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the Advantages of Thorium Reactors (long)

November 2, 2011

ARE ALL POWER PLANTS THE SAME?  In a word, no.  A full answer requires more than a single word.  The one page answer is here.  A friend asked me to explain the advantages of Thorium reactors.  We’ll need a full answer to avoid simple praise or trite condemnation of  some profound energy solutions.   I’ll compare Thorium with other ways to generate electrical power.

The short list of comparisons are these-

  • Safety,
  • Environmental advantages,
  • Ease of installation and site flexibility,
  • Low financial costs,
  • Necessity.

WHAT IS THE THORIUM FUELED REACTOR?Let’s dive into a definition first.  In this case we’re considering the advantages of a reactor cooled with molten salt.  It is fueled with the natural element called Thorium. The reactor converts a pound of Thorium into a pound of nuclear fuel.  The reactor is small compared to most electrical power plants; say about 100 million watts electrical output.  The reactor design has separate fluids to create the uranium fuel and to power the reactor.  Both fluids are molten salts.

Did you get all that and memorize it?  I’ll explain more as we go along.  For now we will simply call this reactor a Liquid Fluoride Thorium Reactor, or LFTR.

HOW ARE THORIUM REACTORS DIFFERENT?  I want to highlight this reactor’s unusual features because this is not your grandpa’s power plant.  LFTR uses a liquid as both a coolant and a fuel.  Though exotic for a reactor, the engine in your car or truck probably does the same thing.  In this reactor, the coolant and fuel are a molten salt that is solid at room temperature.  The salt can be heated until it glows orange hot and yet it doesn’t boil.  Now that’s a little different.  Salt baths are commonly used for treating metals, so we have a lot of industrial experience with them.

The reactor core is the important component that makes the fuel react.  Let me say that again; the fuel cannot sustain a chain reaction without the core.  That is how the reactor got its name.  The reactor is the way we cause the nuclear fuel to react and create heat.

We can put all the fuel in one tank and it will not react.  We can spill the fuel on the floor and it will not react, other than blistering the paint as it cools.  We can mix the FLiBe salt with water or air and the salt sits there.  That is a good thing.  It is very different from other reactors you’ve heard about.

We feed this reactor every few days as the plant operates rather than once a year during a plant shutdown.  We add fuel as we need it so there is not a year’s worth of excess reactivity we need to control. We could build a reactor that we only fueled once, but adding fuel as you go is safer.  It is safer for the same reason you don’t want to drag around a years worth of fuel as you drive your car.

We also clean the fuel every few days as it becomes contaminated rather than removing years of accumulated fission products at one time.  This means that the fuel is continually cleaned of radioactivity.  This is important because the fission products generate heat as they decay.  We limit the amount of heat the fuel will generate by removing the fission products and storing them outside the core.  That is different from your grandfather’s nuclear power.

In the LFTR, the part of the plant that makes electricity runs on a hot gas.  That is more like a jet engine than a steam locomotive.  The LFTR operates at high temperature so it can use air as a coolant rather than needing a large body of water to cool the plant.  That is also very different than most power plants.  All these differences seem unusual at first.

This isn’t your usual generating station.  A Liquid Fluoride Thorium Reactor can fit on a few volleyball courts rather than needing to be as large as a football stadium.  Sure, you could make a LFTR that large.  The good news is you don’t have to.

True, the small power plant doesn’t put out as much power as a huge nuclear plant, but the small plant is much easier to locate and easier to build.  The issue of a small plant versus a large plant is the same issue of a small car versus a bus.  You can fit the small plant were you need it.

One huge advantage of a small power plant is that you can deliver the pieces by truck and have the plant build it in a few months rather than taking many years.  The small LFTR is closer in size to a portable generating station than the huge building we usually think of as a power plant.  Those are a few of the differences, but let’s also look at what the LFTR has in common with conventional plants.


