Molten salt reactors: global savior or a deal with the devil?

The race is on to power the future.

Wikimedia. Image by Brian Kelleher

While all eyes are on so-called “renewable” energy sources to end the world’s dependence on fossil fuels before global warming irrevocably damages the climate’s ability to support our present human civilization and blue-sky alternatives such as fusion energy continue to grab the headlines, a 70 year old technology is quietly experiencing a renaissance.

Led by China, which cannot produce energy fast enough for its growing economy and 1.4 billion people, these reactors are safer than traditional nuclear plants and produce energy from thorium, an element about as common as lead.

Proponents of the technology argue that renewables simply cannot meet the world’s energy needs on the timeline that fossil fuels need to be phased out to prevent environmental catastrophe, particularly with massively growing economies in nations like China.

Many scientists argue that taking existing nuclear plants offline before renewables can replace them is thick-headed. For example, California’s Diablo Canyon plant produces 23% of the golden state’s carbon free electricity, yet it is scheduled to shut down in 2025 because of regulatory favoritism of renewables over nuclear power.

Fewer scientists, however, are arguing for building more nuclear plants. Yet, in many nations, thorium-based molten salt reactors are seen as a safer, less wasteful alternative carbon free energy that faces fewer hurdles in terms of land, water, and storage than wind, solar, hydro, and conventional nuclear fission. From Canada to the EU to Russia and China as well as the USA, investment in the technology is growing, but questions remain about whether such investments are a step backwards into a Cold War trend of waste, radioactivity, and nuclear proliferation that we hope to move on from.

These reactors use molten salt as the primary coolant and sometimes the fuel is dissolved into it as well. Most of these reactors or MSRs are fueled with thorium because of its abundance. Thorium is what is known as a fertile rather than fissile fuel, which means that it cannot produce a nuclear chain reaction on its own, unlike Uranium. Rather, it must be bombarded from outside with neutrons. The thorium will then transmute into an isotope of Uranium (U-233) that acts as the fuel source. In order to start the reaction, therefore, you need something like Uranium or Plutonium produced or extracted elsewhere.

These reactors were looked into decades ago in the United States starting with the aircraft reactor program in the 1940s and 50s and continuing to a long running experiment at Oak Ridge National Lab (ORNL) in the 1960s. Both of these programs ran into a lot of technical challenges and so they were closed down in favor of traditional nuclear fission in 1972. There is a lot of research going on right now to try to overcome challenges both technical and safety related, and, with the promise of carbon free energy, the field is heating up.

MSRs are generally considered safer than traditional nuclear plants. They do not require high pressure water that can result in radioactive steam leaks and are generally self-controlling with increasing temperature, lacking the meltdown potential of uranium plants.

There are two types of MSRs. The first are breeder reactors which produce more fuel than they consume. Essentially, they produce more Uranium than they need, so they are self sustaining. The second are burner reactors. This second burns spent fuel from traditional reactors, helping to solve waste and avoiding the problem of how to fabricate and reprocess fuel for the MSR, since the traditional reactor has already done it.

Nuclear reactors, whether MSR or traditional, are well known for producing enormous quantities of energy. While experimental reactors have produced megawatts of power, current proposed reactor designs in Russia and the EU will produce power in the Gigawatts (billions of watts) from a few cubic meters of fuel, e.g., 2400 MW from a 3.6 high, 3.4 diameter cylinder for the Russian MOSART design, 3000 MW from an 18 cubic meter core in the French MSFR design. By contrast, a typical solar panel produces only about 150 watts per square meter and, mind you, they can’t be stacked like nuclear fuel. They all have to face the Sun. That means, to produce 3000 MW, you would need 20 million square meters of land or 20 square kilometers, nearly 8 square miles.

