Frequently Asked Questions

Q: What is thorium and what makes it special?

A: Thorium is a naturally-occuring mineral that holds large amounts of releasable nuclear energy, similar to uranium. This nuclear energy can be released in a special nuclear reactor designed to use thorium. Thorium is special because it is easier to extract this energy completely than uranium due to some of the chemical and nuclear properties of thorium.

Q: What is a liquid-fluoride reactor?

A: A liquid-fluoride nuclear reactor is different than conventional nuclear reactors that use solid fuel elements. A liquid-fluoride reactor uses a solution of several fluoride salts, typically lithium fluoride, beryllium fluoride, and uranium tetrafluoride, as its basic nuclear fuel. The fluoride salts have a number of advantages over solid fuels. They are impervious to radiation damage, they can be chemically processed in the form that they are in, and they have a high capacity to hold thermal energy (heat). Additional nuclear fuel can be added or withdrawn from the salt solution during normal operation.

Q: Are the salts safe?

A: Very safe. Unlike other coolants considered for high-performance reactors (like liquid sodium) the salts will not react dangerously with air or water. This is because they are already in their most stable chemical form. Their properties do not change even under intense radiation, unlike all solid forms of nuclear fuel.

Q: Have liquid-fluoride reactors been built before?

A: Yes, two liquid-fluoride reactors were built at Oak Ridge National Laboratory in Tennessee in the 1950s and 1960s. These were small research reactors that were built to test the fundamental principles of a liquid-fluoride thorium reactor. The first, which was called the Aircraft Reactor Experiment (ARE) ran for a week in 1957, and the second, the Molten-Salt Reactor Experiment (MSRE) ran between 1965-1969 and validated many of the principals of the fluoride reactor concept.

Q: How does a liquid-fluoride reactor make electricity?

A: Fission reactions take place in the fuel salt, making it hotter. This heat is transferred to a coolant salt outside of the reactor. The coolant salt is then used to heat gas that turns a turbine, which turns an electrical generator, generating electricity.

Q: What is nuclear waste and how does a liquid-fluoride reactor address this issue?

A: So-called “nuclear waste” or spent-nuclear fuel is produced in conventional (solid-core) nuclear reactors because they are unable to extract all of the nuclear energy from their fuel before they have to shutdown. LFTR addresses this issue by using a form of nuclear fuel (liquid-fluoride salts of thorium) that allow complete extraction of nuclear energy from the fuel.

Q: What advantages does a liquid-fluoride thorium reactor offer a utility?

A: Unlike a pressurized-water or boiling-water reactor, a liquid-fluoride thorium reactor operates at high temperature and low pressure. Its high power density means that the reactor vessel itself is much s maller and lighter than an LWR reactor vessel; small enough, in fact, to be mass-produced in a factory rather than constructed onsite. Its inert-gas coolant does not boil in the event of a loss of pressure, and the fuel, blanket, and coolant salts do not react with air or water. All of this means that the containment building of a fluoride reactor can be much smaller than the containment of a light-water reactor of similar power output.

Q: What’s the difference between a “fast-spectrum” reactor and a “thermal-spectrum” reactor?

The basic idea behind nuclear fission is that you can use an electically neutral particle, the neutron, to destabilize a nucleus and cause it to split. This is a big deal because it’s very difficult to get charged particles, like protons and electrons, anywhere near the nucleus–they’re repelled by electrical forces. That’s the basic reason why nuclear fusion is so difficult.

But with the neutron, it’s a different story. It just waltzes right up to a nucleus and hits it, and the nucleus never saw it coming.

Here’s an animated gif of how fission works, and a little movie too.

Now the speed of the neutron when it hits the nucleus has a lot to do with how likely a fission is to occur. One might think, intuitively, that if the neutron is going really fast that it has a better chance of “shattering” the nucleus, but that’s not really how it works. Actually, for the fissile nuclei (such as U-233, U-235, and Pu-239) the SLOWER the neutron is going, the more probable fission is.

So you want slowed-down neutrons to maximize fission. And then from fission comes more neutrons, which continue the reaction. Well, mostly right. Actually, the neutrons borne from fission are going really fast. Really, really fast. And they have to slow down to have a good chance of causing fission. That’s where the moderator comes in.

