Molten salt and traveling wave nuclear reactors

This is part 5 of a series.To read from the beginning 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 evoked by far the greatest enthusiasm. One can speak of a veritable “fan club.” Among the positive features of this reactor type (see below) are its intrinsic safety features and its suitability for utilizing thorium as a fuel source. Thorium is vastly more common than uranium, and promises to greatly simplify the problem posed by so-called nuclear waste.

Currently private firms, universities and government laboratories around the world are engaged in different aspects of MSR development. Private players include  Bill Gates’s Terrapower, Moltex Energy, Terrestrial Energy, Kairos Power LLC, ThorCon Power, Transatomic Power, Flibe Energy, ADNA Corporation, Seaborg Technology and Lightbridge.

The clear leaders in the field of molten salt reactors are the United States – where the MSR was invented – and China. The MSR has been a significant 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 has chosen MSR technology as an essential component of China’s medium- and long-term energy strategy.

The leading figure in China’s MSR effort is Jiang Mianheng, son of the former Chinese president and Communist Party general secretary Jiang Zemin and presently director of the Shanghai branch of the Chinese Academy of Sciences. Jiang Mianheng received his doctorate in electrical engineering from Drexel University in the United States and has headed up a variety of technology programs in China. A 2013 conference presentation by Jiang Mianheng on nuclear energy 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 degrees C – that is continuously circulated through the reactor core. In the core, the liquid-carrying channels are surrounded by neutron-moderating material (mainly graphite), which provides the conditions for fission chain-reactions to occur in the dissolved fuel. Leaving the core at a higher temperature, the fluid runs through a heat exchanger, transferring the extra heat energy to a secondary circuit. It is then recirculated back to core. Along the way the fluid can be processed, various reaction products removed and new material added, as desired. This type of design has numerous advantages, including:

1. A very high degree 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 graphite moderator to interact with, in fact, the neutrons have the wrong energies and cannot trigger fission reactions effectively.

If 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. If for any reason the liquid should become too hot for the reactor to operate safely, a special plug at the bottom of the reactor melts, letting the fluid drop down into emergency dump tanks. All of this happens without human intervention.

In case the fuel-carrying liquid somehow leaks out of the primary circuit, it will quickly cool to below its melting temperature, solidifying and trapping the radioactive substances contained in it. In a certain sense this is the opposite of the dreaded “meltdown” scenario in conventional reactors.

2. MSRs can be made cheaply because they are simple compared with conventional reactors, not requiring large, pressurized containment domes and many complicated additional safety systems found in the conventional systems. Having far fewer systems and parts makes MSRs inherently cheaper. This simplicity also allows MSRs to be smaller in size, which in turn makes them ideal for factory manufacture.

MSRs can run on uranium and existing stockpiles of plutonium and nuclear waste. A variant of an MSR, a liquid fluoride thorium reactor (LFTR), will be able to use abundant thorium as a fuel.

The MSR first emerged in the context of the US effort, launched in the late 1940s, to develop nuclear-powered aircraft. A small prototype, the 8 MWt Molten Salt Reactor Experiment (MSRE), was built and operated from 1957 until 1976 at the Oak Ridge National Laboratory in the United States. Alvin Weinberg, one of the fathers of nuclear energy in the United States, looked upon the MSR as a future workhouse for world 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.

As so often happens these days, this great “American” idea emigrated to China. More precisely: while the MSR has become a big topic again in the United States and elsewhere, China is the only country that is actually going ahead to build one.  Site preparation has begun at Wuwei, Gansu Province, for a 2 MW thermal power molten salt reactor, the TMSR-LF1. It is designed to be a prototype for multipurpose modular MSRs which can be used for hydrogen production and desalination as well as electricity generation. (See the next installment for more about small modular reactors.)

The traveling wave reactor

The traveling wave reactor (TWR) is the brain child 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, by fission reactions, with the process of breeding new fuel, in such a way that the newly-generated fuel contributes in turn to maintaining the fission process.

