Modular Thorium Reactor

[@francistalbot6584]

I have watched it every day for two weeks and I am a nuclear engineer. This is the wave of the future for nuclear energy. This is my dream of clean Nuclear Energy. Let's build MSRs by the thousands. I am a licensed PE in Nuclear Engineering in the state of Maryland and I work for the US NRC. I demand you MSR technology experts bring this new information immediately to US DOE and US NRC immediately to discuss a path forward to bring the MSRs to licensing in the USA.

[@fshu1708]

Francis Talbot, Thank you for your lively interest and support. Thorium MSBRs provide realistic solutions for three of the major problems that face modern society: environmental degradation, energy poverty, and income inequality. You can help to get momentum behind their development and deployment by alerting your fellow nuclear professionals to the potential of these reactors to address the challenge of the 4 S's: Safety (against massive releases of radioactivity), Sustainability (of the fuel supply and the environmental footprint), Security (against weapons proliferation), and $uperiority (of the economics compared to fossil fuels). Only when the experts are united in these judgments, and the public is informed of the practical differences among different alternative energies, will politicians be emboldened to act on behalf of the greater societal good.

[@loktom4068]

Keep dreaming till year 2117.

[@TxFw]

Outstanding discussion thanks to Dr. Shu's thoughtful responses and kindness in taking the time to answer some great questions. I'm so glad I came across this video! I have been a supporter of Thorium MSBRs for a very long time now. Oh the opportunities! This stuff is so good I constantly lose sleep over it.

[@robertweekes5783]

+TxFw I'm glad I'm not the only one losing sleep (and literally dreaming) about thorium!! Check out this bumper sticker I designed - only $4.95 with free shipping - I don't make any money this is for the cause! Let's get a grassroots Thorium movement going - www.makestickers.com/customize.aspx?TID=10804&DN=160419050300&cid=mnsfxzgg2tm3shvlj4zuuico&Referrer=designlibrary.aspx

[@fshu1708]

+TxFw Thank you for your generous comment. The level of the questions and the enthusiasm of the visitors to this website have been a highly gratifying, hopeful, yet humbling experience. As Robert Weekes implies, we can all get more sleep if we work together to convince those with the resources and the power that the technology of thorium MSBRs, invented by ORNL five decades ago, is the practical key to making the transition away from a global economy built around the consumption of fossil fuels. Time is running out unless we take action.

[@TxFw]

+Robert Weekes I have a feeling people would freak out over a sticker showing the radioactive symbol and a traditional nuclear reactor. People need to understand Thorium is different. It's about as radioactive as a banana and I don't see people freaking out about bananas. Thorium will power the world, but the longer people hold on to fear and self-interest, the longer it will take for the movement to reach a critical mass.

[@michaelweaver2627]

The real problem is not engineering. I have faith in engineers when they are given a reasonable problem and enough resources they will accomplish the task eventually. The problem is you never hear any mention of LFTR from politicians who always talk about wind and solar. The only nuclear we can talk about is the nuclear that turns big profits on fuel manufacture and after Fukushima we can't really talk about it at all. The green parties hate nuclear and the establishment seems to have given up on it. From my limited technical understanding LFTR seems to be the way forward and offers huge safety and waste advantages over the current pressurized LW systems but it is not going to go forward unless someone high enough in a developed country's government champions this technology. Maybe the Chinese really are serious about this and they will lead the way but it would be nice if the west also woke up and moved forward.

[@fshu1708]

Michael Weaver, I do not disagree with your analysis, but what do we do about this situation? I hope you can agree to my response to Francis Talbot that somehow we must make our voices heard.

[@fshu1708]

iosef 333. When I said "this kind of reactor," I meant one that uses a thorium-containing blanket salt. If one made the blanket salt from UF4, with U being U-238, it would be possible, of course, to produce Pu-239. And one would not need a robotic hand because one can just pump out some of the blanket salt to extract the Pu-239 chemically. But if one wanted to make Pu-239, there are easier and cheaper designs to do it than with a two-fluid molten salt reactor (2F-MSR). The only reason to build 2F-MSBRs (B for "breeder") is because one wants to do thorium breeding, which has the cleanest fuel cycle possible of all reactor types, and which has the lowest inventory of fuel required to sustain a chain reaction conducted with a thermal neutron spectrum. In my opinion, only such reactors have a chance to replace fossil fuels on the time scale needed for climate change mitigation in this century.

