Energy from Thorium reader Raul Parolari thought that some of our posts should be presented in other languages, so he offered this translation to French.
French translation follows…
Author Archive
Ammoniac Nucléaire
Sunday, December 18th, 2011Nuclear Cement
Monday, November 7th, 2011In the recent Nuclear Ammonia article post, ammonia was illustrated as a fuel that could propel vehicles in a zero carbon era. Despite our best efforts in developing new internal combustion engines and direct ammonia fuel cells, there will continue to be a role for carbonaceous fuels. Gasoline and jet fuel have double the volumetric energy capacity of liquid ammonia. A given fuel tank can only contain half as much ammonia combustion potential energy as gasoline combustion potential energy. Fuel tank size is very important in aircraft. Decades of engineering of airframes and turbine engines have optimized aircraft performance using diesel-like JP8 jet fuel.
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Nuclear Ammonia
Saturday, October 29th, 2011The liquid fluoride thorium reactor (LFTR) has the potential to make electric power cheaper than from coal. Typical costs for electric power bought by US utilities average around 5-6 cents per kilowatt hour generated by coal, hydro, and natural gas sources. Government regulations are requiring utilities to buy solar- and wind-generated power at 20-30 cents/kWh. LFTR’s potential cost advantage of 3 cents/kWh is the economic incentive to stop burning CO2-emitting coal, without economically injurious carbon taxes and politically obscured feed-in tariffs. In this way LFTR can improve both the environment and the economy.
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Renewing the Great Recession
Tuesday, April 19th, 2011We are climbing out the Great Recession. US gross domestic product (GDP) is the value of all produced goods and services in a year. In the Great Recession GDP dropped from an annual rate of $14.48 trillion to $14.03 trillion, a productivity hit of $450 billion, or a 3.2% decrease.
In 2010 the US generated 4.12 PWH (peta watt hours) of electricity to power our economy. That’s 4.12 million thousand kilowatt hours. The average wholesale price of electricity varies from about 4 cents in Texas to 6 cents in New England; let’s say it’s about 5 cents per kWh. The value of 4.12 PWH at 5 cents/kWh is $206 billion, or 1.4% of GDP.
Renewable electric power is expensive. The $2.2 billion Cape Wind project will generate 468 MW of peak power. Average power will be about 30% of that. The capital cost will be $2.2/.468 or $4.7 per peak watt of generating capacity — about the same as today’s new nuclear power plants (which operate 90% of the time). The wind turbines may operate 30% of the time, resulting in a capital cost of $15/watt ($4.7/.30). The capital cost recovery alone (@8% over 40 years) is 15 cents/kWh. To make Cape Wind successful the State of Massachusetts requires utilities to buy wind-generated electricity at 19 cents/kWh, rising annually to 31 cents/kWh — provided all Federal subsidies continue. Similar costs arise in other wind and solar projects.
What would be the impact of 31-cent power on GDP? It raises the cost of the same electricity from 1.4% to 8.7% of GDP, removing (8.7 – 1.4) 7.3% of productivity from our economy.
If you thought the Great Recession was bad with a 3.2% productivity hit, are you ready for a 7.3% GDP hit from 31-cent renewable power?
Liquid Fuel Nuclear Reactors
Sunday, January 9th, 2011The American Physical Society forum on Physics and Society has just published its quarterly newsletter, containing two articles about nuclear power, including one by Robert Hargraves and Ralph Moir, Liquid Fuel Nuclear Reactors.
Today’s familiar pressurized water nuclear reactors use solid fuel — pellets of uranium dioxide in zirconium fuel rods bundled into fuel assemblies. These assemblies are placed within the reactor vessel under water at 160 atmospheres pressure and a temperature of 330°C. This hot water transfers heat from the fissioning fuel to a steam turbine that spins a generator to make electricity. Alvin Weinberg invented the pressurized water reactor (PWR) in 1946 and such units are now used in over 100 commercial power-producing reactors in the US as well as in naval vessels.
Weinberg also pursued research on liquid fuel-reactors, which offer a number of advantages over their solid-fueled counterparts. In this article we review some of the history, potential advantages, potential drawbacks, and current research and development status of liquid-fueled reactors. Our particular emphasis is on the Liquid Fluoride Thorium Reactor (LFTR).
Before describing the characteristics of liquid-fuel reactors we review briefly in this paragraph the situation with PWRs. In a conventional PWR the fuel pellets contain UO2 with fissile U-235 content expensively enriched to 3.5% or more, the remainder being U-238. After about 5 years the fuel must be removed because the fissile material is depleted and neutron-absorbing fission products build up. By that time the fuel has given up less than 1% of the potential energy of the mined uranium, and the fuel rods have become stressed by internal temperature differences, by radiation damage that breaks covalent UO2 bonds, and by fission products that disturb the solid lattice structure (Figure 1). As the rods swell and distort, their zirconium cladding must continue to contain the fuel and fission products while in the reactor and for centuries thereafter in a waste storage repository.
