Future Energy, Thorium and the LFTR reactor
Steve Haigh, August 2013
This video looks at the science, technology and economics of using Thorium in a power generating nuclear reactor.
Thorium is a relatively abundant metal found in the earth’s crust. It is forged in the cores of dying stars and scattered across the galaxy by supernovas. It arrived here 4.5 billions years ago in the formation of the earth. Today, Thorium decay produces a significant amount of the Earth’s internal heat.
Thorium is about as common as lead, 4 times more common than uranium and is produced and usually discarded as a byproduct of mining rare earth metals, used to make magnets for motors, etc.
Thorium is cheap and plentiful but by itself, is not a nuclear fuel.
Natural thorium decays very slowly compared to many other radioactive materials, and the alpha radiation emitted cannot penetrate human skin. Handling small amounts of thorium considered safe.
In today’s light water reactors, it takes 250 tones of natural uranium to produce 1 gigawatt of electricity for 1 year. 1 GW is about the average size of a typical, current generation, nuclear reactor. A 1GW light water reactor produces 35 tons of spent nuclear fuel which is highly radioactive for 10,000 to 1 million years. The USA currently has total electricity generating capacity of 1050 Gigawatts.
By comparison, a theoretical thorium based LFTR type reactor consumes 1 ton of cheap thorium and produces 1 ton of waste, most of which is radioactive for 10 years, the remaining for 300 years.
To replace all coal, gas and conventional nuclear generating capacity in the US (929GW) with LFTR reactors, would theoretically require 929 tons of thorium and produce 929 tons of waste per year. 929 tons of thorium is 82 cubic meters in volume. So to replace all current US coal, gas and nuclear generation capacity requires just 3 standard 40’ shipping containers of thorium per year and produces the same amount of waste.
Currently, the US generates 2,000 tons of nuclear waste per year, to generate 838TWh/year of electricity. To produce the same amount or electricity in a year, a Thorium based LFTR reactor would produce 8.7 tons of waste
Why are we still talking about fission, isn’t that bad?
Many people will turn off at the mention of the word fission; after all, look at Fukushima, Chernobyl, 3 miles island. Why not use solar, wind, and Holy Grail of future energy, fusion? We will, but thorium fission has, in the medium term, the potential to greatly reduce our reliance on fossil fuels. Stay tune for second half of this video when we will look into other energy sources and the economics of thorium based energy.
This video is a work in progress
I am not a nuclear scientist; I’m a computer programmer with a bit of free time on my hands and an interest in energy issues and no agenda to promote. After I started learning about Thorium (mostly on youtube and Wikipedia) and I felt that the subject needed better explanation to the ‘average Joe’ but without dumbing it down to mainstream media levels. BUT THERE ARE ALMOST CERTAINLY GAPING HOLES AND ERRORS IN THIS PRESENTATION. I hope that by circulating it on youtube now and generating some interest, serious nuclear scientists will contact me and provide corrections and clarifications. Then I’ll make an updated video and remove this one.
PART 1: Thorium fuel cycle
Thorium is not a nuclear fuel; it is not a fissile material, meaning it cannot maintain a nuclear chain reaction (and therefore produce energy) by nuclear fission. Thorium must be used in conjunction with uranium, specifically, U233 to produce energy in the thorium fuel cycle, and here’s how:
The reaction starts when a neutron crashes into a uranium-233 atom. U233 is an isotope of uranium. Isotopes are variants of an element. All isotopes of a given element share the same number of protons, they differ in the number of neutrons. The 233 in U233 means there are a total of 233 protons and neutrons in the nucleus of the atom.
Uranium-233 is not a naturally occurring element, it is not mined from the earth like U235 used in conventional nuclear reactors. It is produced by bombarding thorium with neutrons.
If a neutron his a U233 atom, it has a chance of getting ‘captured’ by the nucleus, and if it does, the atom undergoes ‘nuclear fission’. Each atom splits into 2 new atoms, like Xenon, Silver, Palladium, Molybdenum, plus 2 or 3 free neutrons and a lot of energy. It’s this energy in the form of heat that we are ultimately interested in to make electricity.
The 2 new elements that are produced are called ‘fission products’. They are basically unwanted waste products.
It’s the 2 or 3 neutrons that we are interested in because they can go on to hit more U233 atoms and produce a self sustaining chain reaction, producing more heat and more neutrons as long as there is U233 to fuel the reaction.