SAFETY-  Safety comes first.  I’m interested in Thorium because it can help people live better lives. That means safety comes first. For LFTR to be a benefit, the entire system from fabrication to ultimate disposal must be low risk.  In fact, the low risk is a major reason I’m attracted to these power plants.  The LFTR can have an intrinsically safe design.  Intrinsic safety means the plant will safely shut down if-

  1. we physically break the reactor into pieces,
  2. the load goes away and the plant stops providing electrical power to the grid,
  3. we lose internal power in the plant.

Stability- The sort of failures I just listed would cause severe health and safety problems in other designs, but LFTRs have the advantage that they can respond without human intervention.  Beyond that, they can respond to some of these events without intervention of any kind at all.  The plant will respond properly even without an active control system.  I find that feature very attractive from a systems design point of view.  Look, ma, no hands.

How could we break a reactor into pieces and have it remain “safe”?  To do so, the reactor has to operate in its most reactive configuration. That means we won’t get a local power spike if things shift around inside the reactor, even if they shift a lot.

Sure, the power plant has active electronic controls, but it also has a passive control system.  By design, the power from the core drops as the reactor heats up. In fact, if you stop cooling the reactor the power will fall dramatically. This provides intrinsic thermal stability.  Also, if we allow the molten salt to get too hot then the salt will automatically drain into a shutdown tank.  An operator does not have to operate this safety system: it only needs gravity to operate, and gravity is pretty reliable. This automatic drain feature provides walk away safety.

A temperature stable reactor is both safe AND flexible.  The reactor responds to the electrical load and provides more power as the load increases.  Natural stability makes the reactor easier to operate.  For example, you don’t want the reactor to suddenly shut down if the fuel concentration is slightly low, nor do you want a significant power surge if you slightly increase the fuel concentration. Inherent temperature stability provides both safety and an operating cushion.

Clean Fuel-  We create another safety feature by removing fission products from the fuel as the fission products accumulate.  These materials decay rapidly and generate heat.  They must be cooled and isolated for a few hundred years before they are safe to be around.  They don’t belong in a power plant, and they can be removed from the LFTR fuel stream. This means they are not part of an accident scenario if we “break” the reactor.

We need to compare LFTR with conventional nuclear power plants because of the significant difference in the amount and type of nuclear waste they produce.  A conventional nuclear plant starts with fuel concentrated to 3% U235.  That means that 97% of the fuel doesn’t contribute to making electricity but it can form long term high level nuclear waste.

The LFTR could not be more different.  More than 99% of the Thorium found in nature is suitable as nuclear fuel to make electricity. Thorium is added to the reactor as salt, while conventional reactors add uranium as ceramic pellets wrapped in a stainless steel tube called cladding.  The cladding and non-consumed nuclear fuel are the largest source of high level radioactive waste in conventional power plants.

Thorium has none of these materials.  The volume of high level waste from thorium is less than 2% of the waste from conventional spent nuclear fuel.  The nuclear waste from thorium is short lived compared to conventional nuclear waste. I contend that the waste is suitable for open burial once the waste is less reactive than the original ore. That requires a storage period of about 300 years.

No Bombs Here-  Atomic weapons do not directly involve reactor operation, but let’s discuss them anyway.  The simple facts are you can make a weapon out of almost anything given enough time and money.  The Liquid Fluoride Thorium Reactor is no exception.  Given enough time and money it is possible to harvest materials from the reactor to build some kind of weapon. Please understand that simply because something is possible does not mean it is likely.  There are several ways to make an atomic bomb.  Using a LFTR would be one of the hardest.  The other ways are not easy, but they are significantly easier than trying to create weapons grade plutonium or uranium from this fluid fueled reactor.