In the original Teller-Wood proposal, this combined process takes the form of a “burn wave” (“traveling wave”). The reactor is charged with an inner cylinder-shaped core of enriched fission fuel, surrounded concentrically by material from which plutonium can be bred – such as natural uranium, thorium or spent fuel from conventional reactors. The fission chain reaction begins in the core. Excess neutrons radiate out, generating plutonium in the surrounding layer of material. When the plutonium reaches a critical concentration, the chain reaction spreads into the plutonium, generating more neurons which in turn breed plutonium in the next concentric layer, and so on.

The result is a self-sustaining “burn wave” which gradually propagates into the material and continues after the original core is consumed.

This reactor would not need to be refueled every year or two, like conventional reactors, but could theoretically run for a decade or more. Over this period a large portion of the radioactive fission products (“nuclear waste”) would be “incinerated” by the neutron radiation.

Teller and Wood proposed a fully automatic design that would require no human intervention and no active control features once the traveling wave is started. They pointed out that in such a reactor a “runaway” burn wave is physically impossible  (technically speaking because of the time delay involved in the generation of plutonium by beta-decay). Furthermore the reactor could be designed with a large negative temperature coefficient, in such a way that the chain reaction would stop 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, the chain reaction picks up speed and more energy is generated.

A beautiful concept, but the opportunity to actual build a traveling wave reactor came only later, when Bill Gates stepped into the picture. A report by Gates’s TerraPower company to the 2010 International Congress on Advances in Nuclear Power Plants explains:

“The beginnings of TerraPower and its nuclear innovations are found in the deliberations between Bill Gates, Nathan Myhrvold, Lowell Wood and experts during 2006 brainstorming sessions in  Bellevue, Washington. The central focus of the discussions was how to provide sustainable, scalable low-carbon energy for all the  earth’s inhabitants. All forms of energy production were considered, including broad classes of solar and wind. Though these and other technologies were perceived as very important, it became clear that nuclear is the only known technology that can play the needed central role in providing base load power in an environmentally acceptable manner and on any type of relevant time scale.”

The report continues: “A small group, which eventually became TerraPower LLC,  started organized activities in early 2007. The objective was to  make improvements in as many areas of the nuclear enterprise  as possible: safety, waste, efficiency, economics, weapons-proliferation resistance, terrorist-risk reduction, and overall social acceptance. The group considered many types of reactors, including both existing and new concepts. As the assessments progressed,  it became increasingly apparent that the concept of the traveling  wave reactor (TWR), advocated at that time by Lowell Wood, offered improvements in all of these areas.”

Since 2007, TerraPower, in its own laboratories and through multiple partnerships, has carried out design work and experiments, setting the ambitious goal of building a full-scale demonstrator TWR by 2025, to be followed immediately by a commercial version that could be produced quickly in large numbers. In 2015, after four years of negociations, TerraPower signed an agreement with the China National Nuclear Corporation to build a prototype TWR power plant producing 600 MW of electric power. Intensive cooperation developed between Chinese and US nuclear engineers on the project. The demonstration plant was set to be completed at Xiapu in Fujian province by 2023.

At the beginning of last year TerraPower announced that the project had to be cancelled, owning to the new restrictions the Trump administration had placed on technology transfers. Since then, TerraPower and its consortium have been looking for other partners and above all another location to build a demonstration plant. I await more news.

Jonathan Tennenbaum received his PhD in mathematics from the University of California in 1973 at age 22. Also a physicist, linguist and pianist, he’s a former editor of FUSION magazine. He lives in Berlin and travels frequently to Asia and elsewhere, consulting on economics, science and technology. This is part 5 in a series. Click to read part 1 here, part 2 here, part 3 here and part 4 here. Next in the series is the concluding installment describing two more highly promising new reactor designs: the pebble-bed high-temperature reactor and small modular reactors for mass production.