The moderator in a nuclear reactor is the material whose job it is to slow down neutrons without absorbing them. This slowing-down is done by neutrons bouncing off the nuclei of the atoms in the moderating material. For most reactors, moderation takes place in the water that also cools the reactor. For a high-temperature reactor like the liquid-fluoride reactor, graphite (carbon) is used as the moderator.

The neutrons are born from a fission reaction, bounce around in the moderator, slow down, and then cause another fission reaction. This “bouncing-around” process is also called “thermalizing” the neutrons, because by bouncing around in the moderator, the neutrons are brought to the point where they have the same thermal energy as the surrounding material.

This graph shows how likely a fission reaction is based on the speed (kinetic energy) of the neutron that strikes the nucleus is. Cross-section is a concept that corresponds to the probability of interaction–the larger the cross-section, the more the probability of interaction. The energy of the thermalized neutron corresponds to temperature. If a neutron were at the same temperature as the room you’re in (~300 K), it would have an average energy of 0.025 eV. Not very much. If the neutron instead were at the same temperature as the hot fluoride salt in the center of a liquid-fluoride reactor (~1000 K) its average energy would be 0.086 eV. Not much more.

When neutrons are born from the fission reaction, they have energies around 2,000,000 eV, which corresponds to a temperature of 20 billion degrees! That’s much hotter than the center of the Sun! But like hot water poured into snow, when neutrons are that much hotter than their surroundings, they lose energy fast. And most all of that energy is lost through collisions with the nuclei of the moderating material.

So a “thermal-spectrum” reactor is a reactor that has been arranged in such a way so as to optimally “cool” the neutrons so they can cause fission. And as can be seen from the graph, fission is hundreds of times more likely when neutrons are “cooled” down by thermalization/moderation than when they’re “fast”.

So it’s logical to ask at this point, why would anyone want to build anything but a thermal-spectrum reactor? It would seem to have the minimum amount of fuel requirement for a reactor, and it would seem to maximize your chances of getting nuclear reactions. And indeed it does. But there is more to the story.

Uranium is an interesting substance, consisting overwhelmingly (99.3%) of an isotope, uranium-238, that is not fissile. But if uranium-238 captures a neutron it becomes plutonium-239, which is fissile. One more neutron into the plutonium and you get a fission reaction and energy. So you can imagine that it takes two neutrons to “burn” uranium-238.

But there is a very small amount of uranium (0.7%) that consists of the isotope uranium-235, which is fissile and only requires one neutron to fission. Despite constituting such a small fraction of uranium, this U-235 is where nearly all of our nuclear energy comes from today. And the fact that we are burning up this small resource is one of the basic reasons that our nuclear infrastructure is not sustainable. It’s also one of the basic reasons that today’s reactors make so much nuclear waste.

So couldn’t we just burn up the U-238 after the U-235 is gone? Well, to do that, we need to make sure that the fission of Pu-239 (which is what U-238 turns into after it absorbs a neutron) gives off at least two neutrons–one to convert a new U-238 into Pu-239, and another to fission that Pu-239. So how many neutrons does the fission of Pu-239 give off? Well, it all depends on the energy of the neutron that the Pu-239 absorbs. Here’s a graph showing the relationship.

Now this graph shows two lines. One is the line in purple that shows how many neutrons are given off from a fission in Pu-239. As you can see, it’s pretty constant across energies–nearly three neutrons emitted per fission. That seems to indicate there will be plenty of neutrons for fission, conversion, and even some to spare. But the blue line tells a different story. The blue line is the number of neutrons given off per absorption in Pu-239. Why are they different? Because Pu-239 has the unpleasant habit of sometimes just absorbing the neutron that struck it, and not fissioning. This happens more often when the neutron it absorbs is at the slowed-down, thermal energies.

The fact that plutonium-239 likes to eat thermal neutrons and not fission has tremendous implications for our energy future. At thermal neutron energies, the effective number of neutrons given off per absorption isn’t enough to sustain “burning” of U-238. You can see the line dip and weave around the magic 2.0 number at thermal energies (the energies at the left-hand side of the plot). When you account for neutron losses and a number of other things that real reactors must deal with, there’s just not enough neutrons to go around.

