BC: GIVE A LIFT TO LFTRs
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- MSFRs can include a freeze plug at the bottom that has to be actively cooled, usually by a small electric fan. If the cooling fails, say because of a power failure, the fan stops, the plug melts, and the fuel drains to a subcritical easily cooled storage facility. This not only stops the reactor, also the storage tank can more easily shed the decay heat from the short-lived radioactive decay of irradiated nuclear fuels.
- LFTRs can dramatically reduce the long-term radiotoxicity of their reactor wastes. Light water reactors with Uranium fuel have fuel that is 80 to 97% U238. These reactors normally transmute some U238 to Pu239, a toxic transuranic isotope. Plutonium 239 has a half life of 24,000 years, and is the most common transuranic in spent nuclear fuel from light water reactors. Transuranics like Pu239 cause the perception that reactor wastes are an eternal problem. In contrast, the LFTR uses the Thorium fuel cycle, which transmutes Thorium to U233. U233 has two chances to fission in a LFTR. First as U233 (95% fission) and then again as it transmutes to U235 (98%). The fraction of fuel reaching Neptunium 237, the most likely transuranic element, is therefore less than 0.1%. [19] A related advantage is that the LFTR's fuel is relatively pure U233 (perhaps with 1% U232). When these two features are combined, a Thorium fuel cycle reduces the production of transuranic wastes by more than a thousand-fold compared to a conventional once-through uranium-fueled light-water reactor. The LFTR does still produce radioactive fission products in its waste, but they don't last very long - the radiotoxicity of these fission products is dominated by Cesium 137 and Strontium 90. The longer half-life is Cesium: 30.17 years. So, after 30.17 years, decay reduces the radioactivity by a half. Ten half-lives will reduce the radioactivity to two raised to a power of ten, a factor of 1,024. Fission products at that point, in 301.7 years, are less radioactive than natural uranium. Burial in rock or clay is reasonable and safe by that time, because we've always lived with uranium in rock.
- Since the LFTR fuel is liquid, relatively small, simple equipment can continuously remove transmutation products. This immensely simplifies the reactor's behavior, i.e. it is more predictable, thus more easily controlled and safer than a conventional LWR reactor.
- Fluoride combines ionically with almost any transmutation product. This is an MSFR's first level of containment. Fluoride is especially good at holding biologically active "salt loving" wastes such as Cesium 137.
- If there is an accident beyond the design basis for the multiple levels of containment, fluorides do not easily enter the biome. The salts do not burn, explode, or chemically degrade in air or water. The fluoride salts of radioactive actinides and fission products are generally not soluble in water or air.
- The reactor is easy to control at all times. Xenon-135, an important neutron absorber makes solid fueled reactors difficult to control. In a molten fueled reactor, it can be removed at a predictable place, where the fuel is coolest, the pump bowl. In solid-fuel reactors, it remains in the fuel and interferes with reactor control.
- A LFTR operates at or above 650C, well above the 250C Wigner annealing temperature of graphite. This prevents Wigner energy from forming in the graphite moderator. The continual annealing bleeds it off. A sudden release of Wigner energy is thus not possible. Therefore, a Windscale-style graphite-incited fire cannot be caused by the graphite's nonexistent Wigner energy.
- The LFTR resists diversion of its fuel to nuclear weapons. There are two ways: First, the Thorium breeds by converting first to Protactinium, which then decays to U233. If the Protactinium remains in the reactor, small amounts of U232 are also produced. U232 has a decay chain product (Thallium 208) that emits powerful, dangerous gamma rays. These are not a problem inside a reactor, but in a bomb, they complicate bomb manufacture, harm electronics and reveal the bomb's location. [20] On another track, a LFTR doesn't make much spare fuel. It produces at most 9% more fuel than it burns each year, and it's even easier to design a reactor that makes 1% more fuel. With this kind of reactor, building bombs quickly will take power plants out of operation, and this is an easy indication of national intentions.
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A LFTR breeds thorium into uranium-233 fuel. The Earth's crust contains about three times as much Thorium as U238, or 400 times as much as U235 - thorium is about as abundant as lead. It is a byproduct of rare-earth mining, and is normally discarded as waste. Thorium currently (2011) costs only US$ 30/kg. In contrast, the price of Uranium has risen above $100/kg, not including costs for enrichment and fuel fabrication. Using LFTRs, there is enough affordable thorium to satisfy the Earth's energy needs for hundreds of thousands of years. [21] Besides thorium, LFTRs have used all three fuels (U235, U233 and U239). LFTRs have higher neutron fluxes and burnup, and so can utilize spent nuclear fuel.
Conventional reactors consume less than one percent of their uranium fuel, leaving the rest as waste. LFTR consumes 99% of its thorium fuel. The improved fuel efficiency means that 1 tonne of natural thorium in a LFTR produces as much energy as 35 t of enriched uranium in conventional reactors (requiring 250 t of natural uranium), [3] or 4 166 000 tonnes of black coal in a coal power plant.ince all natural thorium can be used as a fuel, and the fuel is in the form of a molten salt instead of solid fuel rods, expensive fuel enrichment and solid fuel rods' validation procedures and fabricating processes are not needed. This greatly decreases LFTR fuel costs.
LFTR's are cleaner: as a fully recycling system, the discharge wastes from a LFTR are predominately fission products, most of which have relatively short half lives compared to longer-lived actinide wastes. [20] This results in a significant reduction in the needed waste containment period in a geologic repository (300 years vs. tens of thousands of years)
The LFTR can "burn" problematic radioactive waste with transuranic elements from traditional solid-fuel nuclear reactors, thus solving the High level waste problem by turning the liability into an asset
LFTRs scale well: Small, 2–8 MW(thermal) or 1–3 MW(electric) versions are possible, enabling submarine or aircraft use
LFTRs have liquid fuels, and therefore there is no need to take apart the reactor just to refuel it. LFTRs can thus refuel without causing a power outage.
A LFTR can react to load changes in less than 60 seconds (unlike "traditional" solid-fuel nuclear power plants), thus it can satisfy both base load and peak load power demands.
The LFTR has very high temperatures. So, it is possible to use very efficient Brayton cycle generating turbines. [11] The thermal efficiency from the high temperature operation reduces fuel use, wastes and the cost of auxiliary equipment (major capital expenses) by 50% or more.
Since the core is not pressurized, it does not need the most expensive item in a light water reactor, a high-pressure reactor vessel for the core. Instead, there is a low-pressure vessel and pipes (for molten salt) constructed of relatively thin materials. Although the metal is an exotic nickel alloy that resists heat and corrosion, Hastelloy-N, the amount needed is relatively small and the thin metal is less expensive to form and weld.
By using liquid salt as the coolant instead of pressurized water a containment structure only slightly bigger than the reactor vessel can be used. Light water reactors use pressurized water which flashes to steam and expands a thousandfold in the case of a leak, necessitating a containment building a thousandfold bigger in volume than the reactor vessel. This gives the LFTR a substantial theoretical advantage in terms of smaller size and lower construction cost.
It can be air-cooled, which is critical for use in many regions where water is scarce
Fission products of a LFTR include stable rare elements such as rhodium, ruthenium, palladium, xenon, neodymium, molybdenum, zirconium and cesium, which are relied heavily on in modern electronics and industrial processes. Medically valuable isotopes such as bismuth-213 and technetium-99m useful for radiotherapy, as well as radionuclides used in powering radioisotope thermoelectric generators are also among the LFTR fission products. Compared to solid fuel oxide waste of traditional nuclear reactors contaminated by transuranic elements, these can be relatively easily extracted from the LFTR waste. ===== |
