Molten salt and traveling wave nuclear reactors

This is part 5 in a series. To read from the start, find Part 1 here, Part 2 here, Part 3 here, and Part 4 here.

The molten salt reactor

Of all the advanced reactor designs, the molten salt reactor (MSR) has generated by far the greatest enthusiasm. One can speak of a true “fan club”. Positive features of this type of reactor (see below) include its intrinsic safety features and its suitability for using thorium as a fuel source. Far more common than uranium, thorium promises to greatly simplify the problem of what is known as nuclear waste.

Currently, private companies, universities and government laboratories around the world are dealing with various aspects of MSR development. Private players include Bill Gates Terrapower, Moltex Energy, Terrestrial Energy, Kairos Power LLC, ThorCon Power, Transatomic Power, Flibe Energy, ADNA Corporation, Seaborg Technology and Lightbridge.

The clear market leaders in the field of molten salt reactors are the USA – where the MSR was invented – and China. The MSR was an important area of ​​technical cooperation between the two countries.

China occupies a unique position not only because it is currently the only country in the world that is actually building a molten salt reactor, but also because the Chinese Academy of Sciences uses the MSR technology as an essential part of the medium and long-term Chinese Technology selected. Concept of energy strategy.

The leading figure in China’s MSR efforts is Jiang Mianheng, son of former Chinese President and General Secretary of the Communist Party Jiang Zemin and currently director of the Shanghai Branch of the Chinese Academy of Sciences. Jiang Mianheng received his PhD in electrical engineering from Drexel University in the United States and directed various technology programs in China. A 2013 conference presentation by Jiang Mianheng on nuclear power in China is available online.

What is the molten salt reactor and why all the interest? The basic idea is to dissolve the nuclear fuel in a liquid – a molten salt at 600-700 ° C – that circulates continuously through the reactor core. In the core, the liquid-carrying channels are surrounded by neutron-modernizing material (mainly graphite), which creates the conditions for fission chain reactions in the dissolved fuel. If the core is left at a higher temperature, the fluid passes through a heat exchanger and transfers the additional thermal energy to a secondary circuit. It is then returned back to the core. On the way, the liquid can be processed, various reaction products removed and new material added if desired. This type of construction has numerous advantages, including:

  1. A very high level of passive (or inherent) safety. The composition of the fuel solution is such that chain reactions can only occur when it is surrounded by the graphite moderator. Without the interaction of the graphite moderator, the neutrons actually have the wrong energies and cannot effectively trigger fission reactions.

As the reactor heats up, the liquid expands, the reactions slow down, and after a certain point the fuel is no longer dense enough for the chain reaction to sustain itself. Should the liquid get too hot for any reason for the reactor to operate safely, a special plug at the bottom of the reactor will melt and drop the liquid into emergency emptying tanks. All of this happens without human intervention.

If the fuel-carrying liquid somehow escapes from the primary circuit, it cools quickly to below its melting temperature and solidifies and traps the radioactive substances it contains. In a sense, this is the opposite of the dreaded “meltdown” scenario in conventional reactors.

  1. MSRs can be manufactured inexpensively because they are simple compared to conventional reactors and do not require the large, pressurized containment vessels and many complicated additional safety systems found in the conventional systems. With far fewer systems and parts, MSRs are inherently cheaper. This simplicity also allows MSRs to be smaller, which in turn makes them ideal for factory manufacturing.

MSRs can be operated with uranium and existing stocks of plutonium and nuclear waste. A variant of an MSR, a liquid fluoride thorium reactor (LFTR), can use ample thorium as fuel.

The MSR was first created as part of US efforts to develop nuclear powered aircraft that started in the late 1940s. A small prototype, the 8 MWt Molten Salt Reactor Experiment (MSRE), was built and operated from 1957 to 1976 at Oak Ridge National Laboratory in the USA. Alvin Weinberg, one of the fathers of nuclear energy in the US, saw the MSR as a future workhouse for global development. Unfortunately, the program was discontinued as part of the process that led to the virtual monopoly of light water reactors in nuclear power generation.

Commissioning of a uranium-233 fuel-operated molten salt reactor on October 10, 1968 in Oak Ridge, Tennessee. Glenn Seaborg, chairman of the US Atomic Energy Commission, is in control. Photo: US Department of Energy / Frank Hoffman

As is so often the case nowadays, this great “American” idea emigrated to China. More specifically, while the MSR has become a big issue again in the US and elsewhere, China is the only country that is actually building one. Site preparation for a 2 MW reactor for molten salt with thermal power, the TMSR-LF1, began in Wuwei, Gansu Province. It is designed as a prototype for modular, multi-purpose MSRs that can be used for the generation and desalination of hydrogen as well as for power generation. (For more information on small modular reactors, see the next issue.)