So at this point we are already producing a lot of energy, simply by packing a lot of U233 into a relatively small space to create what is called ‘criticality’, meaning every fission causes an average of one more fissions.
The problem is, sooner or later, the U233 nuclear fuel will be used up and its expensive to buy more. This is where thorium comes to the rescue.
Thorium in nuclear scientist speak is a ‘fertile’ element. Without getting into details, think of it as fertilizer. You can’t eat it directly, but you need it to grow your food. That’s exactly what thorium does in the thorium fuel cycle, it ‘breeds’ more fuel, namely U233.
When thorium-232 gets hit by a neutron, it has a chance of capturing the neutron, which ‘transmutes’ it into thorium-233 (adds a neutron). Th233 then quickly decays to protactinium-233, which after about 30 days on average, decays into U233. This U233 can be separated from the thorium and protactinium and put back into the core to provide a limitless supply of fuel. The thorium is consumed by this reaction so needs constant replacement.
However, there is one small glitch. The neutrons resulting from the fission of U233 are traveling very fast. Like trying to catch a bullet from a gun, they are very hard for the thorium and uranium atoms to capture. By passing the neutrons through carbon-12 (graphite), the neutrons are slowed down like a speeding bullet going through honey. The neutrons coming out of the carbon-12 are traveling at close to the same speed as the thorium, so like in our honey/bullet analogy, they are very easy to catch. In this state, the neutrons are called ‘thermal neutrons’ meaning they are in thermal equilibrium with their surroundings, the same temperature as the U233 salt. If you remember your physics, you’ll know temperature is measurement of a particles velocity or kinetic energy, heat is simply excited atoms.
And so we have the thorium fuel cycle, cycle because it goes around in a never ending loop where U233 atoms undergo fission to produce energy, neutrons and waste products. Then neutrons and thorium produce more U233. Then by reusing the U233, topping up the thorium and removing the waste products, the system can produce energy for as long as the reactor lasts. And even after that, the U233 and thorium can be used in new reactors.
PART 2: Liquid Fluoride Thorium Reactor, LFTR (lifter)
This video is not about how to design a reactor but it is useful to describe how Thorium is turned into energy in the context of a theoretical reactor. There are several substantially different ways to design a nuclear reactor to use the thorium fuel cycle, but the one that potentially has the most promise and interest at the moment is the Liquid Fluoride Thorium Reactor, or LFTR (pronounced lifter) for short.
In this example of a two fluid LFTR reactor, the cycle starts with a neutron floating in a ‘soup’ of Uranium-233 fuel salt in the ‘fissile core’ of the reactor, the purple block in the drawing. (Fissile core meaning that’s where all the fission takes place). The Uranium-233 fuel salt soup is composed of U233 (a metal at room temperature) dissolved into a molten salt solution. This is not the kind of salt you put on your fish and chips, it composed mostly of beryllium and lithium at about 600°C at atmospheric pressure. Beryllium and lithium are relatively rare, expensive and highly toxic metals, but are not consumed by the reactor, so once the reactor is fueled, the beryllium and lithium rarely needs topping up.
The reactor core runs close to atmospheric pressure which is huge safety advantage over conventional reactors using high pressure water to cool the core.
Also because the reactor runs at high temperature, it is far more efficient in producing electricity from heat than conventional reactors.
From the thorium fuel cycle we know that when a neutron is captured by a U233 atom U233 fission produces 2 new elements called ‘fission products’, 2 or 3 neutrons and lots of heat.
In the LFTR reactor, the U233 molten salt is pumped through a heat exchanger to extract the heat to boil water into steam or heat gas for a gas turbine ,and then onto an electrical generator and onto the power grid. Generating electricity from heat like this is conventional technology in all coal, gas and nuclear power stations today. The secondary coolant going to the turbine is at high pressure but the coolant used is water or a harmless gas such as helium, nitrogen, or carbon dioxide and this part of the system is not radioactive. In the event of an accidental release of the high pressure secondary coolant, no radiation would be released. In a practical system there may be two or more sets of heat exchanges to further isolate the high pressure steam or gas in the turbine from the highly radioactive, low pressure core salts.
The fission products are basically unwanted waste products that need to be removed from the core or they will ‘poison’ the reaction, i.e. absorb neutrons, slow down the chain reaction and eventually kill it.