Courtesy of Japan Atomic Energy Agency R&D Review 2008Figure 1. Solid fuel rods are stressed by fission products, radiation, and heat.
In contrast, fluid fuels are not subjected to the structural stresses of solid fuels: liquid-fuel reactors can operate at atmospheric pressure, obviating the need for containment vessels able to withstand high-pressure steam explosions. Gaseous fission products like xenon bubble out while some fission products precipitate out and so do not absorb neutrons from the chain reaction. Like PWRs, liquid-fuel reactors can be configured to breed more fuel, but in ways that make them more proliferation resistant than the waste generated by conventional PWRs. Spent PWR fuel contains transuranic nuclides such as Pu-239, bred by neutron absorption in U-238, and it is such long-lived transuranics that are a core issue in waste storage concerns. In contrast, liquid-fuel reactors have the potential to reduce storage concerns to a few hundred years as they would produce far fewer transuranic nuclides than a PWR.
History of liquid fuel reactors
The world’s first liquid fuel reactor used uranium sulfate fuel dissolved in water. Eugene Wigner conceived this technology in 1945, Alvin Weinberg built it at Oak Ridge, and Enrico Fermi started it up. The water carries the fuel, moderates neutrons (slows them to take advantage of the high fission cross-section of uranium for thermal-energy neutrons), transfers heat, and expands as the temperature increases, thus lowering moderation and stabilizing the fission rate. Because the hydrogen in ordinary water absorbs neutrons, an aqueous reactor, like a PWR, cannot reach criticality unless fueled with uranium enriched beyond the natural 0.7% isotopic abundance of U-235. Deuterium absorbs few neutrons, so, with heavy water, aqueous reactors can use unenriched uranium. Weinberg’s aqueous reactor fed 140 kW of power into the electric grid for 1000 hours. The intrinsic reactivity control was so effective that shutdown was accomplished simply by turning off the steam turbine generator.
In 1943, Wigner and Weinberg also conceived a liquid fuel thorium-uranium breeder reactor, for which the aqueous reactor discussed above was but the first step. The fundamental premise in such a reactor is that a blanket of thorium Th-232 surrounding the fissile core will absorb neutrons, with some nuclei thus being converted (“transmuted”) to Th-233. Th-233, in turn, beta decays to protactinium-233 and then to U-233, which is itself fissile and can be used to refuel the reactor. Later, as Director of Oak Ridge, Weinberg led the development of the liquid fluoride thorium reactor (LFTR), the subject of this article. Aware of the future effect of carbon dioxide emissions, Weinberg wrote “humankind’s whole future depended on this.” The Molten Salt Reactor Experiment, powered first with U-235 and then U-233, operated successfully over 4 years, through 1969. To facilitate engineering tests, the thorium blanket was not installed; the U-233 used in the core came from other reactors breeding Th?232. The MSRE was a proof-of-principle success. Fission-product xenon gas was continually removed to prevent unwanted neutron absorptions, online refueling was demonstrated, minor corrosion of the reactor vessel was addressed, and chemistry protocols for separation of thorium, uranium, and fission products in the fluid fluorine salts were developed. Unfortunately, the Oak Ridge work was stopped when the Nixon administration decided instead to fund only the solid fuel Liquid sodium Metal cooled Fast Breeder Reactor (LMFBR), which could breed plutonium-239 faster than the LFTR could breed uranium-233.
The Liquid Fluoride Thorium Reactor
A significant advantage of using thorium to breed U-233 is that relatively little plutonium is produced from the Th-232 because six more neutron absorptions are required than is the case with U-238. The U-233 that is bred is also proliferation-resistant in that the neutrons that produce it also produce 0.13% contaminating U-232 which decays eventually to thallium, which itself emits a 2.6 MeV penetrating gamma radiation that would be obvious to detection monitors and hazardous to weapons builders. For example, a year after U-233 separation, a weapons worker one meter from a subcritical 5 kg sphere of it would receive a radiation dose of 4,200 mrem/hr; death becomes probable after 72 hours exposure. Normally the reactor shielding protects workers, but modifying the reactor to separate U-233 would require somehow adding hot cells and remote handling equipment to the reactor and also to facilities for weapons fabrication, transport, and delivery. Attempting to build U-233-based nuclear weapons by modifying a LFTR would be more hazardous, technically challenging and expensive than creating a purpose-built weapons program using uranium enrichment (Pakistan) or plutonium breeding (India, North Korea).
Work on thorium-based reactors is currently being actively pursued in many countries including Germany, India, China, and Canada; India plans to produce 30% of its electricity from thorium by 2050. But all these investigations involve solid fuel forms. Our interest here is with the liquid-fueled form of a thorium-based U-233 breeder reactor.