Here is the point where the road forks, where two paths present themselves, and one was taken, and the other effectively ignored. One path is thorium, the other path is the plutonium fast-breeder.

The path that was taken, or at the very least, the path that the nuclear community has wanted to take for the last sixty years, is the path to the plutonium fast-breeder. Confronted with the data that you can’t get enough neutrons from a thermal-spectrum reactor to “burn” U-238, they began to investigate what happens if you use a “fast-spectrum” reactor. At “fast” energies (the energies on the right-hand side of the plot) things start to look a lot better for plutonium. It makes significantly more neutrons per absorption than 2, and so the “burning” of U-238 looks to be quite feasible. But now you have a different problem, that of buil
ding a fast-spectrum reactor.

But before I go too far, let’s talk about the path not taken–thorium. Thorium is about three times more common than uranium and consists of only one isotope, thorium-232. It has no naturally fissile isotope like U-235, and thorium is not fissile in and of itself. But like U-238, it can be converted into a fissile isotope (U-233) by absorbing a neutron. One more neutron absorption in U-233 causes fission. So again, we ask the question, how many neutrons does the fission of U-233 give off? Is it more than 2? More to the point, is it more than 2 per absorption?

Yes, U-233 not only gives off more than two neutrons per absorption at thermal energies, it gives off significantly more than 2, which is enough to account for the inevitable losses that will occur in a real reactor. This means that a thermal-spectrum reactor can “burn” thorium in a sustained manner and doesn’t need to go to a fast-neutron spectrum. And that has tremendous advantages for safety, economy, and nuclear proliferation.

39 thoughts on “Frequently Asked Questions

  1. Currently most official Energy research is pursuing other approaches for future nuclear energy. Is there any documented analysis of the technical difficulties of LFTR (and/or other related possibilities) that is the basis for this decision? Has there been any attempt to use FOI to extract such information?

  2. Could you expand your answer to answer to "Are salts safe?".
    1. First is is obvious that anything that is 900 degrees is dangerous to a person.
    2. Can you comment on the likely result if the hot salt is exposed to normal air?
    3. What is the like result of water exposed to the salt. I would think a giant plum of steam would result, but would it be radioactive?
    4. Does the hot salt react with concrete?

  3. The WIRED article mentioned need to develop corrosion resistant components of a LFTR. How much of a problem is corrosion for piping?

  4. Practically, how small can they be?
    [IMHO: portability is a huge issue. It is the means by which the industry can eliminate the effect of 'local politics'.]

  5. What is stopping someone from building a working LFTR?

    I understand politics are in the way of some things, but why hasn't Google, or some other mega company built one of these things?

    Kirk, you've said in the past that a working model of a liquid-fluoride thorium reactor was built in the 70's I think. Why don't we fire it back up again?

    Everything about Thorium seems to be hypothetical right now. The world needs to see that it's not just a nerdy science project concept, but something that could actually power their house…

    So again, main question, what is stopping the building of one?

  6. For a typical installation, what would be the distinguishing exterior feature or features? Would it be recognizable as a reactor by the public?

  7. You've often mentioned liquid chloride reactors as waste-burning reactors. What's different about chloride salts and why are they better for this role?

  8. Could you please show the kilogram to megawatt production of Thorium and compare against other nuclear as well coal etc.


  9. You're not helping your cause by glossing over various critical safety issues here.

    Also — s/the salts will not react dangerous/the salts will not react dangerously/.

    One gets the sense that some very large issues with startup are not being explained fully. Questions such as "Why aren't we using thorium now?" and "Is a thorium-fueled reactor economically feasible?" need answers.

  10. "Its inert-gas coolant does not boil in the event of a loss of pressure"

    Wait — which coolant are you speaking of?
    "This heat is transferred to a coolant salt outside of the reactor. The coolant salt is then used to heat gas that turns a turbine…"

    The primary (1st stage) heat transfer is said to be a "coolant salt", presumably a liquid. This is the substance that must circulate through the reactor vessel. I'm no engineer, but that sounds rather more like a liquid than a gas. Were it a gas at low pressure it would have a rather low capacity to transport heat, less than 1/1000th that of an equal volume flow of liquid. I think you mean that the secondary heat-transfer fluid is a gas…. big deal, that's outside the reactor loop and is independent of whatever reactor type one is operating.