[@fuggedupmess]

ok here is my question- HOW do you build a reactor for a single household? can you buy reactors or kits?

[@fshu1708]

Edward Versluys Thanks for your question. Nicholas Xu asked a similar question. Please refer to my answer to him, as well as the subsequent discussion with Sociedad Bienestar Mutuo. It's clear from the sincere curiosity of people on this forum that thorium molten salt reactors have caught the interest of the public. Let's hope for action from government.

[@SupernalOne]

two questions: 1) how is the heat transferred from the molten salt itself into water to make steam to turn turbines and generators? 2) Since the molten fuel must be constantly recirculated through the reactor at 1000 deg F , what kind of pump technology will work under those conditions?

[@fshu1708]

Mark Smed You are right that we need a heat exchanger to transfer the heat obtained from the fuel salt by the blanket salt to a non-radioactive process salt. The heat in the process salt can then (1) drive thermochemical reactions that have to work "uphill" because they are endothermic (absorb heat), or (2) expand a fluid that turns a turbine to generate AC electricity. The fluid in (2) is traditionally water boiling into steam (Rankine cycle), but a better method expands supercirtical carbon dioxide (sCO2) that has a density about a tenth of liquid water and that can still expand like a gas to drive a sCO2 turbine and generate electricity (Brayton cycle). Because it is less corrosive on metal alloys, sCO2 is preferred to supercritical water as a working fluid for turbines. In compactness of equipment and operating temperatures, Brayton cycle systems match well to molten salts. Thus, 2F-MSBRs can generate AC electricity in application (2) more cheaply than turbines powered by the combustion of natural gas. As far as application (1) goes, we can use cheap and clean nuclear heat to replace the current burning of coal or natural gas in many industrial processes that require a reliable source of heat.

[@BienestarMutuo]

That is right !

[@TREDxMUSIC]

As for pumps, I believe it is possible to use normal electromechnical pumps, but corrosion is more of an issue than temperature, so electromagnetic pumps are being looked at.

[@garyk3478]

I take it that separation of U-233 and any purification of the molten salt coolant would take place outside the module shown. If a significant amount of either fluid would to leak and solidify outside the reactor module, would clean-up be possible? Or would the affected area be entombed in concrete? Thank you!

[@RawScience]