The configuration of a LFTR is shown schematically in Figure 2. In a “two-fluid” LFTR a molten eutectic mixture of salts such as LiF and BeF2 containing dissolved UF4 forms the central fissile core. (“Eutectic” refers to a compound that solidifies at a lower temperature than any other compound of the same chemicals.) A separate annular region containing molten Li and Be fluoride salts with dissolved ThF4 forms the fertile blanket. Fission of U-233 (or some other “starter” fissile fuel) dissolved in the fluid core heats it. This heated fissile fluid attains a noncritical geometry as it is pumped through small passages inside a heat exchanger. Excess neutrons are absorbed by Th-232 in the molten salt blanket, breeding U-233 which is continuously removed with fluorine gas and used to refuel the core. Fission products are chemically removed in the waste separator, leaving uranium and transuranics in the molten salt fuel. From the heat exchanger a separate circuit of molten salt heats gases in the closed cycle helium gas turbine which generates power. All three molten salt circuits are at atmospheric pressure.
Figure 2. In a two-fluid liquid fluoride thorium reactor the fission of U-233 in the core heats molten carrier salt (yellow). It attains a noncritical geometry as it is pumped through small passages in a heat exchanger. A separate circuit of molten salt (red), with no radioactive materials, heats gases in the closed cycle helium gas turbine which spins to generate power. Excess neutrons are absorbed by Th-232 in the molten salt blanket (green), breeding U-233 which is removed with fluorine gas. Fission products are chemically removed in the waste separator, leaving uranium and transuranics in the molten salt fuel. All three molten salt circuits are at atmospheric pressure.
LFTRs would reduce waste storage issues from millions of years to a few hundred years. The radiotoxicity of nuclear waste arises from two sources: the highly radioactive fission products from fission and the long-lived actinides from neutron absorption. Thorium and uranium fueled reactors produce essentially the same fission products, whose radiotoxicity in 500 years drops below that of the original ore mined for uranium to power a PWR. A LFTR would create far fewer transuranic actinides than a PWR. After 300 years the LFTR waste radiation would be 10,000 times less than that from a PWR (Figure 3). In practice, some transuranics will leak through the chemical waste separator, but the waste radiotoxicity would be < 1% of that from PWRs. Geological repositories smaller than Yucca mountain would suffice to sequester the waste.
Figure 3. A LFTR produces much less long-lived waste than PWRs. (Adapted from Sylvan David et al, Revisiting the thorium-uranium nuclear fuel cycle, Europhysics news, 38(2), p 25.)
Existing PWR spent fuel can be an asset. A 100 MW LFTR requires 100 kg of fissile material (U-233, U-235, or Pu-239) to start the chain reaction. The world now has 340,000 tonnes of spent PWR fuel, of which 1% is fissile material that could start one 100 MW LFTR per day for 93 years.
A commercial LFTR will make just enough uranium to sustain power generation, so diverting uranium for weapons use would stop the reactor, alerting authorities. A LFTR will have little excess fissile material; U-233 is continuously generated to replace the fissioned U-233, and Th-232 is continuously introduced to replace the Th-232 converted to the U-233. Terrorists could not steal this uranium dissolved in a molten salt solution along with lethally radioactive fission products inside a sealed reactor, which would be subject to the usual IAEA safeguards of physical security, accounting and control of all nuclear materials, surveillance to detect tampering, and intrusive inspections.
It is also possible to configure a liquid-fuel reactor that would involve no U-233 separation. For example, the single fluid denatured molten salt reactor (DMSR) version of a LFTR with no U-233 separation is fed with both thorium and < 20% enriched uranium. It can operate up to 30 years before actinide and fission product buildup requires fuel salt replacement, while consuming only 25% of the uranium a PWR uses.
Starting up LFTRs with plutonium can consume stocks of this weapons-capable material. Thorium fuel would also reduce the need for U-235 enrichment plants, which can be used to make weapons material as easily as power reactor fuel. U-233, at the core of the reactor, is important to LFTR development and testing. With a half-life of only 160,000 years, it is not found in nature. The US has 1,000 kg of nearly irreplaceable U-233 at Oak Ridge. It is now slated to be destroyed by diluting it with U-238 and burying it forever, at a cost of $477 million. This money would be far better invested in LFTR development.
Can LFTR power be cheaper than coal power?
Burning coal for power is the largest source of atmospheric CO2, which drives global warming. We seek alternatives such as burying CO2 or substituting wind, solar, and nuclear power. A source of energy cheaper than coal would dissuade nations from burning coal while affording them a ready supply of electric power.
Can a LFTR produce energy cheaper than is currently achievable by burning coal? Our target cost for energy cheaper than from coal is $0.03/kWh at a capital cost of $2/watt of generating capacity. Coal costs $40 per ton, contributing $0.02/kWh to electrical energy costs. Thorium is plentiful and inexpensive; one ton worth $300,000 can power a 1,000 megawatt LFTR for a year. Fuel costs for thorium would be only $0.00004/kWh.
