The Liquid Fluoride Thorium Reactor
The modern concept of the Liquid-Fluoride Thorium Reactor (LFTR) uses uranium and thorium dissolved in fluoride salts of lithium and beryllium. These salts are chemically stable, impervious to radiation damage, and non-corrosive to the vessels that contain them. Because of their ability to tolerate heavy radiation, excellent temperature properties, minimal fuel loading requirements (i.e., easy of continual refueling) and other inherent factors, LFTR cores can be made much smaller than a typical light water reactor (LWR). In fact, liquid salt reactors, and LFTRs specifically, are listed as an unfunded part of the U.S. Department of Energy's Generation-4 Nuclear Solution Plan.
The Advantages
Some of the many advantages of the LFTR system over other nuclear reactor designs are outlined below. While LWRs can produce U233 from thorium, they will not provide the various advantages outlined below, because of their use of thorium in solid form. It is the unique combination of the thorium cycle and the liquid fluoride reactor that grants all of the following advantages only from the LFTR system.
Safety--LFTRs are designed to take advantage of the physics of the thorium cycle for optimum safety. The fluid in the core is not pressurized, thus eliminating the driving force of radiation release in conventional approaches. The LFTR reactor cannot melt down because of a runaway reaction or other nuclear reactivity accidents (such as at Chernobyl), because any increase in the reactor's operating temperature results in a reduction of reactor power, thus stabilizing the reactor without the need for human intervention. Further, the reactor is designed with a salt plug drain in the bottom of the core vessel. If the fluid gets too hot or for any other reason including power failures, the plug naturally melts, and the fluid dumps into a passively cooled containment vessel where decay heat is removed. This feature prevents any Three Mile Island-type accidents or radiation releases due to accident or sabotage and provides a convenient means to shut down and restart the system quickly and easily.
Proliferation Resistance--For all practical purposes, U233 is worthless as a nuclear weapons material, and indeed no nation has attempted to weaponize U233 because of the abundance of difficulties. U233 is considered an unsuitable choice for nuclear weapons material because whenever U233 is generated, uranium-232 (U232) contamination inevitably occurs. U232 rapidly decays into other elements, including thallium-208, a hard-gamma-ray emitter whose signature is easily detectable. The hard gamma rays from thallium-208 cause ionization of materials destroying the explosives and electronics of a nuclear weapon, and heavy lead shielding is required to protect personnel assembling the warhead. It is possible to generate U233 with little U232 contamination using specialized reactors (such as at the Hanford Site), but not with an LFTR. Any attempt to increase production of U233 in an LFTR reactor will generate U232 contamination and any attempt to steal quantities of U233 results in the reactor shutting down.
Energy Production--Because nearly all of the thorium is used up in an LFTR (versus only about 0.7% of uranium mined for an LWR), the reactor achieves high energy production per metric ton of fuel ore, on the order of 300 times the output of a typical uranium LWR. The LFTR allows much higher operating temperatures than does a typical LWR therefore a higher thermodynamic efficiency. The turbine system believed best suited for its operation is a triple-reheat closed-cycle helium turbine system, which should convert 50% of the reactor heat into electricity compared to today's steam cycle (~25% to 33%). This efficiency gain translates to about 4.11 million barrels of crude oil equivalent per year more than that generated by a steam system. Capital costs are lower due to smaller reactor & turbo-machinery size, low reactor pressures and minimal redundant safety systems. The greater energy production capability of LFTRs means we estimate the cost for electricity from a LFTR plant could be 25% to over 50% less than that from a LWR.
Waste--In theory, LFTRs would produce far less waste along their entire process chain, from ore extraction to nuclear waste storage, than LWRs. A LFTR power plant would generate 4,000 times less mining waste (solids and liquids of similar character to those in uranium mining) and would generate 1,000 to 10,000 times less nuclear waste than an LWR. Additionally, because LFTR burns all of its nuclear fuel, the majority of the waste products (83%) are safe within 10 years, and the remaining waste products (17%) need to be stored in geological isolation for only about 300 years (compared to 10,000 years or more for LWR waste). Additionally, the LFTR can be used to "burn down" waste from an LWR (nearly the entirety of the United States' nuclear waste stockpile) into the standard waste products of an LFTR, so long-term storage of nuclear waste would no longer be needed.