A key feature of the LFTR design is that fission products are separated from the U233 salt solution continuously, without shutting down the reactor, a major advantage over traditional nuclear reactors which need to shut down every few years to reprocess the solid fuel. LFTR reactors need never shut down in their normal working life so providing better overall economy of operation. Fission products are separated from the U233 and salts by a relatively cheap and simple chemical process called pyroprocessing (and I have no idea what that is).
There is no need to make the fuel salt very clean, the purpose is to keep the concentration of fission products and other impurities like oxygen low enough so it is not necessary to clean all the salts all the time.
Nuclear fission in the core continues in a controlled chain reaction moderated by control rods inserted into the core made of neutron absorbing material like silver, indium or cadmium.
Surrounding the U233 fissile core, is a ‘blanket’ of thorium-232, the ‘fertile blanket’. As you may recall from the thorium fuel cycle, this is where thorium is the ‘fertilizer’ that breads U233 fuel. Like the U233, the thorium is dissolved in a hot salt of beryllium and lithium at the same temperature and pressure as the core.
In a simpler one fluid LFTR reactor, the U233 and thorium are mixed together in a single core which is mechanically much simple than the 2 fluid core and blanket design shown here but has greater difficulty of separation of waste products and U233 from the thorium. However it is import to realize this example is not the only LFTR reactor design currently been studied or proposed.
Fast neutrons produced in the core passed through the graphite core wall and slow down to thermal neutrons so they are moving at the same speed as the thorium-232 atoms. Because of this, most of the neutrons are ‘captured’ by the Th232 resulting in high efficiency breading and less leakage of high energy neutrons into the reactor containment walls.
According to the thorium fuel cycle, Th232 plus a neutron transmutes to Th233 which decays to protactinium-233 which decays after a few weeks to U233. A chemical separation system removes the U233 from blanket salts and returns that to the core to provide a continuous supply of fuel forever. The reactor can be designed to be break even in the amount of U233 it produces and consumes, up to about 9% more U233 than it consumes, so the excess can be used to start new LFTR reactors.
Because thorium is used up in this process, it must be periodically replaced at the rate of about 1 ton per year for a 1GW reactor, which is 2.7Kg per day, something you could hold in the palm of your hand.
As a safety feature of this design, there is an emergency drain pipe at the bottom of the core to a ‘dump tank’ of neutron absorbing materials like boron, cadmium, cobalt, titanium, molybdenum, etc. The dump tank is below the core so gravity transports the core salts to the dump tank in the case of an emergency.
The drain pipe is normally blocked by a ‘freeze plug’. This plug is frozen core salts, frozen in this case meaning about 200°C, which is below the melting point of the salts. The freeze plug is kept frozen by a fan. If there is a catastrophic failure of power to the reactor cooling system such as in the Fukushima nuclear plant, and the ‘shit hits the fan’ so to speak, the fan will stop working, the plug will quickly heat up and the core salts will drain into the dump tank. The neutron absorbers will instantly kill the fission chain reaction and in a few hours the core salts will solidify with no release of radiation outside the dump tank.
Additionally, there is an overflow pipe at the top of the core leading to the dump tank. If the core overheats but the freeze plug fan still has power, the core salt liquids will expand though thermal expansion. This will cause core salts to flow out under their own pressure (no pumps) into the dump tank, reducing the amount of fissionable material in the core and cause it to cool down.
These are some of the major advantages of liquid salt reactors over solid fuel reactors in use today. The emergency shutdown system does not require people, computers, pumps, electricity, etc, it is totally passive relying solely on conduction heating, thermal expansion and gravity.
PART 3: LFTR Pros and Cons
The following is a summary of the Wikipedia LFTR page
Thorium-fueled molten salt reactor offers many potential advantages compared to conventional solid uranium fueled light water reactors:
- Inherent safety. If the core salts overheats, they expands considerably due to the liquid nature of the fuel and will push fuel out of the active core region. Also, Thorium absorbs more neutrons if it overheats leaving fewer neutrons in a one fluid reactor to continue the chain reaction, reducing power.
- Stable coolant. Molten fluorides (core and blanket salts) are chemically stable and impervious to radiation. The salts do not burn, explode, or decompose, even under high temperature and radiation. There is no combustible hydrogen production that water coolants have.