The 2009 update of MIT’s Future of Nuclear Power shows that the capital cost of new coal plants is $2.30/watt, compared to LWRs at $4/watt. The median of five cost studies of large molten salt reactors from 1962 to 2002 is $1.98/watt, in 2009 dollars. Costs for scaled-down 100 MW reactors can be similarly low for a number of reasons, six of which we summarize briefly:
Pressure. The LFTR operates at atmospheric pressure, obviating the need for a large containment dome. At atmospheric pressure there is no danger of an explosion.
Safety. Rather than creating safety with multiple defense-in-depth systems, LFTR’s intrinsic safety keeps such costs low. A molten salt reactor cannot melt down because the normal operating state of the core is already molten. The salts are solid at room temperature, so if a reactor vessel, pump, or pipe ruptured they would spill out and solidify. If the temperature rises, stability is intrinsic due to salt expansion. In an emergency an actively cooled solid plug of salt in a drain pipe melts and the fuel flows to a critically safe dump tank. The Oak Ridge MSRE researchers turned the reactor off this way on weekends.
Heat. The high heat capacity of molten salt exceeds that of the water in PWRs or liquid sodium in fast reactors, allowing compact geometries and heat transfer loops utilizing high-nickel metals.
Energy conversion efficiency. High temperatures enable 45% efficient thermal/electrical power conversion using a closed-cycle turbine, compared to 33% typical of existing power plants using traditional Rankine steam cycles. Cooling requirements are nearly halved, reducing costs and making air-cooled LFTRs practical where water is scarce.
Mass production. Commercialization of technology lowers costs as the number of units produced increases due to improvements in labor efficiency, materials, manufacturing technology, and quality. Doubling the number of units produced reduces cost by a percentage termed the learning ratio, which is often about 20%. In The Economic Future of Nuclear Power, University of Chicago economists estimate it at 10% for nuclear power reactors. Reactors of 100 MW size could be factory-produced daily in the way that Boeing Aircraft produces one airplane per day. At a learning ratio of 10%, costs drop 65% in three years.
Ongoing research. New structural materials include silicon-impregnated carbon fiber with chemical vapor infiltrated carbon surfaces. Such compact thin-plate heat exchangers promise reduced size and cost. Operating at 950°C can increase thermal/electrical conversion efficiency beyond 50% and also improve water dissociation to create hydrogen for manufacture of synthetic fuels such that can substitute for gasoline or diesel oil, another use for LFTR technology.
In summary, LFTR capital cost targets of $2/watt are supported by simple fluid fuel handling, high thermal capacity heat exchange fluids, smaller components, low pressure core, high temperature power conversion, simple intrinsic safety, factory production, the learning curve, and technologies already under development. A $2/watt capital cost contributes $0.02/kWh to the power cost. With plentiful thorium fuel, LFTRs may indeed generate electricity at less than $0.03/kWh, underselling power generated by burning coal. Producing one LFTR of 100 MW size per day could phase out all coal burning power plants worldwide in 38 years, ending 10 billion tons per year of CO2 emissions from coal plants.
Development Status of LFTRs
A number of LFTR initiatives are currently active around the world. France supports theoretical work by two dozen scientists at Grenoble and elsewhere. The Czech Republic supports laboratory research in fuel processing at Rez, near Prague. Design for the FUJI molten salt reactor continues in Japan. Russia is modeling and testing components of a molten salt reactor designed to consume plutonium and actinides from PWR spent fuel, and LFTR studies are underway in Canada and the Netherlands. US R&D funding has been relatively insignificant, except for related studies of solid fuel, molten salt cooled reactors at UC Berkeley and Oak Ridge, which hosted a conference to share information on fluoride reactors in September 2010.
Developing LFTRs will require advances in high temperature materials for the reactor vessel, heat exchangers, and piping; chemistry for uranium and fission product separation; and power conversion systems. The International Generation IV Forum budgeted $1 billion over 8 years for molten salt reactor development. We recommend a high priority, 5-year national program to complete prototypes for the LFTR and the simpler DMSR. It may take an additional 5 years of industry participation to achieve capabilities for mass production. Since LFTR development requires chemical engineering expertise and liquid fuel technology is unfamiliar to most nuclear engineers today, nuclear engineering curricula would have to be modified to include exposure to such material. The technical challenges and risks that must be addressed in a prototype development project include control of salt container corrosion, recovery of tritium from neutron irradiated lithium salt, management of structural graphite shrinking and swelling, closed cycle turbine power conversion, and maintainability of chemical processing units for U-233 separation and fission product removal. Energy Secretary Chu expressed historical criticism of the technology in a letter to Senator Jeanne Shaheen (D-NH) answering questions at his confirmation hearings, “One significant drawback of the MSR technology is the corrosive effect of the molten salts on the structural materials used in the reactor vessel and heat exchangers; this issue results in the need to develop advanced corrosion-resistant structural materials and enhanced reactor coolant chemistry control systems”, and “From a non-proliferation standpoint, thorium-fueled reactors present a unique set of challenges because they convert thorium-232 into uranium-233 which is nearly as efficient as plutonium-239 as a weapons material.” He also recognized, however, that “Some potential features of a MSR include smaller reactor size relative to light water reactors due to the higher heat removal capabilities of the molten salts and the ability to simplify the fuel manufacturing process, since the fuel would be dissolved in the molten salt.”
