Molten Salt Nuclear Reactors have a very high negative temperature coefficient leading to a safe and stable control. This is another beauty of the molten salt design. The temperature coefficient is highly negative, leading to a safe design enabling simple and consistent feedback. What does that mean? It means that when the temperature of the fissile core rises, the efficiency of the reaction goes down, leading to less heat generated. There is no risk for a thermal runaway. In contrast, graphite moderated generators can have a positive temperature coefficient which leads to complicated control, necessitating many safety circuits to ensure proper startup, operation and shutdown. Their worst failure mode is they go prompt critical, and no containment vessel can contain the explosion that would occur, so they were built without one. There have been several major accidents in graphite moderated reactors, with the Windscale fire and the Chernobyl disaster probably the best known.
With Molten Salt nuclear Reactors there is no risk for a meltdown, the fuel is already molten, and that is a safe design. The fissile fuel in a Thorium reactor is U-233 in the form of UraniumFluoride (UF4) salt which also contains Lithium and Beryllium to lower the melting point, the operating temperature is around 700C. In its molten form the salt has a very low vapor pressure. The salt flows easily through the heat exchangers and the separators. The salt is very toxic, but since it is completely sealed it is not corrosive. Being a fluid, it is constantly mixed for optimum efficiency. The reactor will never have to be shut down for refueling, it is a continuous flow process. Uranium-235 Nuclear reactors on the other hand have to be shut down for refueling and rebalancing of the fuel rods a little more often than once every two years. The average shutdown is 35 days, or about 5% of the time. Then comes the major problem of safely and securely transporting and reprocessing the spent fuel. In a LFTR the fuel is spent as it is produced, so the fissile inventory is constantly kept at a minimum, and fission products and extra generated U-233 is separated out. this is a much cleaner process than reprocessing spent fuel.
Molten Salt Thorium Reactors are earthquake safe. Thorium reactors have a very simple and compact design where gravity is the only thing needed to stop the nuclear reaction. Conventional nuclear reactors depend on external power to shut down after a SCRAM, where poison rods fall down to halt the reaction. The next figure shows the concept of a Thorium reactor.
The idea is to empty the fissile U-233 core through gravity alone. All that is needed is a melt-plug that is constantly cooled by cold air. In an earthquake or complete electric failure the cold air flow automatically shuts off, and since the fuel is already molten, it will melt the plug, and the molten fluid will run down into channels like pig-iron into heat exchangers that absorb the residual heat.
As we can see the reactor hardened structure is compact, and can be completely earthquake and tsunami proof. What can be sheared off are the steam pipes and external power, but even then the reactor shutdown will be complete safe without any additional power.
For decades, medical researchers have sought treatments for cancer. Now, Alpha Particle Immunotherapy offers a promising treatment for many forms of cancer, and perhaps a cure. Unfortunately, the most promising alpha-emitting medical isotopes, actinium-225 and its daughter, bismuth-213, are not available in sufficient quantity to support current research, much less therapeutic use. In fact, there are only three sources in the world that largely “milk” these isotopes from less than 2 grams of thorium source material. Additional supplies were not forthcoming. Fortunately, scientists and engineers at Idaho National Laboratory identified 40-year-old reactor fuel stored at the lab as a substantial untapped resource and developed Medical Actinium for Therapeutic Treatment, or MATT, which consists of two innovative processes (MATT-CAR and MATT-BAR) to recover this valuable medical isotope.
Thorium based Nuclear Power does not produce much Plutonium-239, which is the preferred material used in nuclear bombs. The higher Plutonium isotopes and other TRansUraniums are about as nasty as they get, need expensive protection against terror attacks, and need to be stored for a very long time.
One anecdote from my youth. The time had come to apply to University, and to my delight I was accepted to Chalmers’ University in Sweden as a Technical Physics major. I felt, maybe I can do my part by becoming a Nuclear Engineer and help solve the energy needs of the future. The Swedes at that time championed the heavy water – natural Uranium program together with the Canadians. Sweden is a non-aligned country, so it was not privy to any atomic secrets, it had to go it alone. They settled on the heavy water moderated natural Uranium process because Sweden had an ambition to produce its own nuclear bomb. Officially this was never talked about, and I was not aware of it at that time. They could have gone with Thorium instead, but a Thorium based nuclear reactor produces very little Plutonium, and what it produces is nearly all Pplutonium-238, not fissile and as such not suitable for bomb making.
