LFTR

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LFTR stands for Liquid Fluorine Thorium Reactor, (pronounced 'Lifter'), a name coined by Kirk Sorenson for his design of a nuclear reactor. For a variety of reasons, this (or a reactor like it) would make an ideal power source for a Mars colony. This article assumes you know what Isotopes are. See Nuclear power for other forms of nuclear power.

Overview Of How It Works

Nuclear reactors use either Uranium 235 (U235), Plutonium 239 (Pl239), or Thorium 232 (Th232). Th232 is about 400 times more common than U235. However, Th232 is not fissile (it will not fission spontaneously), but it IS fertile (it can absorb neutrons and turn into a viable fuel). What must happen is that Th232 absorbs a neutron (either a fast neutron or more easily a slower 'thermal' neutron), and becomes Th233. This is unstable and will beta decay quickly into Protactinium 233. Protactinium is unstable and in about a month will beta decay into Uranium 233 (U233). It turns out that U233 is an ideal nuclear fuel, for reasons explained below.

U233 is extremely fissile, and fissions spontaneously, releasing on average between 2 and 3 neutrons. These will fission further 233 (or U235, or rarely Plutonium 239), which produce more power. Or the neutrons could be absorbed by either Thorium in the reactor fluid itself, or in a separate blanket. This breeds more fuel out of the abundant and cheap Thorium.

Rather than using traditional solid fuel, the Thorium, Protactinium, and Uranium are reacted into Fluorine salts. Fluorine is a ferocious oxidizer and forms very, very stable molecular bonds. In particular these ionic bonds are stable in the high radiation and heat of a reactor core. In addition to the Fluoride salts, salts of Lithium and other elements are included to moderate the reactor. (Moderating the reactor means to slow down fast neutrons in to the slower thermal neutrons. These slow neutrons are more easily absorbed, and thus, fewer neutrons are lost, improving the reactor's neutron economy.)

At the operating temperature, these salts are molten. The salts flow into the reactor core, where the neutron flux induces more fissions, producing heat. The neutrons created breed more Th232 into fuel. The salt flows thru a heat exchange and the heat is used to provide process heat for chemical reactions, or put thru a heat engine to create electricity. The cold salt is circulated back thru the reactor core.

LFTR run at higher temperatures than traditional reactors (~900C rather than ~300 C). This give higher quality heat for process heat, and allows a more efficient heat engine to produce electricity.

Being a liquid provides many advantages over solid fuel:

-- Gases such as Xenon 135 (Xe135) absorbs neutrons. In fact Xe135 absorbs neutron so readily, it is the worst neutron poison we know of. Unfortunately, it is a common fusion product and hinders the neutron economy of traditional reactors and cracks solid fuel cylinders. It is hard to remove Xe from solid fuel pellets. However, in a fluid, it readily can be removed by bubbling Helium gas thru the salt. Removing the Xenon makes the reactor more efficient and more predictable. (The Xenon can then be sold as reaction mass for Ion Thrusters.)

-- As liquids warm, the molecules move further apart. Thus if the LFTR starts to run too hot, the liquid expands and less of it fits into the reactor core. This will automatically slow the reaction, and cool the reactor. Thus it is dynamically stable.

-- The reactor does not require to operate at high pressure like normal pressurized light water reactors. (The high pressure adds expense and increases the danger of a pressure explosion or leak.) Molten salt reactors run at atmospheric pressure.

-- You can do chemical reactions on a liquid (or gas), but not in a solid. In particular, fission products (wastes), can be removed either while the reactor is operating, or in a batch processor later.

-- More Th232 or U233 can be added to the reactor while it is operating, without having to shut it down.

-- Solid fuel expands and cracks meaning it has to be removed when over 99% of the fuel is unburnt. In liquid fuel, it can remain in the reactor until it fissions. Thus, it needs less fuel and the waste is less radioactive.

LFTRs produce less nuclear waste, are proliferation resistant, and safer as described below.

Safety

Oakridge National Laboratories had a (Uranium 235) powered molten salt reactor which ran for 4 years. It had a unique safety feature. A drain at the bottom of the reactor had an electric fan blowing air across a pipe. The salt froze into a solid. However if the reactor lost power, the salt melted and the molten salt drained into smaller, neutron absorbing tanks, which stopped the reaction, and safed the reactor. This was tested hundreds of times since they turned off the test reactor on weekends by stopping this fan. Other safety features include:

-- The salt freezes if it gets cold preventing the movement of the salt in case of a spill.

