Among possible reactor coolants (water, gas, metal and salt), only liquid salts offer the desirable combination of low pressure operation at high temperatures. LFTRs use liquid-fluoride salts as both a coolant and as a carrier for the thorium and thorium-derived fuels. LFTRs operate at near atmospheric pressure offering unmatched safety and greatly simplified reactor designs. In particular, ambient or low reactor operating pressures means no risk of high-pressure atmospheric releases and no need for massive containment structures that can withstand high pressure. LFTRs high temperatures (650 C) enable greater thermal-to-electric conversion efficiencies and use of more compact power conversion systems. Use of thorium fuel in a LFTR generates orders-of-magnitude less mining waste and long-lived transuranic waste than existing light-water reactor (LWR) technology. Thorium is abundant and inexpensive and LFTR’s liquid thorium fuel is easily produced, compared to costly, complex fabrication of solid fuel rods used in legacy water-cooled reactors. The liquid fuel form allows fuel to be added and byproducts to be removed even while the reactor remains online. LFTRs can consume the unused fissile material available in existing spent nuclear fuel waste and weapons stockpiles. Waste products from a LFTR are predominantly fission products rather than actinides, and decay much more rapidly. LFTRs can be produced as modules in a factory and can be made to be air-cooled or water-cooled, land-based or submersible, fixed or mobile, offering more flexible siting, installation and deployment options and with much less visual intrusion.
Liquid versus Solid Fueled Thorium Reactors
Use of thorium in a liquid fuel form is LFTR’s key differentiator from other efforts to employ thorium as a nuclear fuel. Several countries are researching the thorium fuel cycle for use in solid-fueled, water-cooled reactors. For example, LWRs can use thorium in solid fuel form to produce U233, however, they cannot provide the various advantages of the combination of the thorium cycle in liquid-fueled reactors that LFTR technology achieves. Liquid-fluoride reactors use a chemically stable fuel form based on fluoride salts of lithium and beryllium. These salts have exceptional chemical stability, which gives them a remarkable heat storage capacity over a thousand-degree liquid range. Furthermore, the salts’ ionic bonds are unaffected by neutrons or radiation, making them a nearly ideal medium for sustaining a nuclear reaction. Into this liquid salt are mixed salts of uranium for the core salt and of thorium for the blanket salt. Thorium-232 in the blanket salt absorbs neutrons released by fission in the core and is ultimately converted into uranium-233. Uranium-233 is extracted from the blanket salts and is then fed back into the reactor core where fission of the uranium-233 produces high heat for power generation and more neutrons to convert thorium-232 into uranium-233, perpetuating the fuel production cycle. Once a LFTR is operational, thorium is the only input required to perpetuate the thorium-to-uranium fuel cycle. The heat generated from nuclear fission in LFTR is transferred via heat exchangers to a clean coolant salt loop that exits the containment boundary and is then transferred to the working fluid of a gas turbine engine (e.g., a closed-cycle helium or nitrogen turbine) to generate power. The fuel salt, blanket salt and clean coolant salt circuits are each maintained at near atmospheric pressure.
The properties of fluoride salts offer LFTR enhanced safety characteristics over existing reactor technologies. The fluid salt in the core is not pressurized, thus eliminating the fundamental driving force present in the core of legacy water-cooled reactors (like those at Fukushima-Daiichi). The notion of a “meltdown” leading to reactor failure becomes irrelevant in a reactor designed around the use of liquid fuels. The reactivity of the reactor is self-controlling because any increase in the reactor’s operating temperature results in decrease in density of the fuel salt in the core and a reduction of reactor power, thus inherently stabilizing the reactor without the need for human intervention or backup systems. Further, the reactor is designed with a simple salt plug drain in the bottom of the core vessel to completely shut down the core through the natural operation of gravity. If the reactor should lose power or need to be powered down for any reason, the salt plug is simply allowed to melt and the fluid salt to drain via gravity into a passively cooled containment vessel(s) where decay heat is readily removed. This simple 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 reactor quickly and easily.
LFTR is an inherently safe, intrinsically stable and self-regulating design that removes the root causes of today’s reactor accidents and the need for complex redundant safety systems. The core, blanket and primary cooling salt loops are all operated at or near atmospheric pressure and without steam, so a pressurized release, depressurization accident or explosion is impossible. Inherent stability is provided by very negative thermal/reactivity coefficient, i.e., the fuel salt expands as temperature rises, reducing the density and rate of reaction so that a meltdown is impossible. Thus, when the reactor gets hotter, its ability to generate heat goes down. Conversely when it gets cooler, its ability to generate heat goes up. These two effects cause the reactor to be self-stabilizing and to follow the demand for electricity imparted on it by the grid. Xenon byproduct gas levels are also closely controllable so that a slight increase in Xenon gas levels would otherwise absorb needed neutron to terminate the fission reaction in the core.
Because nearly all of the thorium fuel is consumed in a LFTR (versus only about 0.5% of uranium mined for an LWR), the reactor achieves high energy production per metric ton of fuel ore, on the order of 200 times the output of a typical uranium LWR. LFTR can achieve much higher operating temperatures than a typical LWR and, therefore, much higher thermodynamic efficiency. LFTR’s high operating temperatures are well-suited for multi-reheat closed-cycle helium turbine systems, which could convert approximately half of the reactor’s primary heat into electricity compared to today’s steam cycle systems, which can only convert about a third. Capital costs are also lower due to smaller reactor & turbo-machinery size, low reactor pressures and minimal redundant safety systems. We estimate that the greater energy production capability of LFTRs could lower the cost for electricity from a LFTR plant to 25% to 50% less than from a LWR. These efficiency gains also allow more compact system geometries, greatly reducing total utility size and local environmental impacts over traditional reactors.
Thorium is abundant throughout the Earth’s crust. It is the 36th most plentiful element in the crust–four times as common as uranium, 5,000 times as plentiful as gold and generally as abundant as lead. 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 readily accessible. Considering only the readily accessible reserves, 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 a reserve of 3,200 metric tons of processed thorium nitrate buried at a test site in the Nevada desert. This reserve alone is roughly equivalent to 21 billion barrels of crude oil equivalent when used in an LFTR with only minimal processing effort required. Thorium is so energy dense, a mere 6600 tonnes of thorium could replace the combined 5.3 billion tonnes of coal, 31.1 billion barrels of oil, 2.92 trillion cubic meters of natural gas, and 65,000 tonnes of uranium that the world consumes annually. What is more, estimated global thorium reserves could supply the world’s energy needs for thousands of years.
None of the many thousands of warheads in the world’s arsenals are based on the thorium-to-uranium fuel cycle, for a number of reasons. Production of uranium-233 from thorium inherently leads to production of uranium-232, the decay chain of which includes thallium-208, a hard gamma emitter, that would severely ionize critical electronics or explosives and harm technicians in any weaponization effort, and more importantly, would be readily detectable by regulators and security forces. Moreover, isotopic separation of the undesirable U-232 is even more difficult than the already daunting tasks of U-235 enrichment or plutonium breeding currently used to obtain weapons grade materials. For all practical purposes, U233 is highly undesirable as a weapons material, and indeed no nation has weaponized U233 because of the many inherent difficulties of doing so. While other highly-specialized reactors (such as at the Hanford Site) have generated U233 with minimal U232 contamination, any production of U233 in a LFTR will always generate significant U232 contamination. LFTR operation itself is a substantial further deterrent to materials diversion. During operation, the U233-bearing fuel salts are fully contained within a containment structure in an extremely high temperature and radiologically hot e