I was excited to learn about all the possibilities and signed up for a couple of nuclear classes. One lab was to design a safety circuit, then run the heavy water research reactor critical and hopefully watch the reactor shut down from the safety circuit before the system safety circuit shutdown. About that time the word came that U.S. will sell partially enriched uranium at bargain basement prices if Sweden agreed to abandon the heavy water project and sign the nuclear non-proliferation treaty, a treaty being formulated by U.N.
Sweden was in awe about U.N, all the problems of the world were to be solved through it, and it had such a capable General Secretary in Dag Hammarskjöld, a Swede. I looked at the light water, partially enriched Uranium nuclear power plants being developed and decided to have no part with it, not due to safety concerns but it was the design that produced the most nuclear waste of any of the available designs. At that time there was still optimism that fusion would be ready by about the year 2010 or so. The cost of maintaining spent fuel in perpetuity was never considered, so light water reactors became the low cost solution.
India on the other hand refused to join the nuclear non-proliferation treaty, kept their heavy water program going and had by 1974 produced enough plutonium for one nuclear bomb, which they promptly detonated. They still use heavy water moderated reactors, but since India is low on Uranium but rich in Thorium they have now converted one heavy water reactor to Thorium with a Plutonium glow plug. It went on-line in 2011.
They are also developing molten salt Thorium reactors, but full production is still a few years off.
There we have it. We could have gone with Thorium from the beginning, but the cold war was on, and the civilian peaceful use of nuclear energy was still all paid for by nuclear weapons research and development. Once all the bombs we could ever wish for were developed the greatest asset of nuclear power became its greatest liability.
The nuclear waste meant for Yucca
would destine Nevada the sucka
But with Thorium we rid
us of waste that is hid
No need for that waste to be trucka!
Radioactive waste from an LFTR (Liquid Fluoride Thorium Reactor) decays down to background radiation in 300 years instead of a million years for U-235 based reactors. Initially LFTRs produce as much radioactivity as an U-235 based nuclear reactor, since fission converts mass to heat, but the decay products have a much shorter half-life. See the figure below.
Where is the storage for spent nuclear fuel and other nuclear waste now? Look at the map, it is scary.
And these are just the U.S. installations!
Many years ago I studied Engineering at Chalmers’ University in Sweden and I thought I would become a nuclear engineer. Sweden had at that time a peaceful heavy water based nuclear power program together with Canada and India. The advantage with heavy water as moderator is that it can use natural, un-enriched Uranium. One of the end products is of course Plutonium 239, the preferred material to make nuclear bombs, but it could also use Thorium, and the end product is then mostly Plutonium 238, used in space exploration, and we were dreaming big. One of the advantages of Thorium as fuel is that it produces about 0,01% of trans-Uranium waste compared to Uranium as fuel. About that time the U.S. proposed we should abandon the heavy water program and switch to light water enriched Uranium based nuclear power. They would sell the enriched Uranium, and reprocess the spent fuel at cost. They also had the ideal final resting place for the radioactive waste products in Nevada. This was an offer the Swedish government could not refuse, at the height of the cold war. This was in the 1960’s! India on the other hand did refuse, and they eventually got the nuclear bomb. Since that meant Sweden was never going to use Thorium as nuclear fuel, and I could not figure out how to get rid of all the radioactive waste products, I switched my attention back to control engineering.
What did President Trump mean with innovative approaches?
Is this where Thorium comes in!?
Thorium nuclear power is the only realistic solution to power space colonies. To form space colonies, power has to be provided to sustain the colony. This means that Liquid Fluoride Thorium Reactors (LFTR) have to be fully developed and operational here on earth before serious space colony development can even begin. It need to get started in earnest NOW!
Kirk Sorensen has provided an intriguing teaser on the cause for Thorium nuclear energy.
Watch it and enjoy!