- Low pressure operation. Because the coolant salts remain liquid at high temperatures, LFTR cores are designed to operate at low pressures or about 0.6 MPa, the pressure of your house’s water pipes. LFTR coolant salts have very high boiling points. Even a several hundred degree heatup does not cause a meaningful pressure increase. There is no water or hydrogen in the reactor that can cause a large pressure rise or explosion as happened during the Fukushima Daiichi nuclear accident.
- Leak Resistance. Due to the low pressure operation the reactor plumbing is not under high stress from high pressures, so the potential for large leaks is greatly reduced.
- No pressure buildup from fission. LFTRs prevent pressure buildup due to gaseous and volatile fission products. The liquid fuel allows for continuous removal of fission products so they would not be spread in the event of a core breach.
- Easier to control. Xenon-135, an important neutron absorber and makes solid fueled reactors difficult to control. In a molten fueled reactor like a LFTR, xenon-135 can be removed continuously.
- Fail safe core. LFTRs can include a freeze plug at the bottom as discussed already.
- Less long lived waste. LFTRs can dramatically reduce the long-term radiotoxicity of their reactor wastes. Light water reactors have fuel that is more than 95% U-238. These reactors normally transmute part of the U-238 to plutonium with a half life of 24,000 years. In contrast, the LFTR uses the thorium fuel cycle, which transmutes thorium to U-233. U-233 has a 98% chance to fission to more U-233 or U-235 fuel. 2% of remaining fissions can produce radiotoxic waste at about 15 kg per GW per year. These transuranic products (bad shit) are 20x smaller in LFTRs than light water reactors.
- Destruction of existing long lived wastes. LFTRs can use existing nuclear wastes such as plutonium for their initial startup, so they can actually help reduce current stock piles of nuclear waste.
- Proliferation resistance, nuke safety. The LFTR makes is more difficult for diversion of its fuel to nuclear weapons in four ways: First, the thorium-232 breeds a small amounts of U-232 which emits powerful, dangerous gamma rays. These gamma rays are not a problem inside a reactor, but in a bomb, they complicate bomb manufacture, harm electronics and reveal the bomb’s location. Secondly, LFTRs produce very little plutonium, around 15 kg per gigawatt-year of electricity. This plutonium is also mostly Pu-238, which makes it unsuitable for fission bomb building, due to the high heat and spontaneous neutrons emitted. Thirdly, a LFTR doesn’t make much spare fuel. It produces at most 9% more fuel than it burns each year, and it’s even easier to design a reactor that makes 1% more fuel. And finally, use of thorium can reduce or even eliminate the need to enrich uranium. Uranium enrichment is one of the two primary methods by which states have obtained bomb making materials.
Economy and efficiency
Comparison of annual fuel requirements and waste products of a 1GW uranium-fueled LWR and 1GW thorium-fueled LFTR power plant.
- Thorium abundance. A LFTR breeds thorium into uranium-233 fuel. The Earth’s crust contains about four times as much thorium as U-238 (thorium is about as abundant as lead). It is a byproduct of rare-earth mining, normally discarded as waste. Using LFTRs, there is enough affordable thorium to satisfy the global energy needs for hundreds of thousands of years.
- No shortage of natural resources. Sufficient other natural resources such as beryllium, lithium, nickel and molybdenum are available to build thousands of LFTRs.
- Reactor efficiency. Conventional reactors consume less than one percent of the mined uranium, leaving the rest as waste. A LFTR can consume about 99% of its thorium fuel. The improved fuel efficiency means that 1 ton of natural thorium in a LFTR produces as much energy as 35 tons of enriched uranium in conventional reactors (requiring 250 t of natural uranium), or 4,166,000 tones of black coal in a coal power plant.
- Thermodynamic efficiency. LFTRs operating with modern supercritical steam turbines would operate at 45% thermal to electrical efficiency. With future closed gas Brayton cycles, which could be used in a LFTR power plant due to its high temperature operation, the efficiency could be up to 54%. This is 20 to 40% higher than today’s light water reactors (33%), resulting in the same 20 to 40% reduction in fissile and fertile fuel consumption, fission products produced, waste heat rejection for cooling, and reactor thermal power.
- No enrichment and fuel element fabrication. Since 100% of natural thorium can be used as a fuel, and the fuel is in the form of a molten salt instead of solid fuel rods, expensive fuel enrichment and solid fuel rods’ validation procedures and fabricating processes are not needed. This greatly decreases LFTR fuel costs.
