This article is taken from the monthly journal Sciences et Avenir – La Recherche #901 of March 2022.
These are technologies that are growing. Or rather, go back… Molten salt nuclear reactors, or MSRs (Molten Salt Reactor), buried in the 1970s, are making a comeback. In October 2021, CEA and CNRS brought together about a hundred nuclear experts and players in Avignon for a “boot camp” (as the organizers name it) aimed at bringing the MSR community French-style by combining courses, workshops and a conference. The atmosphere was jubilant. Two months earlier, China announced the completion of a thorium-powered MSR prototype in the Gobi Desert. With a production target of 2 megawatts (MW), this is a full-scale test ahead of the launch of the 373 MW “XL” version scheduled for 2030. But behind the big Chinese tree lies a forest of reactors of various sizes, small ones like TerraPower’s 1.2 MW MCFR owned by American billionaire Bill Gates, or medium-sized ones like Terrestrial Energy’s 195 MW IMSR in Canada… In total, more than 70 startups start up in the world. adventures of molten salt nuclear energy.
Technology known for almost 70 years
The strange return of a technology known for almost seventy years. The Oak Ridge National Laboratory in the US even used a prototype between 1964 and 1969 without ever reaching the industrial phase. Despite the absence of major incidents, the US Atomic Energy Commission has drastically cut funding and placed a bet on pressurized water reactors (PWRs), the current industry. “This choice was unscientific, that’s for sure, confie Elsa Merle, who leads the MSFR (Molten Salt Fast Reactor) project of Molten Salt Reactors at the Laboratory of Subatomic Physics and Cosmology of Grenoble.. He was no doubt motivated by the sordid relationship between Oak Ridge, who fiercely defended his project, and the commission, which favored the REP. “
France then follows in the footsteps of the Americans and buys this technology for its nuclear program. Molten-salt reactors are timidly returning to French laboratories with the Bataille law of 1991, which obliges all scientific organizations to work on waste processing. This type of reactor can accurately integrate spent nuclear fuel as fissile material. “For a long time we were only a handful of researchers on this subject, remembers Elsa Merle, who has been doing this since 2004. ASTRID became the flagship recycling project. (prototype of a fast neutron nuclear reactor developed by the KAE, with the reuse of spent nuclear fuel, ed.). With the project frozen in 2019 for budgetary reasons, France found itself without a decision to make nuclear energy a sustainable energy source as it is recyclable. “
In conventional pressurized water reactors, the fuel is in solid form. These are uranium oxide granules packed into tubes assembled into pencils. “In the MSR, fuel is almost like liquid and transparent, like water, explains Daniel Hoyer, director of research at CNRS Grenoble and architect of the MSFR project.. It is a mixture of salts containing fissile material: lithium fluoride or sodium chloride. Everything is brought to a high temperature: from 600 to 700 ° C, depending on the type of salt. Then this heat is transferred through a heat exchanger to a heat carrier, also consisting of molten salts. (see infographic below). “And when you ask Daniel Hoyer about the interest of liquid fuels over solid fuels, the answer explodes: “Liquid is easier to handle using pumps, pipes… That’s why planes fly on kerosene, not coal! “
Fissile substances (thorium, uranium, etc.) dissolved in molten salts bring liquid fuel up to 600°C. This heat is used to convert water to steam or to use supercritical CO2 to drive a turbine and generate electricity. In the event of an accident, fuel can be quickly evacuated to drain tanks, which prevents core meltdown.
Easily adjustable power
Safety is a good example to illustrate the interest in liquid fuels. “The meltdown of the reactor core as a result of an uncontrolled chain reaction, as in Fukushima or Chernobyl, is impossible here, according to Daniel Hoyer. If the reaction is violent, the fuel heats up and the liquid expands. Then the fraction is pumped out to a less active zone, which, by increasing the transparency of the fuel salt, promotes neutron leakage and reduces reactivity. “Another advantage is its controllability. The power of the reactor is controlled by changing the fuel flow, so at a constant temperature, which does not damage the reactor structure, unlike a PWR. It also makes it possible to go faster. “The power of the reactor can increase by 50% in ten seconds, researcher believes. What, for example, to overcome the intermittency of renewable energy sources. “
Liquid propellants are also a real trap: you can put uranium-235, plutonium or even thorium in it, which is more common and better distributed on the planet. MSRs are also potential gravediggers for minor actinides, which are long-lived high-level wastes most problematic among those produced by modern power plants.
Finally, because these reactors operate at very high temperatures, they are more efficient: they produce less waste per kilowatt-hour produced. Furthermore, “The fact that actinides are constantly being recycled means that we do not produce long-lived radioactive residues. “explains Daniel Hoyer. However, it remains a nuclear fuel with associated risks: for example, thorium is highly radiotoxic and difficult to handle.