Other hurdles to LFTR development may be the regulatory environment and the prospect of disruption to current practices in the nuclear industry. The Nuclear Regulatory Commission will need funding to train staff qualified to work with this technology. The nuclear industry and utilities will be shaken by this disruptive technology that changes whole fuel cycle of mining, enrichment, fuel rod fabrication, and refueling. Ultimately, the environmental and human development benefits will be achieved only when the cost of LFTR power really proves to be cheaper than from coal.
References
Robert Hargraves and Ralph Moir, Liquid Fluoride Reactors, American Scientist, July/August 2010
Alvin Martin Weinberg, The first nuclear era: the life and times of a technological fixer. Springer, New York, 1997.
Oak Ridge National Laboratory document repository
S. David, E. Huffer, H. Nifenecker, Revisiting the thorium-uranium nuclear fuel cycle
David LeBlanc, Molten Salt Reactors: A New Beginning for an Old Idea
Ralph Moir, Edward Teller, Thorium fueled underground power plant based on molten salt technology,
http://ralphmoir.com/moir_teller.pdf
Per Peterson, Pebble Bed Advanced High Temperature Reactor, http://www.nuc.berkeley.edu/pb-ahtr/
Oak Ridge National Laboratory, Fluoride Salt-Cooled High-Temperature Reactor Agenda,
https://www.ornl.gov/fhr/agenda.html
A Technology Roadmap for Generation IV Nuclear Systems, http://gif.inel.gov/roadmap/pdfs/gen_iv_roadmap.pdf
Not So Fast With Thorium
Tuesday, December 7th, 2010Keith Schwartztrauber, a self-declared “old nuke”, wrote a letter in the November/December 2010 issue of American Scientist , which had published Liquid Fuel Nuclear Reactors by Robert Hargraves and Ralph Moir.
To the Editors:
Robert Hargraves and Ralph Moir’s article “Liquid Fluoride Thorium Reactors” (July–August) was a pleasure to read but stirred concern in me. I have studied many reactor concepts dating to the 1960s. The authors correctly note advantages to the thorium fuel cycle. Given the commercial failures of the thorium-based high-temperature gas-cooled reactor (HTGR) and the demise of the thorium-based Shippingport light-water breeder reactor (LWBR), however, I don’t envision the liquid fluoride thorium reactor concept playing a central role. The developmental, technical, safety, regulatory and financial challenges are probably insurmountable.
U.S. nuclear reactors are constructed with solid fuel, metal cladding, water coolant, high integrity pressure vessels and piping, and concrete and steel pressure containments. For sound reasons, the Nuclear Regulatory Commission required their designers to assume that the system’s largest pipe could instantly rupture and release reactor coolant to the containment. It was assumed that large fractions of the reactor core fission products and any hydrogen generated would be released. In the case of the HTGR, this included potential graphite-water reactions (yielding hydrogen) and graphite-air reactions (yielding fire) in the core.
With the liquid fluoride thorium reactors (LFTRs), a total loss of coolant is equivalent to a total loss of the liquid core, fuel and blanket materials to the containment. Since the liquid fluoride operates at temperatures of 800 degrees Celsius, it is quite likely that UF4, ThF4 and fission by-products would react with other materials to cause a criticality event, major fires and/or explosions. I find it hard to believe that anyone would endorse building new reactors using such a chemically complex, potentially unsafe, environmentally hazardous, and unproven technology.
Keith Schwartztrauber
Las Vegas, NV
Underground molten salt reactor by Moir and Teller
Drs. Hargraves and Moir respond:
A criticality event would not occur during a total rupture of the reactor vessels because the materials would leave the compact geometry that permits criticality. Also the neutron fission cross sections will reduce as the materials leave the reactor and moderator, thereby hardening the neutron spectra. By design, the reactor room is steel-lined with strongly sloping floors leading to drains. Spilled molten salt would flow to holding tanks designed to treat such an event as “normal” rather than a big “accident.” The continuous chemical processing removes most fission products, especially the gaseous ones that would build up a pressure as they are created, reducing that hazard. The amount of fissile material within an LFTR is a fraction of that within today’s water-cooled power reactors or proposed liquid-metal-cooled fast breeder reactors. The LFTR needs only a low-pressure containment structure, perhaps below ground.
