Liquid Fuel Nuclear Reactors

The 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.

Figure 1. Solid fuel rods are stressed by fission products, radiation, and heat.
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.
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.)
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

Comments

comments


15 Replies to "Liquid Fuel Nuclear Reactors"

  • Bryan Elliott
    January 9, 2011 (11:19 am)
    Reply

    In terms of the lack of a need for a pressure vessel, it should be noted – since most people aren't aware of the properties of the fluoride salts – that even the highest operating temperature of the fuel salt is significantly lower than its boiling point.

    As such, there's never the risk of a boil-off (and the associated pressure spike) in a loss of coolant accident; instead, the freeze plug comes into play, and drains the core into a non-critical, passively cooled configuration.

    When we say, "passively cooled", by the way, we mean that the fuel drains into a matrix of piping that is designed to discourage fission and has enough surface area, thermal conductivity, and thermal mass to dissipate the core's heat and any decay product heat in a worst case scenario – but not so much that the core salt in a normal shutdown freezes to the piping before the piping is properly full.

    If I were to guess, milspec requirements for this drain target would say that the WCS is if the freeze plug has a significantly delayed melt (avg fuel temp ~ 1000C) while the core has triple the expected fission products from high-power operation.

    I'm a programmer, not an engineer, so I don't know how I'd calculate that – but I suspect it means a calandria (so as to spread out the overall initial heat dissipation) into a large, flat matrix of small diameter piping, normally held at vacuum, embedded in high density concrete.

  • sethdayal
    January 9, 2011 (12:12 pm)
    Reply

    $2B/Gw levelizes to 1 cents a kwh leading to 2.0 cents a kwh electricity when financed by world class public power like TVA or Hydro Quebec at current interest rates. It is only the incredibly inefficient corrupt American pirate power scam that requires the high finance rates.

    Westinghouse estimates predicts the factory module built cost of its AP1000 at $1B/Gw with 3 year build times. The current Chinese experience validates this estimate.

    Because of the simplicity of design David LeBlanc estimates DMSR cost at as little as 25% of LWR. That gives DMSR costs in the order or $250M/GW or roughly 1 cent a kwh.

  • Robert Hargraves
    January 9, 2011 (2:15 pm)
    Reply

    Sethdayal, here's my Excel formula for amortizing $2/watt over 40 year lifetime, 8% interest rate, producing power 90% of the time: =PMT(0.08/12,40*12,2/0.9)/0.72 = $0.021460. So I glibly say $2/watt is 2 cents/kWh, for the capital cost.

  • Andrew Jaremko
    January 9, 2011 (8:28 pm)
    Reply

    Robert, thanks for reposting the APS paper. I note that the authors still refer to 'waste' and a 'waste separator' appears in the block diagram. I'd like to see nuclear advocates start using the expression 'fission products' – or even the editorialized 'valuable fission products' – because the fission products are valuable. Their value will ultimately contribute to the profits of businesses smart enough to see them.

  • David L.
    January 10, 2011 (9:51 am)
    Reply

    Seth,

    You said:

    Because of the simplicity of design David LeBlanc estimates DMSR cost at as little as 25% of LWR. That gives DMSR costs in the order or $250M/GW or roughly 1 cent a kwh.

    I think you are mixing up what I may have said. It would be near impossible to get down to 25% of "cheap" LWR. I and others have mentioned 25% to 50% "lower" than LWR. As well, since there has been near zero funding for decades it is very hard for us to formalize our cost estimates. That will be one of the main priorities once modest funding does start to flow. Pretty much everywhere we look we do see potential savings though (size of heat exchangers, pumps, construction times, containment dome etc.).

    David LeBlanc

  • sethdayal
    January 10, 2011 (5:48 pm)
    Reply

    I was using last September's government bond rate of 3.5% for TVA public power. It's now up to 4.5% on 30 years.

    Using a 93% capacity factor on a 40 year 4.5% investment at $2 a watt comes to 1.15 cents a kwh for TVA or Hydro Quebec.

    With the typical discount rate (bond/equity combined) at a pirate utility that 4.5% becomes 12% and the $2 a watt levelizes to 2.6 cents a kwh.

    That $250K/MW is what comes from spewing numbers without thinking. I suppose it would be unreasonable to expect the ultra simple DMSR to be any cheaper under mass production than an CCGT plant at todays $1.2 B/Gw.

  • Thomas Jørgen
    January 17, 2011 (3:30 am)
    Reply

    well, a lot of the costs of a LWR are costs that apply to any thermal cycle plant – you need turbines, cooling, generators and transformers no matter where you get your heat from, after all.
    – The obvious make-or-break cost for this is the cost of the plant that extracts the fission products from the fuel stream – if this can be made cheap, all is well. if it is expensive, that becomes a really serious problem.

  • LarryD
    January 31, 2011 (8:43 pm)
    Reply

    Thomas, check the ORNL documents on this site (in the PDF document repository). The fuel reprocessing unit is a fractional distillation column, not expensive at all.

    The LFTR can run at ~800C, making the turbines more efficient (heat engine efficiency depends on the temperature difference between the hot and cool ends of the cycle).