-- The reactor design is load following. If more power is needed, it automatically heats up. If less power is needed it automatically cools down. If it gets too hot, it automatically slows the reaction.

-- The salt can run at normal pressure. There is no danger of a steam explosion, nor the necessity of a huge containment dome. (It would need a small, room sized containment volume.)

-- There is no water which can flash to steam, or be decomposed into Hydrogen and Oxygen.

-- The salts are not flammable. They do not burn if they touch water. (Some U238 --> Pl239 breeder reactors use molten sodium, which explodes on contact with water.)

Proliferation Resistance

Uranium 233 is not easy to build a bomb out of. Some of it will decay producing hard gamma rays which are much more dangerous than the radiation produced by U235 or Plutonium 239 bombs. These gamma rays are highly penetrating and can be picked up by aircraft or satellites which advertises where the bomb maker is. Further, the hard radiation can damage the electronics of the 'brain' of the bomb. The USA experimented briefly with U233 for atom bombs, but found U235 and Pl239 were cheaper, safer, and easier to work with.

All that said, the fuel for nuclear reactors is totally unsuitable for atom bombs. No bomb has ever been created from civilian nuclear products. Rather the U235 is enriched from natural Uranium, or the Plutonium is bred in special reactors specifically optimized to do this efficiently.

Less Nuclear Waste

Why are nuclear wastes dangerous? Because they send out particles (such as neutrons, electrons, or helium nuclei), or dangerous electromagnetic waves (such as X-rays or gamma rays) jointly known as Radiation. If the isotope has a short half life, it decays ferociously, being highly dangerous. However, because it decays so quickly, it does not stay dangerous for long, soon it has decayed away to stable isotopes and has vanished. If an isotope has a long half life, it will remain in its natural form for millennia. It very rarely decays so it is safe to handle. (There are trace amounts of Thorium in the dirt in your backyard, but you would need a good Geiger counter to detect it. It has a half life of over 14 billion years so it almost never decays.) And there are items which are in-between these two extremes. Isotopes with medium half life are radioactive enough to be too dangerous to spend much time with, but they last long enough to be a concern for many years.

Now after a heavy nucleus such as Thorium decays, it creates 'fission products'. These isotopes are neutron rich, and rapidly undergo beta decays throwing off electrons and gamma rays, until they reach stable nuclei, such as Iodine, Xenon, Lead, or Zirconium. (Sadly Gold, Silver, and Platinum are quite rare fission products.) Fission products have very short half lives so they are wildly, ferociously radioactive. If you touched such waste with your elbow and then ran away, you would receive a fatal dose. Fortunately, because they decay away so quickly, they do not stay this radioactive for long.

Inside a nuclear reactor, we want isotopes to absorb a neutron and fission. That is the lottery win, the big bonus that gives us the heat and energy which we want. However, only 3 isotopes are likely to be created and fission. U233, is the easiest to fission, U235 is in the middle, and Plutonium 239 (Pl239) is the least likely to fission. Inside the neutron rich reactor, atoms are absorbing neutrons, decaying into other isotopes and jiggling around the number of protons and neutrons they have. We WANT them to absorb a neutron and split into two (usually unequal) halves. But sometimes they absorb extra neutrons (becoming larger, heavier nuclei), but don't fission.

Elements Plutonium and heavier are unpleasant. A good number of them have half lives which are in the hundreds or low thousands of years. This is the unhappy medium ground where they are radioactive enough to be dangerous, but have a long enough half life that they must be kept away from animals and people for thousands of years. These 'Transuranic Actinides' are the long lived radioactive wastes which people are so worried about.

-- Now if you start with Uranium 238, the plan is for it to absorb one neutron, turn into Pl239 and fission. There is a 65% chance of this working. If it fails, you may end up with the Transuranic Actinides, which are bad.

-- If you start with U235, and it gets hit by a neutron, there is a 85% chance of a fission. If it fails, it could absorb more neutrons and become Plutonium where it gets another 65% chance. So we have two tries to succeed in fissioning.

