BC: MOLTEN SALT FUELED REACTORS ( MSFR )
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We Should Be Building These ASAP
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Molten-salt Fueled Reactors
The classic MSFR has been very exciting to many nuclear engineers. Its most prominent champion was Alvin Weinberg, who patented the light-water reactor and was a director of the U.S.'s Oak Ridge National Laboratory, a prominent nuclear research center.
Two concepts were investigated. The "two fluid" reactor had a high-neutron-density core that burned U233 from the thorium fuel cycle. A blanket of thorium salts absorbed the neutrons and was eventually transmuted to U233 fuel. The engineers discovered that by carefully sculpting the moderator rods (to get neutron densities similar to a core and blanket), and modifying the fuel reprocessing chemistry, both thorium and uranium salts could coexist in a simpler, less expensive yet efficient "one fluid" reactor.
The power reactor design produced by Weinberg's research group was similar to the MSRE above, which was designed to validate the risky hot, high-neutron-density "kernel" part of the "kernel and blanket" thorium breeder.
The advantages cited by Weinberg and his associates at Oak Ridge National Laboratory include:
1) It's safe to operate and maintain: Molten fluoride salts are mechanically and chemically stable at sea-level pressures at intense heats and radioactivity. Fluoride combines ionically with almost any transmutation product, keeping it out of circulation. Even radioactive noble gases come out in a predictable, containable place, where the fuel is coolest and most dispersed, the pump bowl.
2) There's no high pressure steam in the core, just low-pressure molten salt. This means that the MSR's core cannot have a steam explosion, and does not need the most expensive item in a light water reactor, a high-pressure steam vessel for the core. Instead, there is a vat and low-pressure pipes (for molten salt) constructed of thick sheet metal. The metal is an exotic nickel alloy that resists heat and corrosion, Hastelloy-N, but there is much less of it, and the thin metal is less expensive to form and weld.
3) With fuel reprocessing, the Thorium fuel cycle, so impractical in other types of reactors, produces 0.1% of the long-term high-level radioactive waste of a light-water reactor without reprocessing (all modern reactors in the U.S.). As thorium captures neutrons, it first becomes Th233, which quickly decays to protactinium (Pa233). Pa233 in turn decays to U233 with a half-life of 27 days. U233 is an excellent reactor fuel. As U233 is bombarded by neutrons with a thermal spectrum of speeds, each absorbed neutron either splits the uranium or produces a heavier isotope of uranium, which fission to elements similar to those from U233. These fission products almost all have half-lives less than 30 years. The only source of high-radioactivity long-lived transuranic elements is that a tiny bit of neptunium is produced from the tiny fraction of U236 produced at the tail-end of this process. The neptunium can be separated by the fuel-salt reprocessing, or it is such a tiny fraction that it can be left in the salt and fissioned by excess neutrons. If the transuranics are left in, the separated wastes are pure Uranium fission wastes. All have half-lives less than 30 years. Reprocessed waste from thorium is therefore less radioactive than natural ores in 300 years.
4) The thorium breeder reactor uses low-energy "thermal" neutrons very similar to light water reactors. It is therefore much safer than the touchy fast-neutron breeder reactors that the uranium-to-plutonium fuel cycle requires. The thorium fuel cycle therefore combines safe reactors, a long-term source of abundant fuel, and no need for expensive fuel-enrichment facilities.
5) A molten salt reactor's fuel can be continuously reprocessed with a small adjacent chemical plant. Weinberg's groups at Oak Ridge National Laboratory found that a very small reprocessing facility can service a large 1 GW power plant: All the salt has to be reprocessed, but only every ten days. Society's total inventory of expensive, poisonous radioactives is therefore much less than in a conventional light-water-reactor's fuel cycle, which moves entire cores to recycling plants. Also, everything except fuel and waste stays inside the plant. The reprocessing cycle is:
A sparge of fluorine to remove U233 fuel from the salt. This has to be done before the next step.
A 4-meter-tall molten bismuth column separates protactinium from the fuel salt.
A small storage facility to let the protactinium from the bismuth column decay to U233. With a 27 day half life, ten months of storage assures that 99.9% decays to U233 fuel.
A small vapor-phase fluoride-salt distillation system distills the salts. Each salt has a distinct temperature of vaporization. The light carrier salts evaporate at low temperatures, and form the bulk of the salt. The thorium salts must be separated from the fission wastes at higher temperatures. The amounts involved are about 800 kg of waste per year per GW generated, so the equipment is very small. Salts of long-lived transuranic metals go back into the reactor as fuel.