    The current plans to deal with "waste" surround a glowing core of stupidity and it appears that there's a great deal of value in the thorium cycle concept, but the sloppily presented details abo are filing down my confidence in the information presented here.

  11. @ Sam,

    I understand some of your concerns. I believe that it is not from a lack of ability to answer but from a lack of time and attention. Kirk runs this by himself as well as working a full time job. The point about not reacting to water is focused on not reacting chemically, as happened in Bhopal India. By using a secondary loop of molten salts and a inert gas like nitrogen the reactor can be designed to be passively safe through basic physics rather than multiple "safety systems."

    @ Rob,
    The answers to these questions about why no one is using a LFTR right now have been answered in the Google tech talks linked on the main page. Another part of the answer is the Betamax vs VHS syndrome. Once a particular application of a technology becomes dominant it is risky to work with the "untested" kind even if it is better.

    Another part of the answer is that it takes soooo looongg to burn up the fuel and do the testing. Nuclear fuel takes years to burn – which is a good thing, but it also means that testing cycles are inherently long as well. You need an investor that is here for the long haul to make a difference.

    A final part of the "why not now?" is that the Nuclear Regulatory Commission (NRC) has regulations that are focused on light water reactors. Even a Boiling water reactor had difficulty being licensed because the regulators had a list of questions that "needed answers" and did not understand that some of those questions simply did not apply to the BWRs. Investors in nuclear are aware of these limits in the NRC and are hesitant to invest in a technology that might not be licensed due to its substantial difference from the current LWR technology.

  12. Very interesting article in American Scientist. Obviously the VHS-Beta Max problem is theoretically the cause for this reluctance at DOE, but that would not hold in India. It is my understanding that India has significant reserves of thorium and has been interested in this type of reactor for some time. Why has India not successfully fielded a LFTR yet?

    Thanks again for the thought provoking material


  13. (1) Many claim that spent nuclear fuel can be dissolved in fluoride salts and thus burned in molten salt reactors, if so can spent fuel be dissolved in fluoride salts in a solid form and safely launched into a GTO + 1000 mile orbit? (Before the reactor is turned on)

    (2) How hot can a space based molten salt reactor become? Could one design be developed that could serve as a low efficiency nuclear thermal propulsion engine? IE can liquid hydrogen be run through a very hot molten salt reactor for thrust? (A molten salt bimodal engine) The nuclear aircraft experiment suggests maybe. This would save us from developing two systems, a molten salt reactor and a nuclear thermal rocket based on Nerva

  14. (3)could the nations existing fleet of nuclear power plants be converted over to Radkowsky Thorium cycle using MOX? (4)and is it safe to retrieve spent fuel assembles and place them back into the radkowsky cycle reactor?(5)Then floridize the spent fuel for a molten salt reactor cycle?
    (3) and (4) would be done this decade and as the molten salt reactors come on line we would do (5)

  15. How large can a lftr be made? Would a huge lftr say 4GWe dramatically lower the cost per kwh?

    What is the expected lifetime of a LFTR?

    How much preparation would conventional fuel rods have to undergo in order to "burn" the fissionable components?

  16. From your FAQ it seems like no nuclear waste is produced from the LFTR. But, as energy is not created nor destroyed, I suppose it must be continuously refueled (with thorium as far as I understand). And, if something goes in, something else must come out. So, what is the waste that "comes out" from a LFTR? What safety/storage problems does it pose?
    Thank you very much