-- If we start with U233, then it has a 92% chance of fissioning. If it fails, it could absorb another neutron and fission at U235. If that fails, it could absorb 3 more neutrons and fission at Pl239.

So the upshot is that if we start with Thorium --> U233, we have 3 tries to succeed in fissioning before getting the transuranic actinides.

Now if you take the wastes from a LFTR, they are rich in fission products but quite rare in Plutonium and heavier elements. Fission isotopes (which have short half lives), are wildly dangerous when they are first removed from the reactor. But a LFTR's waste products take 200 to 300 years to become no more dangerous than the natural ores which they came from. 300 years is a long time, but it is much more reasonable to say that we can keep some waste out of the biosphere for 300 years, rather than for hundreds of thousands of years. (Tho stuff that takes hundreds of thousands of years to decay, is really not that dangerous.)

Whereas waste from common reactors take between 1 and 10 million years to decay to natural ore levels of radioactivity. (To be fair, current reactors are extraordinarily inefficient, and burn less than 1% of their fuel. So a lot of their fuel is in the waste products which extends how long they are dangerous. Normal reactors should be redesigned to burn up more of their fuel, and reduce the volume of their wastes.)

In summary: LFTRs produce less waste, and the waste they do produce decays to safety much, much more quickly than normal reactors.

One Fluid Verses Two Fluid Designs

In a single fluid design, the Thorium salt is mixed in with the working fluid. This as the advantage of simplicity, but it makes processing the fuel to pull out the fission products more complex. You would likely do this in a separate facility.

A two fluid design has the Thorium in a separate molten salt circulatory system which surrounds the reactor core. Neutron enter this blanket, and breed the Th232 into Protactinium 233 (Pa233). Pa233 has a different chemistry so it is very easy to extract it from the Thorium salt, and after it decays into U233, it is added to the main reactor. This makes pulling out the fission products easier.

Why LFTR is Suitable For Mars

Apart from being cheaper, safer, and producing less waste, there are three main reasons why LFTRs are ideal for Mars:

-- First pressurized light water reactors require containment domes which have 1,000 times the volume of the reactor itself. This, of course, is very expensive. LFTR can use containment vessels that are the size of large rooms.

-- Second, because high pressure is not needed, it is easier to produce and maintain these reactors than traditional nuclear reactors. (As Martian industry develops, people will want Rare Earth Elements (REE), and Thorium is always found with such ores.)

-- Third, U235 makes up only a small fraction (0.5%) of the natural uranium ore (most is U238). Thus uranium for power plants must be 'enriched', increasing the fraction of U235 to around 4%. This requires fairly advanced industry, and a lot of power. Using thorium in a LFTR requires no enrichment phase to create the fuel. (Thorium has been detected on the planet, by the 2001 Mars Odyssey satellite, and there is no reason not to expect that we can find Thorium rich ores when we reach the planet.)

Some people believe that people can survive on Mars with solar power (approximately half as powerful as on Earth) and wind power (negligible power in Mar's thin, dust filed atmosphere). However, a power rich community will be wealthier and grow faster, than a power starved one.

Difficulties Of Using LFTR

LFTR uses Thorium, Lithium, and Beryllium salts. However, we need to separate the two isotopes of Lithium – Lithium 6 (the version we want) and Lithium 7 (a neutron poison) – which is fairly hard to do. We may wish to have this done on Earth.

LFTR needs corrosion resistant Nickel alloys for the plumbing. Building these are tough for a young industry, and it may be easier to ship these from Earth to start.

If the heat produced by a LFTR is being used as process heat for chemical reactions, or to heat habitats, then it is fine. But if we want to attach a heat engine to it to produce electricity, cooling the cold side of the heat engine is a concern. The atmosphere is so thin, that dumping the waste heat into it will be difficult, and require some clever engineering.

References

The following books,

"Thorium: Energy Cheaper Than Coal" by Robert Hargraves, ISBN: 9-781478-161295. Highly recommended.

"Molten Salt Reactors and Thorium Energy", Edited by Thomas J. Dolan, ISBN: 978-0-08-101126-3. Technical. 58 authors from 23 countries.

"Nuclides & Isotopes: Chart of the Nuclides", Various authors, Published by Knolls Atomic Power Laboratory.

For a non-technical introduction, you can search on YouTube for "LFTR Kirk Sorenson" you should find several videos of various lengths which explain LFTRs.