The traveling wave reactor

The traveling wave reactor (TWR) is the brainchild of Edward Teller and Lowell Wood, both famous veterans of US nuclear weapons development. Teller is popularly known as the “father of the hydrogen bomb”. Both Teller and Lowell Wood played key roles in the Strategic Defense Initiative.

Teller and Wood published their original TWR proposal in 1995.

The basic idea is to combine the process of “burning” nuclear fuel through fission reactions with the process of growing new fuel in such a way that the newly produced fuel in turn helps to maintain the fission process.

In the original Teller-Wood proposal, this combined process takes the form of a “fire wave” (“traveling wave”). The reactor is loaded with an inner cylindrical core of enriched fission fuel that is concentrically surrounded by material from which plutonium can be grown – such as natural uranium, thorium, or spent fuel from conventional reactors. The split chain reaction starts in the core. Excess neutrons radiate and generate plutonium in the surrounding material layer. When the plutonium reaches a critical concentration, the chain reaction spreads into the plutonium, creating more neurons, which in turn grow plutonium in the next concentric layer, and so on.

The result is a self-sustaining “fire wave” that gradually spreads into the material and continues after the original core has been consumed.

Unlike conventional reactors, this reactor would not have to be refueled every or every other year, but could theoretically run for a decade or more. During this period, a large part of the radioactive fission products (“nuclear waste”) would be “burned” by the neutron radiation.

Teller and Wood proposed a fully automated design that does not require human intervention or active control functions once the traveling wave has started. They pointed out that in such a reactor an “out of control” wave of combustion is physically impossible (technically, due to the time lag in the production of plutonium from beta decay). In addition, with a large negative temperature coefficient, the reactor could be designed so that the chain reaction stops by itself when the temperature reaches a certain level. In this case the power level of the reactor would be controlled by the cooling system. When heat is extracted, the reactor cools down, the chain reaction accelerates and more energy is produced.

A nice concept, but the opportunity to actually build a traveling wave reactor didn’t come until later when Bill Gates stepped into the picture. A report from Gates’ TerraPower company to the 2010 International Congress on Advances in Nuclear Power Plant stated:

“The beginnings of TerraPower and its nuclear innovations can be found in discussions between Bill Gates, Nathan Myhrvold, Lowell Wood and experts during the 2006 brainstorming sessions in Bellevue, Washington. The focus of the discussions was the question of how sustainable, scalable, low-carbon energy can be made available to all of the world’s inhabitants. All forms of energy production have been considered, including broad classes of solar and wind. While these and other technologies were seen as very important, it became clear that nuclear power is the only known technology that can play the required central role in delivering base load power in an environmentally sound manner and on every relevant timescale. “

The report continues, “A small group that eventually became TerraPower LLC began organized activities in early 2007. The aim was to make improvements in as many areas of the nuclear company as possible: safety, waste, efficiency, economy, proliferation of weapons resistance, reducing the risk of terrorism and general social acceptance. The group considered many types of reactors, including existing and new concepts. As the evaluation progressed, it became increasingly clear that the concept of the traveling wave reactor (TWR) advocated by Lowell Wood at the time offered improvements in all these areas. “

Since 2007, TerraPower has carried out design work and experiments in its own laboratories and through several partnerships to achieve the ambitious goal of building a full-fledged TWR demonstrator by 2025, which can be immediately followed by a commercial version that can be quickly produced in large numbers. In 2015, after four years of negotiations, TerraPower signed an agreement with the China National Nuclear Corporation to build a prototype TWR power plant with a capacity of 600 MW. An intensive collaboration between Chinese and US nuclear engineers developed during the project. The demonstration plant should be completed by 2023 in Xiapu in Fujian Province.

An excerpt from TerraPower’s advanced Traveling Wave Reactor with nuclear power. Image: TerraPower

Early last year, TerraPower announced that the project had to be canceled due to the new restrictions the Trump administration had placed on technology transfer. Since then, TerraPower and his consortium have been looking for other partners and, above all, for another location to build a demonstration plant. I am waiting for more news.

Jonathan Tennenbaum received his PhD in mathematics from the University of California in 1973 at the age of 22. He is also a physicist, linguist and pianist, and a former editor of FUSION magazine. He lives in Berlin and frequently travels to Asia and elsewhere for advice on business, science and technology. This is part 5 in a series. Click here to read Part 1 here, Part 2 here, Part 3 here, and Part 4 here. Next up is the final episode, which describes two more promising new reactor designs: the silica bed high temperature reactor and small modular reactors for mass production.

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