+Darth Wedgius Dr. Frank Shu's reply response to Darth Wedgius: Darth Wedgius raises three important issues that we address below in reverse order that puts safety first. Issue 3. MSBRS never need to be entombed in concrete (as happened at Chernobyl) because they have a unique safety feature: freeze valves below the bowl of the fuel pumps where the fuel salt is kept frozen by active refrigeration. If electricity to the valve should stop for more than a minute or so for whatever reason, the decay heat of fission products in the fuel salt is enough to melt the freeze plug. The fuel salt in the pump bowl and the rest of the reactor will then drain by gravity to four uninsulated, spherical, dump tanks that contain powdered buffer salt. (Spheres are the strongest mechanical structures for metal of a given thickness.) Because the dump tanks contain no moderators, the mixed salt is well below critical mass in each tank, and the only heat source comes from the radioactivity of the fission products in the dumped fuel salt. This decay heat has been reduced from normal values by the continuous cleaning of the fuel salt during plant operations (see below). The decay heat will try to increase the temperature T of the metal surface of the dump tank and its contents. But because blackbody radiation from the dump tank rises as T4 times the surface area, the decay heat of a properly designed dump tank cannot raise T above the value, about 500 Celsius that would significantly weaken the reinforced concrete in the room holding a dump tank. After a day or so, the decay heat becomes too weak to keep the salt molten against the surface radiation losses, and the salt in the tank will freeze, immobilizing it as a threat to the rest of the plant. When the emergency has passed, covering the dump tanks with opaque insulation cuts the surface losses, and the decay heat can raise the interior to a high enough temperature so that the mixed salt again becomes molten. It can then be drained to hot cells established to clean and restore molten fuel salts as part of normal operations. Even after a severe natural catastrophe such as the one that felled Fukushima, in a week or two, a two-fluid MSBR plant can be restored to working condition. In the most extreme event that one can imagine, some overwhelming terrorist force splits open a dump tank made of metal that is 4 inches thick while the mixed salt (fuel plus buffer) inside is still molten. However, the volume of mixed salt is large, and when it spills its contents onto the steel-plated floor of the dump-tank room, the spilled salt will have a surface area considerably larger than that of the original sphere (which has the smallest area for a given volume of any geometrical shape). Therefore, the steel floor will cool the split salt (transparent to visible and infrared radiation) by blackbody radiation even faster than the dump tank did when it was intact. Although the concrete floor beneath the steel plating can suffer damage, the mixed salt will freeze before it can escape the domed reactor building. Such an event may destroy the plant, but no large-scale release of radioactivity will occur to the neighboring environment. Issue 2: If blanket salt were to leak from the pool that is heated by the fuel salt in its channels, the blanket salt would freeze and seal the leak because it lacks sufficient decay heat to keep the salt molten. If the fuel salt channels were to spring a leak, the blanket salt, which is kept at a higher pressure because of its greater density and the way we pump the two salts, would push into the fuel salt channels and displace some fuel salt out of the core. The core would then lack a critical mass, and the fission reactions would automatically come to a halt. Mixing of blanket salt with fuel salt would complicate clean-up because thorium behaves chemically similarly to lanthanides, which are a large fraction of the fission products in the fuel salt. (Chemical processing is much simpler in a two-fluid MSBR, where the thorium and uranium are kept separate from one another, than a one-fluid MSBR, where the thorium and uranium are mixed from the start; see below.) After replacement of the defective core with a new module, the plant can still run in a hybrid mode (intermediate between 1 & 2 fluids), but breeding may be impaired until all the thorium in the fuel salt has been converted to U-233. Issue 1a: Yes, the separation of U-233, in the form of UF4, bred from Th-232 in the form of ThF4 in the pool of blanket salt, occurs outside of the module. A quantity of blanket salt (a few hundred liters per day in a typical plant) is extracted from the pool and fluorinated by sparging fluorine gas F2 through the molten salt. The fluorination by F2 converts liquid UF4 to gaseous UF6, while leaving the thorium as ThF4 because thorium does not have any valence state other than +4. The gaseous UF6 bubbles out of the isolated blanket salt (which goes back to the pool) and passes through a cooled powder of NaF-BeF2. The NaF-BeF2 powder adsorbs the gaseous UF6. Sparging H2 gas into the powder then reduces UF6 back to UF4 via the release of 2HF gas, i.e., the mixture becomes the NaF-BeF2-UF4 that is the fuel salt. Electrolysis splits the reaction product 2HF into the F2 and H2 needed to process the next batch of blanket salt. A part of the recovered NaF-BeF2-UF4 is added to make up the amount burnt in the core, with the excess transported as fuel for a growing fleet of thorium MSBRs. Issue 1b: Cleaning the fuel salt of its fission products begins also by fluorinating it (several liters per day in a typical plant) to extract the liquid UF4 as gaseous UF6. But now, volatile fission products, made gaseous by fluoride volatility, accompany the gaseous UF6. Passing the gaseous stream through powdered NaF-BeF2 adsorbs out the UF6, while the remaining stream of volatile fission-product fluorides are condensed elsewhere for eventual treatment and disposal. The fluorinated fuel salt left behind now contains only NaF-BeF2 and dissolved non-volatile fission products. If one pumps down the vapor pressure of this mixture, while the fission products continue to add decay heat, the NaF-BeF2 will boil and evaporate away (to be recondensed elsewhere), leaving behind a much smaller volume of non-volatile fission products to be disposed together with their volatile counterparts. Thus, MSBRs handle all nuclear “waste” on site for burial for 300 yr or for radioisotope reuse. In reality, the evaporated and recondensed NaF-BeF2 will contain small amounts of radioactive and stable CsF and SrF2, which behave chemically similarly to NaF and BeF2. Fortunately, none of the isotopes of Cs or Sr have large capture-cross sections for neutrons, so when we add back the separated UF6, converted to UF4 by reduction with H2, to the NaF-BeF2, we recover fuel salt that is contaminated with minor amounts of CsF and SrF2, too small to have much of an adverse effect on the breeding ratio. We supplement any missing UF4 because of prior nuclear fission by the amount bred in the pool mentioned in Issue 1a.

