Liquid fluoride reactor

A liquid-fluoride reactor (a specific example of a molten salt reactor) is a nuclear reactor wherein the nuclear materials are fluoride salts dissolved in a solution of other fluoride salts. While the reactor can be used to consume any fissile material, it is particularly efficient^[1] using the Thorium fuel cycle. This is because the liquid-fluoride reactor can continuously reprocess its nuclear material relatively easily, compared to solid-fueled nuclear reactors. This aids breeding, because the breeding blanket is already in a liquid form amenable to reprocessing. In Thorium's fuel cycle, thorium must be exposed to neutrons to convert to protactinium, then be isolated from neutrons to decay to uranium-233, and then be exposed to neutrons again for U-233 fission and energy release.

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Many others (including the notable Dr. Alvin Weinberg, inventor of the light-water reactor) consider fluid-fueled reactors the only practical way to exploit the potential of thorium. The preponderance of solid-fueled reactors probably has a lot to do with why thorium hasn't caught hold in the solid-core nuclear world.

An MSR based on chloride salts has many of the same advantages. However, the larger, less-dense atoms of chlorine causes the reactor to be a fast breeder. Theoretically, it wastes even fewer neutrons and breeds more efficiently, though it may be less safe. It would require a salt with an isotopically-separated Chlorine, Cl37, to prevent neutronic activation of the chlorine into sulfur which would form corrosive sulfur chloride.

Advantages of the Liquid-Fluoride Reaction Fueled with Thorium

The basic advantage of thorium over uranium is that it can be nearly completely consumed in a thermal-spectrum reactor. Uranium cannot--it requires a fast-spectrum reactor for consumption of the abundant U-238 isotope (99.3% of natural uranium).

Since thorium can be almost completely consumed, it can be used in a way that produces no waste product "heavier" than neptunium-237.

One can use thorium in solid-core or in liquid-fueled reactors. The basic advantage of using the liquid-fueled reactors is that gaseous fission products (especially Xe-135) can be continuously removed (improving neutron efficiency) and that the intermediate breeding product of thorium (protactinium-233) can be removed online and isolated from the neutron flux until it decays to uranium-233 (the fissile fuel). Also that a U-233 fueled reactor will inevitably produce uranium-232, which has a decay product that is a strong gamma emitter (Gamma_radiation) and renders the uranium nearly worthless in a nuclear weapon.

The fact that the reactor core is not pressurized means that the reactor vessel can be lightweight weldments, and the containment doesn't need expansion cubage. This hugely reduces the cost of the reactor and land area. The lack of pressure means that the core can't explode. The savings and safety advantages multiply throughout the installation.

Another savings is that the reactor operates at much higher temperatures: 650C. The lower the salt mixture's melting point, the greater the efficiency. With FlLi7Be salts the conversion can use a two-stage regenerated helium turbine, about 1.4 times as efficient as steam. In a conventional LWR plant, the turbine, etc. are about half of the plant's cost. The efficiencies also multiply through the thermal part of the design, reducing core size, fuel use, etc, as well as heat exchanger sizes and site cooling requirements.

Fuel burn-up is very high, 98%. 1 Gw-year of electricity needs about 800kg of Thorium. Thorium currently costs $60/Kg, and is 3 times more abundant than Uranium in the crust.

The thorium cycle breeds fuel, and therefore needs no enrichment plants, a major savings of investment capital over Uranium schemes.

The reactors can be thermal, or nearly so. This means that they can be designed to be inherently safe. i.e. much safer than fast plutonium breeders.

Waste is 0.1% of the once-through uranium cycle. Transuranic wastes can almost be completely eliminated by careful design, so that all off-site wastes are fission products with half-lives of 30 years or less. This means that waste disposal is a 300-year problem, not a 200,000 year problem.

Reprocessing costs: The plant only has to remove 80kg/year of wastes from the several tons of fuel salt in a 1Gw plant. The salt is already fluid. Reprocessing can be quite a small item, maybe as little as a couple of electrically-heated graphite retorts to vapor-refine the salt. The staffing will probably be the main cost.

An important deployment savings is that the fuel does not need to be tested. Testing of a single nuclear fuel design fabricated from solid materials can often take ten years, and millions of dollars. Molten salt mixtures have already been validated.

The fuel cycle is less convenient for weapons construction than Uranium or plutonium cycles. Because of the Thorium cycle's low breeding ratio, removing U233 for use in bombs will actually shut down power plants.

Disadvantages

Molten salt fluoride reactors, nevertheless, present a number of design challenges. Known issues include:

Since it uses unfabricated fuel, basically just a mixture of chemicals, current reactor vendors don't want to develop it. They derive their long-term profits from sales of fabricated fuel assemblies.

Uncooled graphite moderators can cause some geometries of this reactor to increase in reactivity with higher temperatures, making such designs unsafe. Careful design may fix this, however.

High neutron fluxes and temperatures in a compact MSR core can rapidly change the shape of a graphite moderator element, to require refurbishing in as little as four years. Eliminating graphite from sealed piping was a major incentive to switch to a single-fluid design.^[2] Most MSR designs do not use graphite as a structural material, and arrange for it to be easy to replace. At least one design used graphite balls floating in salt, which could be removed and inspected continuously without shutting down the reactor. ^[3]

A safe thorium breeder reactor using slow thermal-energy neutrons also has a low breeding rate. Each year it can only breed thorium into about 109% of the U233 fuel it consumes. This means that obtaining enough U233 for a new reactor can take eight years or more, which would slow deployment of this type of reactor. Most practical, fast deployment plans would start the new Thorium reactors with Plutonium from existing light-water reactor wastes or decommissioned nuclear weapons. This scheme also decreases society's stock of high-level wastes.

The high neutron density in the core rapidly transmutes most isotopes of Lithium to Tritium, a radioactive isotope of Hydrogen. In an MSR, the Tritium forms Hydrogen Fluoride (HF). Tritium HF is a corrosive, chemically poisonous, radiotoxic gas. All MSR designs used very expensive isotopically purified Lithium 7 for their carrier salts in order to reduce Tritium formation as far as possible. The MSRE proved that this worked. Some slow corrosion occurs even in the exotic nickel alloy, Hastelloy-N used for the reactor. The corrosion is more extreme if the reactor is exposed to hydrogen which forms corrosive HF gas. Mere exposure to water-vapor causes uptake of corrosive amounts of Hydrogen, so practical MSRs operate the salt under a blanket of dry inert gas, usually helium.

When cold, the fuel salts radiogenically produce poisonous fluorine gas. The salts should be defueled and wastes removed before extended shutdowns. Unfortunately, this was discovered the unpleasant way, while the MSRE was shut-down over a 20-year period.

The salt mixture is toxic, and water-soluble. The reactor design must therefore isolate the salt from the biome. This is a normal reactor safety requirement anyway.

References and links

1. ^ Moir, R.W; Cost of Electricity from MSRs; Lawrence Livermore National Laboratory, U.S.
2. ^ Energy from Thorium blog->Reactor Design->Graphite and Two-Fluid vs. One-Fluid LFRs Viewed 6/2007
3. ^ ORNL-4548: Molten-Salt Reactor Program: Semiannual Progress Report for Period Ending February 28, 1970, pg. 57 (U.S.'s Oak Ridge National Laboratory Report Series)

• "Fluid Fuel Reactors", 1958
• Energy from Thorium's Document Repository Contains scanned versions of many of the U.S. government engineering reports. This repository is the main reference for the molten-salt fueled reactor's technical discussion.
• Fluoride volatility
• "Costs and Advantages of LFR"