Many of the LFTR chemical processes were pioneered at Oak Ridge and Argonne national laboratories and are used in the aluminum and uranium fuel manufacturing industries, but not within today’s U.S. power reactors. Acceptance of LFTR, “the chemist’s reactor,” will require new skills within the NRC, the nuclear industry and the utilities. These LFTR safety features require validation that can only be achieved by a concerted research and development effort—estimated to cost less than a billion dollars excluding new reactor construction—to bring the technology to a level exceeding that already demonstrated at Oak Ridge.
LFTR discourages weapons proliferation
Saturday, October 2nd, 2010In response to the Liquid Fuel Nuclear Reactor article by Robert Hargraves and Ralph Moir, the September/October 2010 issue of American Scientist printed an exchange of letters initiated by Cameron Reed of Alma College.
U-233 is created in the Th-232 blanket
To the Editors:
Robert Hargraves and Ralph Moirs article Liquid Fluoride Thorium Reactors (July August 2010) presents a strong case for developing such plants. The relative simplicity of their construction and operation, their inherent safety and their lack of plutonium production are powerful advantages that should be carefully considered. However, their nonproliferation potential may not be quite as promising as they imply.
The authors, just as Mujid Kazimi did previously (see, Thorium Fuel for Nuclear Energy, American Scientist, Sept.Oct. 2003), suggest that U-232 in the U-233 produced by thorium reactors makes proliferation unlikely, due to the formers prolific and high-energy gamma-ray emissions. Neither article quantified how much U-232 is produced, making the claim difficult to judge. The first reaction in the production of U-232 has an extremely small cross-section for neutrons below about 6 mega-electron volts. As only a small fraction of neutrons generated in fissions are this energetic, the production rate of U-232 is very low.
U-233 is an excellent fuel for a fission weapon. It has a considerably smaller bare critical mass than U-235, about 15 kilograms versus 45 kilograms. This can be made significantly smaller perhaps halved by use of a lightweight beryllium tamper. Unlike the plutonium present in spent fuel, U-233 is immune to predetonation problems in even a crude gun-type bomb due to its low rate of spontaneous fission. It is a fairly copious alpha decayer, a property that can lead to premature detonation if the core is contaminated by light elements. But because the rate of alpha decay is only about one-sixth of that of Pu-239, this might not represent an insurmountable purification problem for would-be bombmakers. Perhaps liquid-fluoride thorium reactors could be engineered to enhance production of U-232 as a nonproliferation measure even if that produced a performance penalty.
Cameron Reed
Alma College
U-232 is produced in the fuel core
Drs. Hargraves and Moir respond:
A commercial reactor will make just enough uranium to sustain power generation. Diverting any would stop the reactor, alerting authorities to a breach. Certainly terrorists could not steal uranium-233 dissolved in a molten salt solution along with lethally radioactive fission products inside a sealed reactor. International Atomic Energy Agency (IAEA) safeguards would require security, accounting of all nuclear materials, surveillance and intrusive inspections. It is conceivable that a nation or revolutionary group might expel IAEA observers, stop a LFTR and attempt to remove U-233. Skilled engineers would need to modify the radioactive reactor’s fluorination equipment to separate uranium from the fuel salt. U-233 produced in a liquid-fluoride thorium reactor (LFTR) is a poor choice for nuclear weapons because the neutrons that produce U-233 also produces 0.13 percent contaminating U-232, whose decay products emit 2.6 mega-electron volt, penetrating gamma radiation. That would be hazardous to weapons builders and obvious to detection monitors. The U-232 decays via a cascade of elements to thallium, which emits the radiation. A year after U-233 separation, a weapons worker one meter from a subcritical 5-kilogram sphere would receive a radiation dose of 4,200 millirems per hour, compared to 0.3 millirems per hour from plutonium. Death becomes probable after 72 hours of exposure. After 10 years, this radiation triples. U-232 cannot be removed chemically. Centrifuge separation would make the equipment too radioactive to maintain. Conceivably, nuclear experts might try to devise chemistry to remove the intermediate elements of the U-232 decay chain before thallium is formed. But at-risk nations could be limited to using a LFTR variant with no chemical processing capability.
Deploying LFTRs will decrease, not increase risks of nuclear weapons proliferation. Kick-starting LFTRs with plutonium can consume existing stocks of that weapons-capable material. Using thorium fuel reduces the need for U-235 enrichment plants, which can make weapons material as well as power reactor fuel. This energy source is cheaper than coal, can increase prosperity and can reduce the potential for wars over resource competition.
US DOE nuclear power R&D nearing $0
Thursday, September 9th, 2010
US advanced nuclear fission R&D is starving
The US DOE invested $16 billion into R&D for the liquid sodium cooled fast breeder reactor. The work carried out by Argonne National Labs on this Integral Fast Reactor was cancelled during the Clinton administration, under the guise that IFR would increase nuclear weapons proliferation. That was the wrong reason. A good reason is that LFTR is a better technology.
