These MSR designs build on some 390 reactor-years experienced with sodium-cooled fast neutron reactors over five decades and in eight countries, according to

I also am interested in thorium as an energy source, so I queried Bard regarding one company that is utilizing this material and MSR technology– Thorcon.

What is the status of the Thorcon project in Indonesia?


As of October, 2023, the Thorcon project in Indonesia is still in the planning and development phase. In July 2022, the Indonesian government gave ThorCon the green light to proceed with its demonstration plant. Site feasibility and grid integration studies are currently underway, and preparations for pre-fission testing of the primary power loop have begun.

The ThorCon demonstration plant will be a 500 MW molten salt reactor. It will be built and tested at PT PAL's shipyard in Surabaya, Indonesia. Once testing is successfully completed, ThorCon expects Indonesia's Bapeten regulator to refine its regulations and issue a type license for the design. This would allow ThorCon to build additional commercial power plants in Indonesia.

ThorCon has signed memoranda of understanding (MOUs) with several

key Indonesian stakeholders, including the state-owned utility PLN, the state-owned shipyard PT PAL, and the Governor of Bangka Belitung Province, where the demonstration plant is planned to be located.

ThorCon has also raised a significant amount of funding to support

the development of its demonstration plant. In July 2022, ThorCon

announced that it had successfully completed an important round of

funding, which will provide the funds required to bring the program to the point where construction on the pre-fission test platform is

ready to begin.

Overall, the Thorcon project in Indonesia is progressing well, but it is still too early to say when the demonstration plant will be completed and operational.


One difference between the Kairos Power system and Thorcon MSR systems is that the latter plans to utilize thorium whereas the former has no plans to use thorium. Kairos Power utilizes a pebble-bed reactor core that has been developed by other firms such as X-Energy, NuScale and Terrestrial Energy. Thorcon reactors are self-contained modular units built in a shipyard and towed to the service site by boat. The construction technology is quite impressive.

See the Wikipedia article on Pebble-bed reactors. One disadvantage of pebble-bed reactors vs MSR is the ability of the latter to use a higher percentage of the fuel and somewhat easier removal of byproducts.

A brief history of the MSR can be found here.

Terrestrial Energy is a Canadian-based company working on fourth-generation

nuclear technology in the UK. The company’s Integral Molten Salt Reactor (IMSR) is a Generation IV nuclear reactor that uses molten salt as a coolant and fuel carrier.

Abilene Christian University has a program to develop an experimental MSR. Two key technologies of these advanced reactors are using molten salt as the coolant and providing fuel in a liquid form.

The most advanced MSR project is that of Terrapower:

What is the status of the Terrapower reactor project?


As of October, 2023, the TerraPower reactor project is on track to begin construction in 2025 and start operations in 2030. TerraPower has purchased land in Kemmerer, Wyoming, where the first Natrium reactor demonstration project will be built. The company has also awarded supplier contracts and announced an MOU with Centrus Energy to ensure that the demonstration reactor has access to the high-assay, low-enriched uranium (HALEU) fuel it needs.

The Natrium reactor is a sodium-cooled fast reactor that uses molten salt as an energy storage system. It is designed to be safer, more efficient, and more flexible than existing nuclear reactors. The demonstration project is part of the U.S. Department of Energy's Advanced Reactor Demonstration Program (ARDP), which is working to accelerate the development and deployment of advanced nuclear reactors in the United States. TerraPower is also working on a variety of other projects related to nuclear energy, including:

  • Developing a traveling wave reactor (TWR) design, which is a type of fast reactor that can operate on depleted uranium and produce less waste than existing reactors.
  • Working with other companies to develop HALEU production capabilities in the United States.
  • Developing new technologies to improve the safety and performance of nuclear reactors.

TerraPower is a leading company in the advanced nuclear energy industry, and its projects have the potential to revolutionize the way we generate electricity.


Terrapower uses well-known and thoroughly researched technology. It is backed by Bill Gates, who studied the technology for years, and given his technical background, I doubt that he would be behind a project that is unlikely to succeed.

As I researched, I was somewhat impressed by the touted fuel efficiency of the Dual Fluid MSR reactor. Here is Bard's response to my query about Dual Fluid technology:

What is the status of the Dual Fluid reactor project in Canada?