  17. Looks like Kirk and Co. may be a bit busy to respond to all these questions so I'll take a shot at a few.
    @ Michele: All nuclear fission produces fission products, period. However, these decay at extraordinary rates, on the order of 50% in a week and 83% in a year. They're all virtually harmless within a couple hundred years. If you have a system such as the LFTR where multiple neutron reactions must occur to create long lived actnides (Pu, Am, etc.) you have a VERY small inventory of such problem atoms. This eliminates the concern of 10K+ year storage areas. Storing rapidly decaying fission products for two to three centuries is an easily accomplished engineering feat. We have buildings around the world to prove this point.
    @ Paul: A huge LFTR is not so much of an issue with the reactor as with the utility. Such a power output is massive overkill for all but the largest of cities and would represent a huge financial risk for a new technology. Could it be built, sure. The point is that this reactor design is so flexible that greater gains will probably be realized by manufacturing them within an assembly line and allowing for learning curves to take effect. As for lifetime, many specs call for 30 year refurbishments, somewhat akin to getting a full workup on your car at 100K miles, but afterwords you can expect another solid 30 years out of the LFTR… rinse and repeat. The important point is that since a huge reactor pressure vessel isn't needed (the primary concern in current reactor lifetime specs) for the LFTR, there's no reason to decommission an UNPRESSURIZED liquid reactor such as this, just keep it well maintained.
    As for the fuel rods, that's a longer answer I'll have to get to tomorrow. Till then…

  18. Ok, mass frustration rules the day. I attempted to post a four paragraph reply today to several questions and lost it. I forgot to save a backup to Word before I submitted so let this be a lesson to all. BACKUP before submitting!

    Ok, got that off my chest. I'm going to answer a couple different questions for my own sanity, I'll get back to the others soon enough.

    @Jason: No there are no flammable chemicals produced or used with the any of the Molten Salt Reactors (MSR). The flouride salts are relatively inert and do NOT energetically react with water or air. No burst of radioactive steam is going to occur if a main pipe was to break either. The salt would ooze (low pressure) onto the containment floor and channel down to the drain tanks to cool and solidify. This stuff isn't like sodium, it would be a pretty boring show if a core breach occurred. ORNL ran tests back in the day to prove this.
    Pure U233 in a weapon? Sure, it's fissile so it can do the job. However, no such weapon in sixty plus years of a nuclear armed world… what gives? The fact is, U233 has to be manufactured from Thorium (it's all manmade in a sense) and this manufacturing process always introduces contamination in the form of U232. U232 renders a weapons program a bust. Its decay chain contains hard gamma emitters that destroy/disable electronics and create serious heat handling issues. Not to mention the danger to those having to deal with these weapons- they'd become more and more dangerous as storage times increased for decades; not good from a weapons production point of view. This is most desirable however from a proliferation resistance point of view. Thus, no U233 bombs despite the abundance of Thorium. Instead North Korea and others are much more content to modify research reactors to cobble together some Pu239, which has none of these problems.
    @ Craig
    Power calculations you ask? We love those here… take a look:
    Fission of U233 atom= 200 MeV (all decays and gamma emissions considered but ignoring neutrinos)
    1.602×10^-13 Joules/MeV gives our conversion to Joules from electron volts (call this Y)
    Avagadros Number 6.023×10^23 for conversion of atom to mole (call this A)
    So the energy content of 1 mole of U233 (233 grams) is 200 * Y * A = 19.2 TJ (10^12 Joules)of energy.
    Now 1 Kg of U233 fully fissioned gives us 1000/233* 19.2 TJ= 82.4 TJ of thermal energy.
    Now convert this to Megawatt Hours (3600 seconds in an hour since a watt is 1 Joule per second) at 3.6×10^9 Joules. Thus 82.4×10^12 / 3.6×10^9 = 22,888 MWh(t) per Kg! With a conversion rate of ~50% for high temperature LFTR operations, you get ~ 11,444 MWh(e) p/Kg. If you continue these calcs for a GwYr then you end up with less than 1 Metric tonne p/GWy of electrical power production! Since a tonne of thorium fits into a milk crate with room to spare, you begin to grasp the meaning of power density.
    As for coal, assume 3 KwH(e) /kg… to get to 11,444 MwH(e) you need (remembering to convert from Kwh to MwH) 11,444,000/3 = 3,814,600 kg of the best coal (bitminous)! Conversion rate of 1 to 3.8 million for U233 to coal. Mind blowing really.

  19. @steven rappolee:
    I'm not sure an MSR would work well sans atmosphere – very low external coolant mass, and all – but something like a nuclear lightbulb might work.

    Basically, it's a double-walled sphere. The inner wall is fused quartz filled with a critical mass of gaseous UF6, and cooled by expanding hydrogen. The outer shell is UV reflective and filled with hydrogen gas (UV absorbtive).