  • Denton
    March 6, 2011 (2:09 am)
    Reply

    Comparing thorium/uranium 233 fuel to uranium 238/235 fuel leads me to three questions:

    1. Why is reactor fuel composed of 97% uranium 238? This isotope is not a fissile energy source. It is also not a useful fertile material which can be transmuted into a fissile energy source. Only a tiny amount (0.5%) is transmuted to plutonium 239 which is a fissile energy source. Uranium 238 while contributing nothing produces radioactive byproducts. It also multiplies the waste mass by a factor of a hundred while not itself being significantly radioactive. Why not eliminate the uranium 238 content and make fuel with only uranium 235? Is there a technical reason to not do this, or is it strictly a security issue of producing large quantities of weapons grade uranium 235?

    2. Many of the advantages of thorium as a fuel source come from its use in liquid form. Wouldn't uranium 235 have the same advantages if it were used in a molten salt form?

    3. Another advantage of thorium is that a reactor using it functions as a breeder reactor. Is there no practical breeder option using uranium 238 as the fertile material? Is this not done because it would require a fast neutron reactor which is difficult to stabilize?

  • Robert Hargraves
    March 7, 2011 (7:58 am)
    Reply

    Denton,

    LWR fuel is enriched from natural 0.7% U-235 to 3+%, the remainder being U-238. Enrichment is costly, so the LWR designers trade off enrichment against intervals between shutdowns for refueling. As enrichment gets less expensive (centrifuges & lasers) I expect enrichment levels will rise.

    Absorption of neutrons by U-238 creates Pu-239, which is significant; it provides 1/3 the power near the end of the fuel rod life. Neutron absorption also creates the other transuranic elements that contribute to the long period of radioactivity of spent fuel.

    The U-238 does not fission, so there are no waste fission products from it. Most of it remains U-238 in spent fuel.

    The maximum enrichment for domestic nuclear power fuel is limited by IAEC rules to 20%, so that the material is not suitable for explosive weapons. Nuclear submarines use fuel enriched to ~90% and can operate for decades with no refueling.

    U-235 in molten salt can make a fine reactor. This was demonstrated in the Oak Ridge ARE and MSRE experiments. It has the advantages of compactness, high temperature, online refueling, online fission product removal, etc.

    U-238/Pu-239 fast breeders existed in the US, eg EBR-II. A dozen or so such fast breeder reactors exist, using solid fuel. Russia is exporting its BN-800. A fast breeder has about ten times the fissile material of a thermal reactor. I'm not sure how a molten salt U-238/Pu-239 fast breeder could work; there are issues about Pu solubility.

  • Denton
    March 9, 2011 (2:44 am)
    Reply

    Thank you for the reply.

    Given the impressive list of LFTR advantages, I fail to see why venture capitalists aren't climbing over each other to build them. Has anyone with deep pockets been asked if they would invest? Where is the logjam preventing progress? Is it a financial problem of too slow of a return on too much capital investment? Is it a technological issue of not knowing how to build LFTRs? Is there a regulatory problem in getting NRC approval? Is the current nuclear industry blocking progress to protect its revenue streams? Is it fear of the public’s reaction to nuclear power? What needs to be targeted to move this forward?

    In advocating for LFTRs, you are trying to get the nuclear industry to change both its fuel source, and reactor design. Is it possible to change just the fuel source, and adapt it to existing reactors? Could fuel assemblies be redesigned for thorium 232/uranium 233? What is the primary problem in doing this?

  • Denton
    March 10, 2011 (6:43 am)
    Reply

    To try to answer part of my own question, in a LFTR thorium 232 transmutes in three steps to uranium 233 when it is hit by a fast neutron (still unclear as to whether this happens with a thermal neutron as well). Since commercial reactors are water moderated to produce thermal neutrons there would be no breeding of new uranium 233.

    Even if thorium did absorb a neutron and decay into protactinium, the protactinium must be removed from the reactor before it absorbs another neutron and transmutes into uranium 234.

    So thorium is not at all compatible with current reactors.

  • Robert Hargraves
    March 11, 2011 (10:42 am)
    Reply

    Denton, Lightbridge (nee Thorium Power) is testing solid fuel that incorporates thorium. Thorium can convert to Pa to U-233 in a LWR at thermal energies. One design uses Pu for the fissile material. The breeding ratios are < 1, so the thorium is sort of a fuel life extender.

    The LFTR is quite different, using liquid fuel.

  • Bryan Frazer
    March 29, 2011 (9:52 am)
    Reply

    Given the mess in Japan's nuclear industry and how it affects thinking of the other 400+ owners of current reactors this would seem to be the time to get on a thorium bandwagon. Does the arms industry still run the power show?

    Given the 1000 plus billionaires on the planet one would think their humanitarian ideals could kick in here.

    I am seeing that China, Russia and India will have thorium based reactors while the West slumbers in nuclear waste.

    Now is the time for all good people to come to the aid of the planet.

    How can regular folks invest in thorium based industries?

  • Bruce in Jacksonvill
    September 28, 2012 (2:45 am)
    Reply

    I see different calculations of costs and comparisons. Most appear to be very close. A 1100 MWe plant using four 225 MWe reactors and Brayton Cycle turbines including the one time fuel charge at current corporate bond rates for 30 years would cost $19.60 per MWh for capital expense. This is a very cost effective plant if the cost estimates are correct.


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