6) With continuous reprocessing, a molten-salt-fueled reactor has more than 97% burn-up of fuel. This is very efficient, compared to any system, anywhere. Light water reactors burn up about 2% of fuel on a once-through fuel cycle (current practice, 2007).
7) With salt distillation, an MSFR can burn Plutonium, or even fluoridated nuclear waste from light water reactors.
8) The molten-salt-fueled reactor operates much hotter than LWR reactors, from 650 °C on conservative designs, to as hot as 950 °C on aggressive designs. So very efficient Brayton cycle (gas turbine) generators are possible. This is also very efficient, a goal of "generation IV reactors" that has already been achieved by MSRs. This reduces fuel use and auxiliary equipment (major capital expenses) by 50% or more.
9) MSRs work in small sizes, as well as large, so a utility could easily build several small reactors (say 100 MWe) from income, reducing interest expense and business risks.
Molten salt fuel reactors are not experimental. Several have been constructed and operated at 650 °C temperatures for extended times, with simple, practical validated designs. There's no need for new science at all, and very little risk in engineering new, larger or modular designs.
The reactor, like all nuclear plants, has little effect on biomes. In particular, it uses only small amounts of land, relatively small amounts of construction, and the waste is separated from the biome, unlike both fossil and renewable energy projects.
Combining the above, some form of molten-salt thorium breeder could be the most efficient well-developed energy source known, whether measured by cost per kW, capital cost or social costs.
There are some design and social advantages:
Thorium's fuel cycle resists proliferation in two ways:
It is verifiable because the epithermal thorium breeder produces only at most 9% more fuel than it burns in each year. Building bombs quickly will take power plants out of operation.
Also, an easy variation of the thorium fuel cycle would contaminate the Th232 breeding material with chemically inseparable Th230. The Th230 breeds into U232, which has a powerful gamma emitter in its decay chain (Tl-208) that makes the reactor fuel U233/U232 impractical in a bomb, because it harms electronics.
Thorium is more abundant than uranium. The Earth's crust has about three times as much.
Thorium is cheap. Currently, it costs US$ 30/kg.
Control of the salt's corrosivity is easy. The uranium buffers the salt, forming more UF4 from UF3 as more F is present. UF3 can be regenerated by adding small amounts of metallic beryllium to absorb F. In the MSRE, a beryllium rod was inserted into the salt until the Uf3 was the correct concentration. [1]
Extensive validation (fuel rod design validation normally takes years and prevents effective deployment of new nuclear technologies)is not needed. The fuel is molten, chemical reprocessing eliminates reaction products, and there are tested fuel mixtures, notably FLi7BeU.
There is no need for fuel fabrication. This greatly reduces the MSR's fuel expenses. It poses a business challenge, because reactor manufacturers customarily get their long-term profits from fuel fabrication. A government agency could, however, type-license a design, which utilities could replicate.
Molten-fuel reactors can be made inherently safe: Tested fuel-salt mixtures have negative reactivity coefficients, so that they decrease power generation as they get too hot. Most fuel-salt reactor vessels also have a freeze plug at the bottom that has to be actively cooled. If the cooling fails, the fuel drains to a subcritical storage facility.
Continuous reprocessing simplifies numerous reactor design and operating issues. For example, the poisoning effects from xenon-135 are not present. Neutron poisoning from fission products is continuously mitigated. Transuranics, the frighteningly long-lived "wastes" of light water reactors, are burned as fuel.
A fuel-salt reactor is mechanically and neutronically simpler than light-water reactors. There are only two items in the core: fuel salts and moderators. This reduces concerns with moderating interactions with positive void coefficients as water boils, chemical interactions, etc.
Coolant and piping need never enter the high-neutron-flux zone, because the fuel is used to cool the core. The fuel is cooled in low-neutron-flux heat-exchangers outside the core. This reduces worries about neutron effects on pipes, testing, development issues, etc.
The salt distillation process means that chemical separation and recycling of fission products, say for nuclear batteries, is actually cheap. Xenon and other valuable transmuted noble gases separate out of the molten fuel in the pump-bowl. Any transuranics go right back into the fuel for burn-up.
Molten salt 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.
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. |