Low Pressure and Inert-  The LFTR is a low pressure reactor, and that significantly adds to safety.  It operates at a few atmospheres pressure, or about the pressure inside a soccer ball or your car tire.  This low internal pressure eliminates the need for the thick walled reactor pressure vessel that is used on water cooled reactors.  The low pressure and inert materials eliminate the potential for a steam explosion that is inherent in the design of a pressurized water reactor.  They eliminate the potential of a sodium fire that is inherent in a sodium cooled fast reactor, or the lithium fire that is inherent in many fusion reactor designs. Because salts are so chemically stable, there are simply fewer ways to create an explosive accident with a salt cooled reactor.

Easy to Package-  The light, compact, and passively stable power plant is easily hardened to withstand earthquakes.  The plant does not have to be located at the edge of a lake or ocean, so it is much less likely to be damaged by a Tsunami.

Air cooling is unusually reliable.  Some locations have run out of water, but I’ve never heard of a site which ran out of air.  (Yes, a sandstorm or volcanic ash eruption would be an exception.  Those events usually offer considerable warning.)  There are other advantages.


Low Carbon Footprint-  Despite its considerable size, a conventional nuclear power plant has a very low carbon footprint that lies between the footprint of hydro power and wind.  All three have a smaller footprint than solar power.  The LFTR is better than conventional nuclear, and the differences in carbon footprint are sizable.  The average person living the western lifestyle consumes the energy contained in a cube of coal that is 5 meters on a side.  For comparison, the same energy is contained in a golf ball sized sphere of Thorium.  Most of the carbon footprint from conventional nuclear power comes from mining, refining and concentrating uranium fuel.  Thorium is much more plentiful than uranium and is a waste byproduct of mining rare-earth minerals.

Unlike conventional reactors, the LFTR runs on natural Thorium without isotopic concentration. The massive concrete structure of a conventional power plant also adds to its carbon footprint.  The LFTR power plant does not require a concrete building designed to contain a violent steam explosion.  All of these differences significantly reduce the carbon footprint of LFTR.

Small Power Plants are Easier to Build-  Small size simply makes it easier to locate a LFTR where power is needed.  The LFTR and conventional nuclear plants produce similar energy per area, and both are quite different from alternative energy sources.   A 100 mega watt LFTR takes an area the size of 3 tennis courts, or about half an acre.  A wind farm takes 50 times more area, and that excludes the backup power plant it requires.

A solar power plant takes 48 thousand times more area, and that excludes the backup power plant it requires.  Note that the area required to provide power to three quarter of a million people changed from the size of a house lot to the area of a mid-sized city.  It is simply easier to license a small site than to license a much much larger one.  Rail access is not required, but road access is necessary for delivery and periodic maintenance.

High Efficiency Reduces Waste Heat-  High thermal efficiency means that less heat is released into the environment for each watt of electricity we produce.  Compared to electrical energy from LFTR, a coal fired power plant releases 12 percent more heat to the environment.  A conventional nuclear power plant releases 66 percent more heat to the environment than a modern LFTR.  A solar photovoltaic generator releases 81 times more heat to the environment.  I thought we were trying to keep the planet cool?

Ease of Installation and Site Flexibility-  Delivering the parts of a nuclear plant by truck is a small benefit.  The small plant does not require water-front access for delivery of materials by barge or for plant cooling.  Air cooling allows the plant to be located on the side of a mountain or in the desert, though some water is required for the human operators. The revolutionary aspect of a small reactor is the fact that the power plant come from a factory in standard pre-assembled modules.  The conventional reactor is built on site the way a stick-built house is built, one part at a time.  In contrast, the LFTR is assembled on site like a mobile home.  This means that site preparation and fabrication occur in parallel at different locations.  Like mobile homes, the reactors are built on a continuous basis and the next customer simply selects a delivery date.

The LFTR may be small, but it is not weak.  The semi-buried design minimizes the exposure of the plant to accidental storms and deliberate attack.


Cost is the result of the previous factors, and each has a cumulative effect.