The Dual Fluid reactor project in Canada is still in the early stages of development. The Canadian-German nuclear technology company Dual Fluid was founded in 2021 to bring its technology to serial production status. In 2023, Dual Fluid signed a memorandum of understanding with TRIUMF, Canada's particle accelerator center, to advance the development of a small modular reactor (SMR) based on Dual Fluid technology. The Dual Fluid reactor is a new type of nuclear reactor that uses two different fluids for fuel and cooling. This design is said to have a number of advantages over traditional nuclear reactors, including:

  • Increased efficiency and safety
  • Ability to use processed nuclear waste as fuel
  • Reduced costs

Dual Fluid expects to have a prototype reactor launched by 2030, with serial production starting around 2034.

The Dual Fluid reactor project is receiving support from a number of Canadian government agencies and private investors. The Canadian government has provided Dual Fluid with C$20 million in funding through its Nuclear Research and Development Program. The company has also raised over C$100 million in private investment.

The Dual Fluid reactor project is a promising new development in the nuclear energy sector. If successful, the Dual Fluid reactor could provide a clean, safe, and affordable source of energy for Canada and the world.


Dual Fluid technology was developed in Germany. The Dual Fluid reactor, touted as a Generation V design, uses liquid lead as coolant but molten salt in the reactor core. The Rwanda Atomic Energy Board (RAEB) signed an agreement on September 2023 with Dual Fluid to collaborate on the development of a demonstration Dual Fluid nuclear reactor in Rwanda. The demonstration reactor is expected to be operational by 2026.

Today‘s Pressurized Water Reactors can only convert a rather small proportion of the exploited uranium (1%) into energy. A Dual Fluid reactor can burn any fissionable material, including thorium or natural uranium, as well as processed radioactive waste from conventional reactors and can burn the fuel more completely. This also means less radioactive waste, both in terms of quantity and half-life of the remaining actinides.

A core meltdown or uncontrolled power excursion is impossible with this design, and it generates electricity at about half the cost of fossil-fuel plants, according to the company.

One technology that Bard highlighted and that I had not was High-temperature Gas-cooled Reactors. Since my knowledge here was somewhat limited, I decided to asl Bard for help:

What companies are working on High-temperature Gas-cooled nuclear reactors?


The following companies are working on high-temperature gas-cooled

nuclear reactors (HTGRs):

  • X-energy (United States)
  • NuScale Power (United States)
  • Terrestrial Energy (Canada)
  • GE Hitachi Nuclear Energy (United States/Japan)
  • Mitsubishi Heavy Industries (Japan)
  • China National Nuclear Corporation (CNNC)
  • State Nuclear Power Technology Corporation (SNPTC) (China)

These companies are developing a variety of HTGR designs, including pebble bed reactors and prismatic block reactors. HTGRs have a number of advantages over traditional light water reactors, including:

  • Higher operating temperatures, which allows them to be used for industrial heat applications, such as hydrogen production and steelmaking
  • Greater safety features, such as passive cooling systems
  • Smaller footprints, which makes them more suitable for deployment in urban areas

HTGRs are still in the early stages of development, but they have the potential to play a significant role in the future of clean energy. In addition to the companies listed above, there are a number of other organizations working on HTGR development, including universities, research institutes, and government agencies.

The US Department of Energy (DOE) is also supporting the development of HTGRs through its Next Generation Nuclear Plant (NGNP) program. The DOE's goal is to develop and deploy a commercial HTGR demonstration plant by the early 2030s. The program is associated with Idaho National Laboratory (INL).

HTGRs have the potential to play a significant role in helping countries to meet their climate goals. They can provide a reliable and low-carbon source of baseload electricity, as well as heat for industrial applications.


Here is a link to more information on the Gas-Cooled Fast Reactor.

In August 2011 INL published a report on the Modular HTGR Safety Basis and App0roach.

Also see the X-Energy website.

A more detailed description of pebble-bed reactor technology (2002) is available at OSTI.

An excellent source of information on HTGRs, as well as all of these technologies is Their information is expansive but often somewhat outdated.

Here is a link to their information on Generation IV Reactors.

Also see DOE's Next Generation Nuclear Plant (NGNP) program.

Thanks to Bard, this phase of my research into nuclear reactor technology is fairly complete. Later, I will discuss other topics such as nuclear waste issues, the effects of previous nuclear incidents, as well as safety and regulatory concerns. Much work on reactors is being done in Russia, China, and India. I may discuss some of these projects as well.

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