[@garyk3478]

***** Thank you for the reply! So a leak of the fuel salt or blanket salt during a chemical processing stage could require permanent shutdown of the plant without a clean-up to restore operation being a viable option, but would not escape to the greater environment. Would that be accurate?

[@RawScience]

Dr. Shu's response: 2nd Response to Darth Wedgius Small spills during chemical processing of blanket salt and fuel salt are never a threat to the power plant because the hot cells do not have to be located close to the reactor. The amounts being handled in any hot cell at a given time are parceled into small enough pieces so the decay heat cannot compromise the physical structure of a hot cell. Human are protected from the radioactivity by heavily shielded containment boxes from which they can manipulate materials and tools remotely.

[@garyk3478]

***** So a processing call can be compromised without losing the plant. Thank you for the reply!

[@threejaguar]

Processing can be done while the plant is running. Xe gas just bubbles out of the fuel, and can be recovered and sold, so that major solid fuel headache goes away. Chemically processing FLiBe on the fly is a damned sight easier than processing solid fuel rods. You get all of the ash products out of the FLibe, and valuable ash products, like rare earth and Paladium can be easily separated, instead of dumping them all into Hanford-style bubbling pools of strontium-90 heated boiling waste.

[@robertweekes5783]

Interesting concept, could the graphite house the molten material all by itself? Can it withstand heat & corrosion?

[@RawScience]

Your question will be forwarded and answered by Dr. Shu.

[@RawScience]

Per Dr. Shu: Good questions! It has been known since the early work of Oak Ridge National Lab that graphite, which ORNL used as the moderator in the MSRE, is resistant to corrosion by molten fluoride salts. As long as there is no oxygen or steam in the system, nuclear grade-graphite can withstand temperatures in excess of 2800 Celsius, which is the temperature to which carbon is brought in order to make it graphitic. In our design we also use graphite as a container to house the fuel and blanket fluoride salts that flow through a core module in two sets of channels. To prevent the fuel and blanket salts from mixing, it is necessary to seal the graphite of its pores, which we know how to do. The "plumbing difficulty" that prevented ORNL from realizing a workable two-fluid design is solved by making the blanket salt carrying thorium circulate through a big pool to which the blanket salt channels are open.

[@AndrewDodson_AdamsAtoms]

Graphite will probably not make a good material considering that it becomes a large block of nuclear waste after a few years of irradiation within a reactor. It does not conduct heat as well as a metal, and also has issues with shrinking and swelling under neutron flux. It would be ok for a prototype though!

[@RawScience]

Andrew, here is the response from Dr. Shu: The issues raised by Andrew Dodson are complex, and do not have simple answers. It is true that graphite suffers from dimensional changes by fast-neutron bombardment, but similar damage affects all solids, including all metal alloys. Lowering the rate of damage depends on getting the neutron energy spectrum as soft as possible, i.e., by having more graphite with which fast fission neutrons collide. In our modular design for a two-fluid molten-salt breeder reactor (MSBR), this amelioration occurs by having more non-structural graphite between core modules than exists in the parts that have structural purpose, which are much more expensive to fabricate. In this way, it should be possible to extend the service life of the more expensive graphite from a few years to a few decades. Nevertheless, Dodson is correct that sooner or later the graphite components in a MSR core, just as used fuel rods in a LWR core, become high-level nuclear waste that must be disposed. But the carbon of graphite, unlike the metal of fuel rods, can be ground up and oxidized under a different kind of molten salt to become carbon dioxide that is only very slightly radioactive. This greatly reduces the volume of high-level nuclear waste (fission products) that one has to deal with. These fission products embedded in the graphitic components of the reactor are no different in quality, and much smaller in quantity, than those that must be cleaned periodically from the fuel salt if one is to have greater than unity breeding. Thus, the infrastructure that must be established for any fleet of operational modular thorium power-reactors could be used to dispose of both sources of high-level nuclear waste, which are orders of magnitude smaller in volume and duration of dangerous radioactivity for MSRs than LWRs. Finally, while it is true that graphite does not conduct as well as liquid sodium (used in fast reactors that breed for plutonium), reactor-grade graphite is much more thermally conductive than most metals (not counting aluminum, copper, silver, or gold, which are not used in nuclear reactors). Thus, we propose to use graphite even in our commercial MSBR design, and not just in a test prototype, for three functions: (1) structural separation of flowing fuel and blanket salts, (2) heat transfer from fuel salt to blanket salt, and (3) moderation of fast fission neutrons without absorbing them. (The absorption of neutrons by most metals requires keeping metal parts, even nuts and bolts, in the core to a minimum.)

