Liquid Fluoride Thorium Reactor - Advantages - Economy and Efficiency

Economy and Efficiency

• Thorium abundance. A LFTR breeds thorium into uranium-233 fuel. The Earth's crust contains about three to four times as much thorium as U-238 (thorium is about as abundant as lead). It is a byproduct of rare-earth mining, normally discarded as waste. Using LFTRs, there is enough affordable thorium to satisfy the global energy needs for hundreds of thousands of years.
• No shortage of natural resources. Sufficient other natural resources such as beryllium, lithium, nickel and molybdenum are available to build thousands of LFTRs.
• Reactor efficiency. Conventional reactors consume less than one percent of the mined uranium, leaving the rest as waste. With well working reprocessing LFTR may consume about 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), or 4,166,000 tonnes of black coal in a coal power plant.
• Thermodynamic efficiency. LFTRs operating with modern supercritical steam turbines would operate at 45% thermal to electrical efficiency. With future closed gas Brayton cycles, which could be used in a LFTR power plant due to its high temperature operation, the efficiency could be up to 54%. This is 20 to 40% higher than today's light water reactors (33%), resulting in the same 20 to 40% reduction in fissile and fertile fuel consumption, fission products produced, waste heat rejection for cooling, and reactor thermal power.
• No enrichment and fuel fabrication. Since 100% of 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. Even if the LFTR is started up on enriched uranium, it only needs this enrichment once just to get started. After startup, no further enrichment is required.
• Lower fuel cost. The salts are fairly inexpensive compared to solid fuel production. For example, while beryllium is quite expensive per kg, the amount of beryllium required for a large 1 GWe reactor is quite small. ORNL's MSBR required 5.1 tonnes of beryllium metal, as 26 tonnes of BeF₂. At a price of $147/kg BeF₂, this inventory would cost less than $4 million, a modest cost for a multi billion dollar power plant. Consequently a beryllium price increase over the level assumed here has little effect in the total cost of the power plant. The cost of enriched lithium-7 is less certain, at $120–800/kg LiF. and an inventory (again based on the MSBR system) of 17.9 tonnes lithium-7 as 66.5 tonnes LiF makes between $8 million and $53 million for the LiF. Adding the 99.1 tonnes of thorium @ $30/kg adds only $3 million. Fissile material is more expensive, especially if expensively reprocessed plutonium is used, at a cost of $100 per gram fissile plutonium. With an startup fissile charge of only 1.5 tonnes, made possible through the soft neutron spectrum this makes $150 million. Adding everything up brings the total cost of the one time fuel charge at $165 to $210 million. This is similar to the cost of a first core for a light water reactor. Depending on the details of reprocessing the salt inventory once can last for decades, whereas the LWR needs a completely new core every 4 to 6 years (1/3 is replaced every 12 to 24 months). ORNL's own estimate for the total salt cost of even the more expensive 3 loop system, was much lower, only around $30 million, which is less than 100 million in today's money.
• LFTRs are cleaner: as a fully recycling system, the discharge wastes from a LFTR are predominantly fission products, most of which have relatively short half lives compared to longer-lived actinide wastes. This results in a significant reduction in the needed waste containment period in a geologic repository. After 300 years the radiotoxicity of the thorium fuel cycle waste is 10,000 times less than that of the uranium/plutonium fuel cycle waste.
• Low corrosion, long lasting materials. ORNL developed a special alloy, Hastelloy N, for the MSRE. They later modified the alloy for improved resistance to radiation damage and tellurium embrittlement, by adding some titanium and niobium, respectively. This resulted in very low corrosion rates compared to light water reactors,. Graphite is completely inert in redox controlled fluoride melts, and while it needs to be replaced every 4–30 years (depending on core power density) due to fast neutron radiation damage, the estimated cost of replacement graphite is very low, 0.01 cent per kWh (0.1 mill/kWh, or $0.1/MWh) in 1969 dollars. Taking into account inflation, in today's dollars this is 0.03 cents per kWh.
• Destruction of existing long lived nuclear wastes. The LFTR can "burn" problematic radioactive waste with transuranic elements from traditional solid-fuel nuclear reactors without producing new transuranic waste in the process, thus solving the long term high level waste problem by turning the liability into an asset.
• Less fissile fuel needed. Because LFTRs are thermal spectrum reactors, they need much less fissile fuel to get started. Only 1-2 tonnes of fissile are required to start up a single fluid LFTR, and potentially as low as 0.4 ton for a two fluid design. In comparison, solid fueled fast breeder reactors need at least 8 tonnes of fissile fuel to start the reactor. While fast reactors can theoretically start up very well on the transuranic waste, their high fissile fuel startup makes this very expensive.
• No downtime for refueling. LFTRs have liquid fuels, and therefore there is no need to shutdown and take apart the reactor just to refuel it. LFTRs can thus refuel without causing a power outage (online refueling).
• Load following. As the LFTR does not have xenon poisoning, there is no problem reducing the power in times of low demand for electricity and turn back on at any time.
• No high pressure vessel. 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. Thicker components can be made of a lower cost, high strength high temperature steel, such as Incolloy 800H, that is then clad with a corrosion resistant layer on the salt contacting side, one such metal is the more expensive but highly fluoride resistant Hastelloy N. This reduces cost of the metals, and is commonly used in light water reactors, that use high strength steel and a stainless steel liner for corrosion protection against the hot pressurized water.
• Excellent heat transfer. Liquid fluoride salts, especially LiF based salts, have good heat transfer properties. Fuel salt such as LiF-ThF₄ has a volumetric heat capacity that is around 22% higher than water, FLiBe has around 12% higher heat capacity than water. In addition, the LiF based salts have a thermal conductivity around twice that of the hot pressurized water in a pressurized water reactor. This results in efficient heat transfer and a compact primary loop. Compared to helium, a competing high temperature reactor coolant, the difference is even bigger. The fuel salt has over 200 times higher volumetric heat capacity as hot pressurized helium and over 3 times the thermal conductivity. A molten salt loop will use piping of 1/5 the diameter, and pumps 1/20 the power, of those required for high-pressure helium, while staying at atmospheric pressure
• Smaller, low pressure containment. 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. The LFTR containment can not only be smaller in physical size, its containment is also inherently low pressure. There are no sources of stored energy that could cause a rapid pressure rise (such as hydrogen or steam) in the containment. This gives the LFTR a substantial theoretical advantage not only in terms of inherent safety, but also in terms of smaller size, lower materials use, and lower construction cost.
• Air cooling. A high temperature power cycle can be air-cooled at little loss in efficiency, which is critical for use in many regions where water is scarce. No need for large water cooling towers used in conventional steam-powered systems would also decrease power plant construction costs.
• LFTRs scale well. Given the diminished scale of LFTRs, no need for big containment, pressure vessel and cooling towers for the air-cooled designs, LFTR small modular reactors lend themselves well to factory mass production. According to American Scientist, a reasonable cost projection is that 100 MW reactors could be factory produced for around $200 million.
• From waste to resource. There are suggestions that it might be possible to reuse some of the fission products. However, compared to the produced energy, the value of the fission products is low, and chemical purification is expensive.