  • Rapid installation decreases financial uncertainty.  The installation site can be prepared quickly because the power plant is small and simple.  Large cooling towers, cooling canals, rail lines, waste handling pools or heavy containment buildings are not required.  The size and location of the electrical demand does not need to be forecast eight years in advance.  This reduces the risk that the plant would be built and then sit idle if the local economy has a downturn.
  • Quick delivery means that construction interest costs are low.  The plant quickly begins to generate revenue rather than having capital costs tied up for many years during fabrication, construction  and certification of a custom plant.
  • License approval is easier because the power plants are standardized.  There are many identical units with a long learning curve.
  • The modular plant can be removed and reinstalled at another site if the political climate forces the plant to close.  No one wants a project to fail, but it is reassuring that substantial costs can be recovered if the project is cancelled.
  • Spent fuel presents a long term financial risk.  LFTR presents significantly decreased fuel storage and handling costs compared with conventional nuclear plants.  The low cost is due to high fuel efficiency and the vastly reduced amount of high level radioactive waste. The wastes are easily processed to remove medical materials and other precious  isotopes.
  • The air cooled Brayton cycle allows the plant to rapidly follow a changing electrical load.  Plants that can follow a changing electrical demand receive higher prices for each watt they generate compared to plants that can only provide base-load power.
  • The plants must be built and must also be removed at the end of their lifetime.  There are several decommissioning options.  The power core could be replaced and the electrical switchyard reused. To do so, the unit would be defueled and the empty reactor and moderator assembly would be removed for burial.  If regulations allow, the unit would be defueled and the empty reactor and moderator assembly entombed in place.  This keeps decommissioning cost low.
  • Thorium provides an inexpensive nuclear fuel.  (Thorium is about as abundant as Tin.)  Thorium is considered a waste material in rare earth mining and known supplies are sufficient to provide the world’s current population with electrical power for several thousand years. We have more fuel than we need for current demand and we haven’t looked very hard to find more. This means the long term price of Thorium should be stable
  • The reactor can be designed to start with minimal outside power.  In contrast, some power plants require up to 15% of their rated power in order to start operation.  A cluster of LFTRs might have one reactor equipped for a “black start”, and “black start” power plants earn higher fees.  Black start power often comes from hydro plants and combustion turbines.  LFTRs add to that option which conventional nuclear plants cannot.

WHAT NEED DOES THIS POWER PLANT SATISFY?  A packaged reactor can provide safe and environmentally responsible power to the first world and the third world.   In the industrial west it means lower energy costs and more employment.  In the third world it means millions of lives.  It means income and food for a family.  If we provide an alternative, then we might stop stripping our planet’s forests to provide cooking fuel.

WHAT IS THE BEST FEATURE OF THIS REACTOR?  I’ve described the improvements that Thorium reactors offer over other power sources.  The best feature is not one of them, but all of them in combination.  The best feature is that molten salt reactors offer many improvements without major technical weaknesses.

WHY DON’T WE HAVE THESE REACTORS MAKING POWER NOW?  That answer belongs in another post.

S.F.  Doug, this one is for you.


Here is TLFTR in 5 minutes on Youtube.  Give it a view.

4 Comments leave one →
  1. November 3, 2011 10:58 am

    Great article Rob! It answered a lot of questions but raised many more – like how do you clean the fission products out of the reactor while it’s running? Sounds like an uncomfortable job.


    • November 3, 2011 6:13 pm

      Thank you, Doug. Part of the salt flow is removed and cleaned on a continuous basis. The uranium is removed from the salt by fluorination, and reintroduced to the reactor. The contaminated salt can be separated from the fusion products by electrochemical separation through a metal bath, or by distillation. People are not in the chemical area of the plant.


  2. November 9, 2011 1:55 pm

    Hi Rob,
    You should leave a link to your excellent Lftr Ed materials on *Google+* in my H.Op. – there’s an MSR-hungry tech audience there that has a high degree of political activism.

    For example, check out the below about a way for a “US Industrial Renascence”!
    Kim L Johnson



  1. Disruptive Thinkers and Thorium Power « SlowFacts

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