[@AndrewDodson_AdamsAtoms]

Is it possible to recycle the graphite into new moderator? I have wondered this for a while, and Thorcon makes the claim that this is easily done. www.c4tx.org/thorcon/pub/exec_summary.pdf I am very honored by your attention to my comments. I wish you much success. If you ever need an electrical engineer, I have been in thorium molten salt reactor groups for the past 2 years.

[@demering]

This video:Thorium Reactor Me who plays alot of mindustry: *Kaboom?*

[@tempoletiongaming]

You are not alone dude

[@RavenClan148]

I'm researching thorium reactors for a paper. Is there a major disadvantage of thorium? The materials I have found state the conversion of power plants to thorium would be too expensive and it still produces radioactive waste (at a lesser scale). But that doesn't seem like a good enough answer compared to the benefits. Is there something I'm missing? Thank you in advance.

[@RawScience]

Response to Turner Davis: You are really raising two issues here: (1) burners versus breeders, and (2) thorium versus uranium. On issue 1, these are the facts. Let us abbreviate million metric ton = MMT and year = yr. According to the Nuclear Energy Association, the proven reserves of uranium in the world is 5.5 MMT. The once-through rate usage of the fleet of present-day LWRs, which are burners, that supply 4.8% of the world’s primary energy supply is 0.070 MMT/yr. Simple division then yields that the proven supplies of uranium can last 5.5/0.070 = 79 yr if nuclear remains to hold its present percentage of primary energy use. Climate change would then overwhelm us in a few decades. On the other hand, if we build enough LWRs to supply 100% of the world’s primary energy use, the uranium would last only (4.8%/100%)79 yr = 3.9 yr. Clearly, therefore, burners are unsustainable if nuclear energy is to supply anything close to 100% of the world’s primary energy use. We must go to breeders that consume nearly 100% of the natural fuel rather than 1%. If uranium is that fuel, and the reactors are fast uranium/plutonium breeders, proven reserves could last (100%/1%) 3.9 yr = 390 yr, which would give us enough time to develop fusion as the ultimate energy resource. On the other hand, if we were to go to the thorium fuel cycle, the proven reserves are 6.4 MMT according to the World Nuclear Association. Theoretically, there should be 4 times as much Th as U, but to be conservative, let us stick to proven reserves. In that case, thorium breeder reactors would supply (6.4 MMT/5.5 MMT) 390 yr = 440 yr of today’s primary energy use, not much different from 390 yr. In either case, one needs to convert a 4.8% fleet of present-day LWRs to 100% fleet of breeder reactors. So you are right that the argument about conversion expense is specious. In either case, most of the expense, necessary to make nuclear energy sustainable over the long term, is the difference in cost between a large fleet of breeders and a small fleet of burners, not on whether we choose to use uranium/plutonium cycle versus the thorium cycle. The merits of the latter case should be argued on whether thorium (thermal spectrum) breeders are safer, superior (in cost), and securer (in weapons proliferation) than uranium/plutonium (fast spectrum) breeders. That argument requires another post.

[@noahv7288]

+Sunlight Paladin protactinium 233 is the problem. Its a intermediate stage to breeding thorium 232 into uranium 233 it lasts 27 days and has to be filtered from the fissionable material because it will gobble up neutrons and not fission. There are modern chemical filter processes to remove and separate the stuff but back when they started research on thorium and molten salt reactors it was an insurmountable problem.