Supply--Thorium is abundant in the Earth's crust. It is the 36th most plentiful element in the crust--four times as common as uranium and 5,000 times as plentiful as gold. According to the U.S. Geological Survey's 2006 Mineral Yearbook, the United States is estimated to have 300,000 tons of thorium reserves (about 20% of the world's supply), more than half of which is easily extractable. Considering only the readily accessible portion, this national resource translates to nearly 1 trillion barrels of crude oil equivalent--five times the entire oil reserves of Saudi Arabia. In addition to the naturally occurring reserves, the United States currently has 3,200 metric tons of processed thorium nitrate buried in the Nevada desert. That supply is roughly equivalent to 21 billion barrels of crude oil equivalent when used in an LFTR with only minimal processing effort.
Secondary Products--Because an LFTR is so energy dense, the electricity and excess heat from the reactor can be used to fuel other industries beyond electricity production, including economical desalinization of water, cracking of hydrogen from water or hydrocarbons, generation of ammonia for fertilizer and fuel cells, and extraction of hydrocarbons from oil shale and tar sands. Additionally, the nuclear waste products from the LFTR include stable rhodium and ruthenium, rare elements needed in modern electronics; technetium-99, which offers great promise as a catalyst similar to platinum; iodine-131 and cesium-137 for medical applications; strontium-90 for radioisotope power; and xenon, used in commercial products and industrial processes.
The Risks
While LFTRs offer much promise, several economic and engineering issues need to be addressed before this technology can become a reality.
Thorium as a Fuel--Thorium has never actually been continually processed for fuel in a fully operational liquid fluoride reactor. The MSRE used U233 as a fuel, but the U233 was generated in another reactor. A follow-on reactor design was planned to do the full-system tests, which the MSRE was too cost-constrained to perform, but it was never funded. A prototype reactor based on the ORNL design work would need to be built and the continuous thorium cycle processing validated as the fuel source in an operational LFTR.
Turbine System--The gas turbo-machinery is similar engineering to the well-developed open-cycle turbine (e.g., jet aircraft engine). However, this kind of closed-cycle electric generation system has never been built. A new triple-reheat closed-cycle Brayton system would need to be built and tested along with the LFTR. However, this is a minimal engineering risk in obtaining the overall efficiency of the electricity generation system. If the close cycle turbine system proves not to be economically viable, a steam system can be used.
Cost of Thorium--The price of thorium ore is difficult to quantify. On one hand, some will argue that it is expensive, citing the lack of demand and the consequently limited market supply. On the other hand, the case can be made that thorium is nearly worthless in light of the U.S. government's decision to essentially "throw away" 3,200 metric tons of processed thorium by burying it in the Nevada desert. We cannot predict how the price of thorium would be affected if the world's thorium reserves were exploited for use in LFTRs. However, thorium does not incur a cost of enrichment as uranium does, mostly due to the fact that natural thorium occurs only in one isotope. We believe that if the world's thorium supplies were exploited for energy, its price would drop to be comparable to--or even lower than--current uranium ore prices.
Cost of Thorium Reactors--Even though a full-scale LFTR has never been built, we expect the lifecycle cost of thorium reactors could be at least 30% to 50% less than equivalent-power uranium-based LWRs. Nevertheless, the engineering, fabrication and licensing of any energy-dense endeavourer is never certain and subject to many outside factors. Because of the various advantages afforded by the LFTR technology, we expect there will be a reduced regulatory burden, which would lessen costs and accelerate startups. For full-scale construction of LFTRs, factory-built modular construction can be used to provide scalable reactors from 100-kilowatt to multi-gigawatt production. This flexibility in site location eliminates the largest risk facing new U.S. commercial power plants today. Further, LFTRs have operational cost advantages over both types of reactors currently licensed. Unlike pressurized water reactors, LFTRs will not have to be shut down for extensive periods for refueling. Unlike boiling water reactors, LFTRs do not radioactively contaminate the turbines used for electrical generation, which should translate into significantly reduced operational and maintenance costs for this portion of the power plant and reduced amounts of low-level waste for end-of-life disposal.
This article is taken from (www.energyfromthorium.com) with permission from Kirk Sorensen
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