- Lower startup costs. For a 1GW two fluid LFTR, the startup costs could be something like this:
- Start-up cost for a 1GW two fluid LFTR
|Item||Amount||Cost / Kg||Total cost||Comment|
|Uranium 233||700Kg||$100,000||$70M||Assumes expensively produced from enriched plutonium. Could be a lot cheaper if using nuclear waste from existing reactors.|
|Beryllium as BeF2||26,000Kg||$147||$3.8M|
|Lithium as LiF||66,500Kg||$120–800||$8-53M||Cost according to wikipedia. Battery Grade Lithium Fluoride 99.95% on alibaba.com is $10-19 / Kg|
This is about half the cost of a first core for a light water reactor and the salt inventory can last for decades, whereas the LWR needs a completely new core every 4 to 6 years.
- LFTRs are cleaner: as a fully recycling system, the discharge wastes from a LFTR are predominantly fission products, most of which have relatively short half lives. This results in a significant reduction in waste containment.
- Low corrosion, long lasting materials.
- No downtime for refueling. LFTRs have liquid fuels, and therefore there is no need to shutdown and take apart the reactor just to refuel it. LFTRs can refuel without causing a power outage.
- Load following. As the LFTR does not have xenon poisoning, there is no problem reducing the power in times of low demand for electricity and turn back on at any time.
- No high pressure vessel. Since the core is not pressurized, it does not need the most expensive item in a light water reactor, a high-pressure reactor vessel for the core. Instead, there is a low-pressure vessel and pipes (for molten salt) constructed of relatively thin materials.
- Excellent heat transfer. Liquid fluoride salts, especially LiF based salts, have good heat transfer properties. The LiF based salts have a thermal conductivity around twice that of the hot pressurized water. This results in efficient heat transfer and a compact heat exchanger.
- Air cooling. A high temperature power cycle can be air-cooled at little loss in efficiency, which is critical for use in many regions where water is scarce. No need for large water cooling towers used in conventional steam-powered systems.
- From waste to resource. It may be possible to extract some of the fission products such as xenon and neodymium for sale.
LFTR’s are quite unlike today’s operating commercial power reactors. These differences create design challenges and trade-offs:
- Startup fuel—Uranium-233 is not a naturally occurring element. It must be manufactured from thorium so a reactor fueled by a fissionable product, like waste plutonium, must be built to manufacture U233 to start a LFTR.
- Salts freezing—The fluoride salt mixtures have melting points, ranging from 300 to over 600 degrees Celsius, depending on the mixture. The salts, especially those with beryllium fluoride, are very viscous close to their freezing point. This requires careful design and freeze protection in the containment and heat exchangers. Uncontrolled freezing must be prevented or the reactor would be damaged like ice in your car’s radiator.
- Beryllium toxicity—The proposed salt mixture FLiBe, contains large amounts of beryllium, a poisonous element. The salt in the primary cooling loops must be isolated from workers and the environment to prevent beryllium poisoning.
- Waste management—There is also a need to manage the waste, which is still very radioactive, even though it is hazardous for a shorter period.
- Noble metal buildup—Some radioactive fission products, such as noble metals, don’t form salts, but deposit on the pipes. Novel equipment, such as nickel-wool sponge cartridges, must be developed to filter and trap the noble metals to prevent build up over time.
- Limited graphite lifetime—Compact designs have a limited lifetime for the graphite moderator and fuel / breeding loop separator. Under the influence of fast neutrons, the graphite first shrinks, then expands indefinitely until it becomes very weak and can crack
- Proliferation risk from reprocessing—While a lifter produces far less radioactive waste than a LWR, it still does produce some, which could fall into the wrong hands. See Wikipedia for more detailed explanation.
- Corrosion and Radiation damage to reactor—More research is needed on the long term effects of LFTR operation on the materials used for its construction.
- Long term fuel salt storage issues—If the fluoride fuel salts are stored in solid form over many decades, radiation can cause the release of corrosive fluorine gas, and uranium hexafluoride.
- Business model—Today’s solid fuelled reactor vendors make long term revenues by fuel fabrication. Without any fuel to fabricate and sell, a LFTR would adopt a different business model.
PART 4: The big energy picture