The graph comes from a presentation by Von Hipple, who got the data from the IAEA, which obtained it from DOE. The data are adjusted to $2011 to compare to the DOE 2011 budget items.
$103 million is the largest budget item for advanced nuclear power, funding the prolonged Next Generation Nuclear Plant (NGNP) project. It is a high temperature gas reactor with TRISO fuel form (triple barrier fuel grains) in either pebbles or hexagonal compacts. The choice will be made as soon as the competition between Westinghouse (pebble bed) or General Atomics ends. One advantage of NGNP over LWRs is the high temperature for applications such as dissociating hydrogen from water and achieving higher burnup leading to less waste. Can NGNP scale up to meet the goals of Aim High?
There is still a $22 million budget item that is principally for continuing research in fast reactors, such as IFR.
Although there is $40 million allocated to Advanced Fuel Cycles, there is none for closed fuel cycles! And none for liquid fuels.
One more thing to note is that there is no budget item at all for GNEP, the Global Nuclear Energy Program. GNEP was a multinational program to guarantee fuel supplies to all nations, without the possibility of cutting off fuel as a sanction for bad behavior. This would discourage nations from building uranium enrichment facilities. [In the last few months Jordan, Vietnam, and South Korea have joined the list of countries planning to build centrifuge uranium enrichment plants.] GNEP also provided for the return of the spent fuel to the supplier nations, so as not to provide a reason to build fuel reprocessing facilities. [The US just OK'd India's plans to reprocess US-sourced fuel, since we can't take it back. South Korea says it will reprocess spent fuel because they have no more room for the waste.] The DOE GNEP budget was zeroed at the beginning of the current administration, and all mentions of GNEP were removed from the DOE web site.
Are we spending enough on R&D for advanced nuclear power, such as LFTR?
Blue Ribbon Commission learns about thorium
Tuesday, September 7th, 2010
On August 30 Robert Hargraves presented a ten-minute version of Aim High to the Reactor and Fuel Cycle Subcommittee of the President’s Blue Ribbon Commission on America’s Nuclear Future. All the presentations are posted here by the commission.
The commission will not recommend any specific technology such as LFTR, but this presentation might nudge them closer to recommending policy changes for NRC that would facilitate SMR (small and medium reactor) licensing, and also support technology neutral licensing, so that technologies differing from today’s standard light water reactors might be approved.
Here is the text of the presentation, one paragraph per slide.
Energy cheaper than from coal
Sunday, July 11th, 2010
When economic well-being measured by the gross domestic product exceeds a threshold, birthrate drops sharply.
Global warming now threatens irreversible climate damage, ending glacial water flows needed to sustain food production for hundreds of millions of people, and shrinking the polar cold water regions of the ocean where algae start the ocean food chain. Atmospheric CO2 dissolving into the ocean acidifies it, killing corals and stressing ocean life. Demand for biofuels increases destruction of CO2 absorbing forests and jungles.
Burning coal for power is the largest source of atmospheric CO2, which drives global warming. Airborne coal soot causes 24,000 annual deaths in the US and 400,000 in China. We seek alternatives such as burying CO2, or substituting wind, solar, and nuclear power.
The world population growing from 6.7 to 9 billion will increase resource competition, exacerbating environment stress. Yet the OECD nations, with adequate energy supplies, have birthrates lower than needed for population replacement. Nations with GDP per capita over $7,500 have sustainable birthrates. Electricity for water, sanitation, lighting, cooking, refrigeration, communications, health care, and industry contributes to economic development. Those nations with per capita electricity of 2,000 kWh/year (1/6 US use and an average power of 230 W) do achieve GDP of $7,500 per capita, which leads to sustainable birthrates.
Taxing carbon seeks to encourage energy sources that do not emit CO2, yet this has not been effective in Europe. Developing countries will not agree to carbon taxes and forgo an advantage they perceive led to prosperity in OECD nations. Alternatively, a source of energy cheaper than from coal would dissuade all nations from burning coal, without imposing tariffs or taxes that reduce economic productivity. Affordable electric power can also help developing nations reach modest levels of prosperity and lifestyles that include sustainable birthrates.
The objective for energy cheaper than from coal is $0.03/kWh and a capital cost of $2/watt of generating capacity. How can the liquid fluoride thorium reactor produce energy cheaper than from coal?
Fuel costs. Thorium fuel is plentiful and inexpensive; one ton worth $300,000 can power a 1,000 megawatt LFTR for a year – enough power for a city. Just 500 tons would supply all US electric energy for a year. The US government has 3,752 tons stored in the desert. US Geological Survey estimates reserves of 300,000 tons, and Thorium Energy claims 1.8 million tons of ore on 1,400 acres of Lemhi Pass, Idaho. Fuel costs for thorium would be $0.00004/kWh, compared to coal at $0.03/kWh.