    Essentially, the hydrogen gas is passed through the cooling loops in the quartz, into the outer shell, through a turbine / compressor, and the the outer wall to radiate heat. Because of the high temperatures (the core will operate at about 12,000 C, the coolant around 1000C), the main cooling mode will be radiative.

    Not sure it's more suitable than LFTR, though; the nuclear lightbulb was originally meant to be a propulsion design: heat the hydrogen via UV and push it through a nozzle.

  20. People do not worry about the danger of molten iron in a steel mill.

    People and all other live things have always had internal radioactivity in them since life began. The worries about radioactivity and nuclear waste in the world are far too excessive and have become a false secular religion. Every type of power plant puts radioactive elements in the air and on the ground including many geothermal and hydroelectric power plants. Natural gas fired and coal fired power plants release far more radioactivity into the air and onto the ground than nuclear power plants do.

    Chernobyl proved, not that reactors are super dangerous, but that they are less dangerous and deadly than chemical plant explosions. At this point only 51 people can be possible said to have died because of the explosions at Chernobyl, the worst possible nuclear power reactor event, and many of those were caused, not by the explosion, but by authorities not taking proper precautions after the explosion.

    All soil and rock on the earth has built in radioactive elements, and there are many places where people have lived in highly elevated radiation areas for thousands of years, even excluding those that live at high altitudes.

    There is no reason to treat radioactive elements and differently because they were once in a nuclear reactor, which actually eliminates many tons of radioactive elements and has not produced a net ton of radioactive elements. Fusion reactors, if built, can produce net amounts of radioactive elements far better and cheaper from even non radioactive elements. They could produce unlimited amounts of fissionable plutonium from uranium because hydrogen is, relatively, unlimited in the universe.

    Since the human race is not proposing to take up all the radioactive elements in all rocks and soil on the earth an put them in a "safe" place that is protected for millions of years. There is no reason to do the same for any radioactive element from reactors, especially not the ones that are unchanged by the reactor. Even the ones that are changed by the reactor do not represent an important danger more significant than the potassium in all life, including humans, if the possible level of exposure is less or only a few times more. People go up in airplanes or live in Denver Colorado where radioactivity from space is much more. Live cells and multicellular organisms have survived and repaired damage from internal radioactivity for billions of years, but have also repaired the far more dangerous and prevalent damage from oxygen and other chemicals and elements such as copper and silver and iron.

    There is an absolutely false belief that people can be perfectly safe. Another such false belief is that radioactivity is too dangerous to tolerate in any amount, whilst not knowing or forgetting that it is built into life and the earth and space and cannot be avoided.

    Almost all the monies spent to protect people from radioactive wastes would extend millions of times more lives by being used to build safer water pipe systems or doing more immunizations against disease.

    The worries about nuclear reactors and the efforts to stop them because of the pretext of the excessive danger of radioactive wastes had killed more people by exposing them to the fumes of fires of other sources of energy and put them in danger of lack of heat and food. Ethanol fuel production from food crops would not be needed if coal were made into gasoline at less cost and effort, and power was produced by nuclear reactors.

    It is known how to make reactors that cannot have any chance of a run away reaction under any circumstances. Carlo Rubbia invented one and others have been invented. Even Chernobyl type reactors could be modified so. ..HG..

  21. There are several LFTR programs underway at present. Of note, a Japanese consortium have started work on a small scale LFTR with a view to building a larger scale version when they have analysed the data.
    India has plans for a number of LFTRs because of the NPT. Their view is sod the uranium brigade, they're going with thorium. An abundance of supply helps too!
    There is also a program in the CIS. I don't know how far along that one is.

  22. Very nice update!
    Can't help noticing how solid core nuclear fission reactors were used for naval propulsion before for civilian electricity generation.
    About LFTR, can you make the small one first?

  23. I understand the purpose of this web site is to push the Liquid Salt version of thorium reactors, but to get the public thinking about thorium as a fuel, why not pressure goverments running CANDUs to use thorium and get some good press out there? Combining the safety of a CANU with the half-life of thorium seems like a perfect fit – unless I am missing something obvious here.

    What about building some small research LFTRs for schools ala the Candian SLOWPOKE and getting the next generation of nuclear scientists comfortable with the process?

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