Pipe corrosion still under investigation
Behind these promises, aspects remain to be explored, including the chemistry of molten salts, to ensure that the pipes transporting the salts will not corrode. The laboratory of the Institute of Nuclear Physics in Orsay is engaged in research in the field of decontamination and corrosion. In this regard, researchers are not starting from scratch. Molten salts have been used in industry for decades. Another key point: the heat exchanger between the radioactive salt and the rest. Complex for development, “It has to be very efficient and very compact because the heat exchanger needs as little fuel as possible. “, summarizes Daniel Hoyer. Finally, there is a regulatory aspect, an agreement from the Office of Nuclear Safety (ASN) is necessary to start the reactor. There is still much to be done on this side. ASN replies that it does not have elements “because there is no project in France “.
But this can change very quickly. Young French startup Naarea is developing a molten salt reactor that will be 3D printed using new materials such as graphene. This method of construction, also used in several American projects, makes it possible, among other things, to ensure the tightness of a system that handles liquids without the need for welding. “Or even make the shape of the reactor more complexes that promote heat exchange with the surrounding air “adds Axel Loro, a physicist at CNRS.
This technology, which will simplify reactor architecture, is particularly interesting because Naarea focuses on very small reactors. This is in line with a fundamental trend in the nuclear industry: the creation of a series of low power modular reactors – less than 300 MW. About 70 different concepts are in development around the world. “Unlike cathedral power plants, this sector is opting for standardization and industrial production of small reactors. explains Michel Derdeve, professor at the Institute of Political Studies in Paris. This is a gap in concept that corresponds to the idea of a delocalized economy. “
Small divisions for isolated regions
Since 2020, the world’s first floating nuclear power plant has been operating in the Russian port of Pevek in Eastern Siberia. This plant supplies electricity to the region with two reactors of only 35 megawatts (MW) each, enough for a city of 100,000 people to replace a coal-fired power plant, thus preventing the release of 50,000 tons of CO2 in year. This is the first Small Modular Reactor, or SMR for Small Modular Reactor, in operation today. Now that their production has been put into series, four more barges are soon to anchor in this region of Chukotka, and construction of an onshore project will begin in the remote city of Ust-Kuyga in Yakutia. In the USA, Canada, China, Europe, the movement towards small reactors “This solution enables the decarbonization of isolated regions, mining sites and integration into small power grids.”, explains Michel Berthelemy, an analyst at the Nuclear Energy Agency (OECD). In the United States, Nuscale thus obtained the go-ahead to build 12 77 MW modules at the Idaho National Laboratory site. Commissioning in 2029.
“The nuclear industry began with small light water reactors, explains Michel Berthelemy, Analyst at the Nuclear Energy Agency (OECD). Then, to increase their power and achieve economies of scale, the size of the reactors was increased. The return to small reactors today aims to prioritize the series effect. “At the same time, the increase in burn rate also made it possible to produce more power with less fuel. But this excess energy required more cooling infrastructure. Small reactors of several megawatts, such as the Naarea, are cooled only by natural convection. , i.e. atmospheric air, which should allow in the future to install them on cargo ships or aircraft … So far, in addition to huge EPR, small reactors are an opportunity to locally reduce emissions of polluting gases, but if they are not properly monitored, this will be due to the dangerous possible spread of radioactive materials.
“Thousands of mini-reactors by 2030” by Jean-Luc Alexandre, CEO of Naarea.
“If we want to bring energy offers as close as possible to the needs of users, we must develop reactors with a capacity of one to several tens of megawatts (MW) that will supply boats, islands, industrial facilities … These, even most importantly, do not need more than 50 MW . The Naarea reactor works with mining waste and spent fuel that comes out of today’s power plants after four to five years of use. Considered waste today, they can be reused after being processed in our molten salt reactor. Already used radioactive material will be split by 99%. Waste volume will be minimized and long-lived radioactive elements eliminated.
Our goal is not to sell reactors, but only the generated electricity. We will restore the reactors at the end of their lives in order to regenerate them. This is a security guarantee. We want to produce energy that is cheaper than coal, but that will be paid at market prices by rich countries and at quasi-subsidized rates by poor countries. This would support their local economy, such as water pumps, and stimulate agriculture. We are currently developing a “digital twin”, a simulation that virtually reproduces the laws of physics and significantly reduces development time. Among the many tests that we have to carry out, we must, for example, study the behavior of graphene, a material that has exceptional resistance to the corrosive effects of various salts. Already used in marine paints, but not yet used in the nuclear industry. We hope to produce thousands of reactors by 2030.”
Interview with Sylvie Roy