Thorium vs. Molten Salt Reactor
- Dec 19, 2018 11:25 pm GMT
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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.