Consider that the United States in 2020 used 93 quadrillion Btus of energy or about 27 trillion kWh (including all sources, not just electricity). Getting away from gas cars and machinery to battery powered will only increase that demand, promising an unbelievable energy crunch (that will only be worse in the EU where gas automobiles are being phased out.) About 1000 MSFR cores could supply all US electricity needs. This suggest that thorium as well as other fertile elements can rightly be considered superfuels.

Yet, there is a downside to MSRs in that they are nuclear reactors. They produce radioactive isotope byproducts just like ordinary reactors such as Cesium, Iodine, and Technetium, all of which will remain radioactive for hundreds of thousands or millions of years. Proponents of the reactors argue that this waste can be reprocessed, however, to a much smaller volume than existing reactors, and, therefore, is a reasonable risk to take. We already have huge amounts of spent nuclear fuel that is currently just sitting at the nuclear plants where it is produced, with no national plan for how to store or deal with it. (The US proposed repository within Nevada’s Yucca mountain has been in political limbo for decades now.) MSRs conceivably could burn up that fuel, turning it into useful energy and less dangerous isotopes. Thus, from that perspective, the byproducts of MSRs are safer than what we are currently tolerating — accidents waiting to happen as occurred at Japan’s Fukishima when a tsunami tore through the plant, spilling radioactive waste all over the region.

A more disturbing problem is that, as nuclear reactors, they also produce nuclear fuel that could potentially be used to make bombs. The byproduct of thorium bombardment is Protactinium-233 and Uranium-233. One could, conceivably, separate out the Uranium. A declassified 1960s memo showed that U-233 is very suitable for making nuclear weapons, similar to plutonium-239 and perhaps superior to U-235. Indeed, the Soviet Union produced a weapon that had a mixture of U-235 and U-233. There are technical complications in obtaining the U-233 out of the liquid fueled reactor and potential contamination with U-232 which would prevent it from being sufficiently reactive but there are proposed methods of bypassing the production of U-232 entirely, which would then result in pure U-233.

The number of nuclear nations has continued to grow over the past 80 years and preventing easy access to fissile material is still the best way to stop nations from obtaining them. While one could argue that no nation is really safe with them, it is clear that some have much better safety protocols and stable leadership than others. Still, it isn’t clear that we should abandon MSRs because of the risk of proliferation, but, rather, tightly control the technology as we have with traditional nuclear and other dangerous tech.

The remaining problem with MSRs, and the one that I think is most serious, is whether they are really a time saver, given the urgency of the climate crisis. No one currently has a working production reactor. The EU and USA have no good regulations that would control the fuel or waste, and we have a growing economy in safe “green” energy. Moreover, nations in Europe such as Denmark and Germany are progressing to produce all their electricity from renewables already. Does it make sense to go down this potentially dark path in a panic or should we simply fill the seas, prairies, and deserts with solar panels and wind turbines? Molten salt, after all, can store solar energy just as well as nuclear and is already being used for that purpose.

While the land area required for solar (not even counting wind) to meet our needs is large, consider that the US has about 4 million miles of roads. At a standard width for two lanes of 24 feet, we have conservatively 18000 square miles of tarmac just from roads. That amount of area devoted to solar would provide more than double current energy needs. If we are willing to devote that much to the construction and maintenance of asphalt, why not safe, cheap power?

China and Russia may have good reasons for wanting to build these reactors, and more power to them, literally. China is probably closest to it right now. The USA and EU are much further behind, and, while Canada has recently thrown millions into the fray, it will fall short of a complete solution without appropriate sources of fissile material in that nation.

My own take on the issue is that we should build MSRs as burners but only to get rid of our existing waste and possibly for use in space where solar is impractical, but anything more, such as breeder plants, may be a deal with the devil, diverting precious momentum from the green revolution, resulting in more radioactive material being produced apart from existing sources, and expanding on cold war technology that deserves to be left to history. Moreover, we will not get what we want in return which is a world free of fossil fuels ASAP.

Serp, Jérôme, et al. “The molten salt reactor (MSR) in generation IV: overview and perspectives.” Progress in Nuclear Energy 77 (2014): 308–319.