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Al Weinberg's best reactor design after the PWR

Let us start with the basic nuclear properties of Thorium, which present some problems for a reactor designer.

How is Thorium a Fuel?
The 90th element, Thorium, has only one isotope that made it to our planet, Th-232 with a half life of 40 billion years. It has no fission cross section with neutrons below fusion energies and is not itself a fuel. It does have a good absorption cross section to make Th-233 which decays in 22 minutes into Protactinium-233, which is also useless as a fuel. That decays in 27 days to Uranium-233, an excellent fission fuel.

No Plutonium.
The good thing is that U-233 is 6 neutrons away in any absorption chain from Plutonium-239. If you can get some U-233 from another source then your reactor will make only the faintest traces of Plutonium.

How to start a Thorium reactor.
The conversion of Thorium into fuel needs a large flow of neutrons from something else to make U-233. One could just kick off with some enriched Uranium but the operation is then polluted with Plutoniums which take 50 years to burn out. A better deal is to put Thorium rods in an ordinary PWR and recycle them often to remove the U-233. Even better would be to use copious Fusion neutrons to breed the fuel as shown in the Fusion Hybrid library.

Al Weinberg's best civilian reactor.
Alvin Weinberg was the leading US reactor engineer at Oak Ridge National Laboratory in Tennessee, and his lab tested many concepts.
He knew that the water cooled reactors all had the same flaw that if all pumping power failed then the reactor would have a melt down. Pellets of solid fuel must be packaged in some metallic sleeve to make fuel rods and cooled by water or high pressure, fast moving gases like Helium or Carbon Dioxide.

Liquid Fluorine Salt Fuels
He began to think about liquid fuels which would also be their own coolant. This solved a slew of problems at a stroke. But what could dissolve all these metals and all the fission by products in one liquid?
The answer is element 9, Fluorine-19. This is one electron short of the closed and highly stable form of the noble gas, Neon. Fluorine will react with almost every single element, including Gold, in its search for completion. This also makes it very fickle and it will always swap to a more willing partner.
The lightest metallic element, Lithium is pleased to oblige and share its outer electron, leaving it more like the noble gas Helium. Molten Lithium-Fluoride salt is the perfect solvent for nuclear fuels. It is a clear, greeny-blue liquid with similar physical properties to water. With the right proportions of Thorium and U-233 you have a breeder reactor which is already melted down.

Safety of Liquid Fuels
The reactor salt is pumped around at 700-900 C but freezes solid, if it leaks or is set aside for processing, at 350 C. If the pumps fail, the reactor starts to heat up but this opens temperature sensitive valves to disperse the liquid to holding tanks which then have too little fuel to be reactors. Fix the pumps and restart the reactor. How is that for fantastic safety? A freeze-up instead of a melt-down!

Keeping Clean
The waste problem simplifies also. As the liquid is pumped around, high pressure Helium can be bubbled through. The radioactive noble gases leave with the bubbles, to be frozen out elsewhere. Xenon is a strong neutron absorber and poisons PWR reactors where it is trapped in the fuel rods.
Some other metallic fluorides also choose to leave. Bubbling more Fluorine through the the liquid allows some salts to grab more fluorine and become volatile gases instead of molten salts. They leave for processing. Many strongly radioactive fission products leave this way, and others could be removed by further chemical processing.
The fuel salt can be kept clean of the most radioactive isotopes and their heat load. The real wastes, Caesium and Strontium,can be separated and reduced to small packages for 500 year storage. No geological storage.

One of the World's Safest Reactors
The Liquid Fuelled Thorium Breeder cannot melt down, uses no inflammable materials, is totally safe in engineering accidents, cannot lose the reactor in an incident, and cannot release significant radiation form the reactor. The engineering is simpler in many ways, having no structural component in the reactor chamber.
The background here shows the cunning two salt version of the reactor, with Thorium in the outer chamber and the Uranium in the core.
There is a few years work to do to taking Weinberg's basic design forward to the full blown complete plant described here.

All Boxes ticked except for the Politics
Weinberg's pilot plant at ORNL was closed in 1969 because it did not make good bomb material. The Fast reactor was seen as the only way to consume excess Plutonium and higher Actinides and captured all the funding.

The nuclear freeze caused by Jimmy Carter's bungling of the Three Mile Island incident which released no significant radiation outside the plant, put nuclear power in limbo for the EU and USA till now. No further research on the Thorium Breeder has been done.

The US, EU, and UK Governments refuse to start a new Thorium project. The Chinese, India, and five privately funded projects are making progress.

The Economics of how private finance and the nuclear industry can produce working Thorium MSRs in 10-15 years are similar to the efforts to build Small Fusion Reactors. This is discussed under Innovation on Economics Ave.

The UK is determined to have a £32Bn high speed electric train set with illusory benefits by 2035 but has contributed nothing to the Thorium Breeder.

NUCLEAR primer
Radium Beryllium neutron source
A Neutron source.
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Trinity Library: Thorium
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Moir n Teller on MSRs
Select Reports
Xrays in LIFE Fusion Hybrid
Moir: Thorium hybrid lecture.