NASA relies on pu-238 to power long-lasting spacecraft batteries that transform heat into electricity. With foreign and domestic supplies dwindling, NASA officials are worried the shortage will prevent the agency from sending spacecraft to the outer planets and other destinations where sunlight is scarce. Thorium reactors produce PU-238 as a “free” byproduct. In 2009 Congress denied a request to produce more Pu-238 by traditional means, instead relying on Russia to sell us the plutonium. (Remember the Russian reset?) Russia made their last delivery in 2010. PU-238 production has since been restarted by converting Ne-237 to Pu-238 at a cost of over 8 million dollars per kilogram. The Ceres-Dawn spacecraft used over 22 Kg of Pu-238 as electricity generator.
To get the best efficiency of generating Pu-238 out of a molten salt Liquid Fluoride Thorium Reactor, the excess U-233 and TRansUranium products have to be extracted continuously while the reactor is running, and this technology is not yet implemented, but is necessary to implement before we can also have Thorium power on the moon, and Thorium Power is the only viable solution if we are ever going to have a moon colony, so we should get to it.
A Thorium based fast breeder nuclear reactor produces much less TRansUranium waste, 0.012% waste products compared to a Uranium-235/238 fast breeder with between 3 and 8% U235. The Thorium process has a much higher efficiency of fission than the Uranium process. See the figure below.
Pu = Plutonium, Am = Americum, Cm = Curium, all TRansUraniums, nasty stuff.
With Thorium based Nuclear power, there are few storage problems, with traditional U235 power long tern storage is an immense and urgent problem, and has been since the 1960’s. At that time Sweden had a heavy water U-238 nuclear power program going using unenriched Uranium, but abandoned it in favor of traditional U-235 enriched nuclear power, because U.S. promised to provide the material and take care of the reprocessing and final storage of all nuclear waste at cost if Sweden joined the nuclear proliferation treaty. Reprocessing was to be done in Washington State, and one of the final storage sites mentioned was Yucca Mountain in Nevada, having the ideal geological properties.
Time went by and in 1982 – Congress passed the Nuclear Waste Policy Act, requiring the establishment of a deep geologic repository for nuclear waste storage and isolation. Yucca Mountain was high on the list out of of 9 possible sites.
Time goes by, and Congress is still not able to decide on a solution. Meanwhile, TRU’s from spent and reprocessed fuel is piling up in less than ideal locations. There is now more than a quarter million metric tons of highly radioactive waste in storage near nuclear power plants and weapons production facilities. Thorium based nuclear power would go a long way to alleviate this problem.
Thorium is a by-product of mining heavy metals and rare earth metals. The price is the cost of extracting and refining, which can be as low as $40/Kg. No extra mining is required for extracting the Thorium, and we all know that mining is a major source of pollution.
The first thing we must realize is that rare earth metals are not all that rare. They are a thousand times or more abundant than gold or platinum in the earth crust and easy to mine, but more difficult to refine. Thorium and Uranium will be mined together with rare earth metals.
U.S. used to be the major supplier for rare earth metals, which was fine up to around 1984. Then the U.S. regulators determined that Uranium and Thorium contained in the ore made the ore radioactive, so they decided to make rare metal ore a “source material” with all what that meant for record keeping and control. This made mining in the U.S. unprofitable so in 2001 the last mine closed down. China had no scruples, such as human and environmental concerns, so they took over the rare earth metals mining and in 2010 controlled over 95% of the world supply, which was in line with their long term plan of controlling the world by 2025. Luckily this has now been rectified with U.S. and Australian mines reopened, but the U.S. mined ore is still shipped to China for refining. However, in July 2019, President Trump activated Section 303 of the Defense Production Act to declare domestic production capability for rare earth elements and other critical minerals “essential to the national defense.” Domestic refining is scheduled to begin late 2020.
So, why is this important? Just take a look at all the uses for rare earth metals. The most sought after pays all the cost of mining and refining, and the rest are readily available at nominal cost.
The Chinese almost got away with it, and that was but one reason the trade negotiations were so complicated and hard fought, but necessary. Donald Trump fought for reciprocity and fair competition. Since the onset of the COVID -19 pandemic, originating in Wuhan, China, it has become more and more obvious that China can no longer be allowed to be single source supplier of anything.