Capital costs. The 2009 update of MIT’s Future of Nuclear Power shows new coal plants cost $2.30/watt and PWR nuclear plants cost of $4.00/watt. The median of five cost studies of molten salt reactors from 1962 to 2002 is $1.98/watt, in 2009 dollars. The following are fundamental reasons that LFTR plants will be less costly than coal or PWR plants.
Pressure. The LFTR operates at atmospheric pressure, without a massive reactor vessel pressurized to 160 atmospheres, and without a large containment dome needed to contain any accidentally released radioactive materials propelled by pressurized steam. One concept for the smaller LFTR containment structure is a concrete building below grade, with a concrete cap at grade level to resist aircraft impact.
Safety. PWRs are safe because of defense in depth – multiple, independent, redundant systems engineered to control faults. LFTR’s intrinsic safety keeps such costs low. A molten salt reactor can’t melt down because the core is already molten — its normal operating state. The salts are solid at room temperature, so if a reactor vessel, pump, or pipe ruptured the salts would spill out and solidify. There is no explosion potential because the pressure in the reactor is atmospheric. If the temperature of the salt rises too high, a solid plug of salt in a drain pipe melts and the fuel drains to a dump tank; the Oak Ridge researchers turned the reactor off this way on weekends.
Heat. The LFTR safely operates at high temperatures. Salt remains liquid below 1400°C; internal graphite core structures maintain integrity even above this. Molten salt heat capacity exceeds that of the water in PWRs or liquid sodium in LMFBRs, allowing more compact heat transfer loops. The molten salt heat exchange loop components of high-nickel metals such as Hastelloy-N are qualified up to 750°C.
Helium gas (green) is successively heated by 700°C molten salt (red) from a LFTR heat exchanger as it passes through high, medium, and low pressure turbines (T). The gas cycles back through three successive compressors (C), cooled by fluid (blue) that transfers rejected heat externally. The recuperator (R) transfers some energy from the compression cycle back to the expansion cycle. The generators (G) produce electricity. (Diagram courtesy of Per Peterson of UC Berkeley.)
Brayton Cycle. The triple reheat closed cycle Brayton turbine achieves a 45% efficiency of conversion from thermal to electric power, compared to 33% typical of existing nuclear and coal power plants using traditional Rankine steam cycles. The Brayton rejected heat to power ratio is thus 1.2 (55/45) rather than Rankine’s 2.0 (67/33) so the cooling requirements are nearly halved, reducing cooling tower costs and making air cooled LFTRs practical in arid regions where water is scarce. This compact Brayton turbine machinery is a quarter the mass, suggesting a similar cost reduction.
Boeing, producing one $200 million airplane per day, is a model for LFTR production.
Mass production. Commercialization of technology leads to lower costs as the number of units increase. Experience benefits arise from work specialization, new processes, product standardization, new technologies, and product redesign. Business economists observe that doubling the number of units produced reduces cost by a percentage termed the learning ratio, seen in the early aircraft industry to be 20%. Today Moore’s law in the computer industry illustrates a learning ratio of 50%. In The Economic Future of Nuclear Power University of Chicago economists estimate the learning ratio is 10% for nuclear power reactors. Boeing, producing one $200 million airplane per day, is a model for LFTR production. Reactors of 100 MW size costing $200 million can be factory produced. Manufacturing more, smaller reactors traverses the learning curve more rapidly. Producing one per day for 3 years creates 1095 production experiences, reducing costs 65%
Research. Cost reductions are presaged by current engineering research. Compact, thin-plate heat exchangers may reduce fluid inventories, size, and cost. Possible new materials include silicon impregnated carbon fiber with chemical vapor infiltrated carbon surfaces and higher temperature nickel alloys. Operating at 950°C can increase thermal/electrical conversion efficiency beyond 50%, and also improve water dissociation to create hydrogen for manufacture of synthetic fuels such as methanol or dimethyl ether that can substitute for gasoline or diesel oil, another use for LFTR technology.
In summary, LFTR capital cost targets of $2/watt are supported by simple fluid fuel handling, high thermal capacity heat exchange fluids, smaller components, low pressure core, high temperature Brayton gas turbine power conversion, simple intrinsic safety, factory production, the learning curve, and new technologies already under development. A levelized $2/watt capital cost contributes $0.02/kWh to the power cost. With plentiful, inexpensive thorium fuel, LFTR can generate electricity at <$0.03/kWh, underselling power generated by burning coal. Producing one LFTR of 100 MW size per day could phase out all coal burning power plants worldwide in 38 years, ending 10 billion tons of CO2 emissions from coal plants now supplying 1,400 GW of electric power. Low LFTR costs are vital to this coal replacement strategy, achievable if this goal is maintained during every design choice. Inexpensive electric power can also assist developing economies to improve prosperity, encouraging lifestyles with sustainable birthrates.
