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Thorium vs. Molten Salt Reactor

To a limited group of technophiles and nuclear technology enthusiasts, thorium has become a unicorn. But does thorium really represent nuclear innovation?

 

Back in the 1950s and 1960s, the scientists at Oak Ridge National Laboratory in the USA developed the Molten Salt Reactor design – a liquid salt fueled and cooled nuclear reactor system. They designed it, they prototyped it, and they operated it. The experiment was called the Molten Salt Reactor Experiment, or MSRE. The MSRE used a thorium fuel cycle.  It used a lithium beryllium fluoride coolant salt mixture, called FLiBe. It used a graphite moderator. It used a special material called Hastelloy N – a nickel alloy developed specifically to withstand the harsh environment.

 

The experiment was a great success. It proved that this liquid fuel system could facilitate nuclear fission, and that it was tremendously stable, and easy to operate. Dick Engel, the project manager, even called it “boring” because the engineers had virtually nothing to do while it operated.

 

At the rudiments of the technology lay the liquid fuel. Liquid nuclear fuel-coolant, the MSRE discovered, was a much more efficient mechanism for capturing the immense heat from fission than solid fuel/water coolant. Salt coolant was a much more versatile coolant, with a huge thermal range, compared to a water coolant, and capable of storing and easily conveying that immense heat from fission.

 

The thorium-232/uranium-233 fuel cycle that was used in the MSRE was a departure from the uranium-235/uranium-238/plutonium-239 fuel cycle that was being used in the Light Water Reactor design, also invented by the Americans. The LWR was being used in the US Navy submarine program, and by the mid-1950s, started to be used in commercial power plants. Thorium, it was projected, could have some advantages over uranium, particularly in a liquid fuel application.

 

In order to make thorium fuel, Th232 must either be blended with U235 or Pu239, or it must be bombarded with neutrons to make a supply of U233, which is also fissile. The Th232 and U233 is then blended to create a fuel that is capable of achieving criticality. Since the dawn of the atomic age, there have been a small handful of commercial applications of a thorium fuel cycle.

 

In order to make commercial nuclear fuel, U235, which is about 0.7% of naturally-occurring uranium, must be concentrated to between 3% and 5% of the uranium fuel element. This is not so easily achieved either, but there is a multi-decade legacy of uranium enrichment. The fuel cycle is well-understood by regulators, operators and the supply chain.

 

What are the advantages of thorium?

 

Thorium is abundant. That is certainly an advantage it has over uranium. It is abundant and broadly geographically dispersed and easy to extract from nature. Unlike uranium, thorium is found in great concentrations right on the surface of the earth, most commonly, in black sand beaches.

 

Thorium is not fissile, which means that thorium by itself could never possibly be weaponized. However, because it is not fissile, it means that thorium always requires fissile material to make fuel, and that creates new proliferation risks.

 

This is where the actual advantages of thorium end. All the other advantages commonly attributed to thorium are actually advantages of a Molten Salt Reactor – not of thorium itself. These virtues became conflated with the Molten Salt Reactor design. Because of the fact that thorium fuel was used, enthusiasts rediscovering this technology 40 years later have misplaced the rudiments of the innovation.

 

Molten Salt Reactors have tremendous safety, waste and proliferation virtues, which translate into substantial commercial virtues. The following is a non-exhaustive list:

 

  • Fluoride salts have an approximately 1,000C range in which they stay liquid – neither freezing nor boiling;
  • Fluoride salts operate naturally at high temperature, obviating the need for immense pressure in a reactor vessel;
  • Fluoride salts are chemically very stable and inert, eliminating the risk of chemical explosions in a reactor system;
  • A liquid fuel is inherently easier and cheaper to chemically process, thereby creating a pathway for total nuclear waste elimination.

 

There are many others. These advantages are specific to Molten Salt Reactors, and not to thorium fuel.

 

The thorium enthusiasts will certainly find this controversial. However, if the goal is eliminating energy poverty and pollution, one must accurately assess the source terms of nuclear innovation.  The mystical nature of thorium has served its purpose by attracting all walks of life to develop an interest in advanced nuclear technology – including myself.  Now the market must focus on the most pragmatic way of commercializing true nuclear innovation.

Canon Bryan's picture

Thank Canon for the Post!

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Bob Meinetz's picture
Bob Meinetz on Dec 21, 2018 4:28 pm GMT

"The MSRE used a thorium fuel cycle..."

Canon, that the MSRE was a breeder using a thorium fuel cycle is a common misconception. The fuel used in the Molten Salt Reactor Experiment at Oak Ridge was uranium tetrafluoride (UF4) with various mixtures of the isotopes U-233 and  U-235. Though thorium was never added to the molten salt mixture of the MSRE, a followup breeder was planned but never materialized. After a 2006 paper by Edward Teller and Ralph Moir again proposed the idea, test programs were launched in the U.S., China, and Russia.

In the paper, thorium was chosen not because of perceived "mystical" qualities but practicality. There are a limited number of radioactive isotopes suitable for use in fission reactors; U-233 is one with good proliferation resistance, and one which can be easily bred from Th-232.

If you feel thorium isn't a viable candidate, what would you suggest as an alternative?

Canon Bryan's picture
Canon Bryan on Dec 26, 2018 3:33 pm GMT

Thank you for this.

U233 is part of the thorium cycle. The thorium cycle is any fuel that contemplates Th232-U233 and their decay chains, etc.

U235 is part of the uranium cycle. The uranium cycle consists of fuels that contemplate U235-U238-Pu239. U233 cannot be manufactured easily from U235 or U238, nor would anyone want to. They would use thorium to do that.

Yes, both fuel cycles were used in MSRE, but clearly most proponents of the technology have focused their attention on the thorium cycle fuel.

There are some proliferation resistance characteristics gained by using thorium, it is true. The absence of Pu239 and heavier actinides. However, one is still left with added proliferation risks:

- U233 unusued fuel, and its decay products.

- One must face the added proliferation risk of Pu233 for 28 days.

- Because thorium is not fissile, the only way to make fuel out of it is to add fissile material. That would typically be plutonium. That adds proliferation risk.

Also, I think it would be strange to call the breeding of U233 "easy". It is by no means easy. Unless you think it is easy to come by a controlled neutron source.

Bob Meinetz's picture
Bob Meinetz on Dec 26, 2018 9:09 pm GMT

Canon, how U-233 can be fabricated was irrelevant to the purpose of the Molten Salt Reactor Experiment, which also used U-235 and plutonium as fuel. The experiment's purpose was to determine whether combining a radioactive source with a molten salt mixture might be a practical and safe way to generate electricity.

A followup molten-salt breeder was planned when ORNL Director Alvin Weinberg was fired by Richard Nixon in 1973. There were several reasons why thorium had been selected as the most likely candidate. In particular, the production of U-233 from thorium within the reactor vessel also resulted in the production of several hard gamma emitters, which would likely make any attempted theft of the reactor mixture fatal.

Both U-235 and Pu-239 worked well in the MSRE, but were rejected because they could potentially be diverted en route to the reactor. In theory, U-235 could be added once to a thorium-fueled breeder to initiate fission, after which the reactor could run for decades without the addition of more.

If thieves were able to successfully extract U-233 from a molten salt mixture it would make for poor bombmaking material. Several weapons tests have been performed using U-233 alone, or in combination with U-235/plutonium, all with disappointing results.

Thus, I have yet to see evidence thorium is not the most pragmatic and safe way for commercializing molten salt reactor technology. Whether or not that represents true nuclear innovation remains to be seen.

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