[@RawScience]

Dr. Frank Shu's respones to Noah V: Noah V raises a valuable issue in regard to the questions raised by Sunlight Paladin. However, noah v's comment applies to the ORNL two-fluid MSBR design and not to the one for which Frank Shu received patents in the US, China, Japan, and Russia (so far). In a thorium reactor, neutrons leaving the core irradiating a blanket containing Th-232 create U-233. The desired nuclear transformations are Th-232 + n --> Th-233 + gamma --> Pa-233 + electron + antineutrino (beta decay) --> U-233 + electron + antineutrino (another beta decay). The first beta decay occurs quickly and presents no problems. The second beta decay has a half-life of 27 days and is problematical because Pa-233 has a relatively large neutron-capture cross-section via Pa-233 + n --> Pa-234 + gamma, with the Pa-234 ultimately decaying to U-234 that is not a fissile. In a power reactor that has a high flux of neutrons, Fermi pointed out that the latter transformation (called r-process in astrophysics), which absorbs a neutron without producing a fissile, threatens greater than unity breeding by the beta-decay transformation of Pa-233 to U-233 (called s-process in astrophysics). Wigner responded to Fermi’s criticism of a thorium fuel cycle in thermal-spectrum power reactors (where neutron capture cross sections are especially large) with the idea of a two-fluid reactor where the fuel elements take a liquid form (so one can clean the fuel of fission products that also threaten > 1 breeding), as well as a thorium blanket that is a liquid, so that one could quickly remove the Pa-233 chemically from the reactor (say, on a 1 week or shorter time scale). Removing Pa-233 from the reactor allows it leisurely to decay into U-233 without the danger of capturing another neutron. Wigner’s original suggestion was to use water slurries, but ORNL under the direction of Alvin Weinberg, improved the idea by making both fluids into molten fluoride salts. As discussed in the response to jarmo vitalo, the ORNL strategy met with two further criticisms: (1) disbelief that existing chemical extraction techniques could treat the required volumes so quickly (tens of thousands of liters of blanket salt per week), and later (2) belief that producing U-233 in a nearly pure form might allow illegal weapons production (a slight contamination of U-233 by U-234 would not mar weaponization). Although noah v’s suggestion of using modern extraction techniques may or may not answer the first objection, it does not address the second. Adoption of a large pool that holds the blanket salt gives a simple and effective solution to both problems because effective irradiation of Pa-233 by neutrons occurs only in a thin shell surrounding the core. In most of the convectively overturning pool (about 8x larger than the effective irradiating shell in our proposed design), the Pa-233 is shielded by the thorium in the blanket. This design effectively removes most of the Pa-233 from neutron irradiation, allowing the Pa-233 to decay into U-233 without chemical or physical extraction from the blanket, which is now the whole pool. By not having so much graphite moderation in the core that the neutron spectrum becomes highly thermal, we can automatically prevent weapons proliferation. An epithermal spectrum that contains an appreciable flux of neutrons with energy greater than 6.8 MeV can have one fast neutron come into a Th-232 nucleus and knock two neutrons out via Th-232 + n --> Th-231 + 2n. The Th-231 will quickly decay to Pa-231, which has a very long half-life of 32,800 years (by alpha decay). Thus, the fate of Pa-231 in a reactor environment will inevitably be to capture a neutron to become Pa-232 via Pa-231 + n --> Pa-232 + gamma. The half-life of Pa-232 against beta decay is short enough (1.3 day) and its cross-section against neutron capture is small enough so that its most likely fate is the s-process beta-decay: Pa-232 --> U-232 + electron + antineutrino. In its radioactive decay chain, the U-232 produces long-lasting energetic gamma-ray emission that provides a powerful deterrent against weapons production. Notice that the initial target for producing both U-233 and U-232 is Th-232 in the blanket salt. Thus, in a properly designed two-fluid thorium MSBR, there is an unavoidable rate at which U-232 is produced relative to U-233, given by the ratio of the cross-sections of the (n, 2n) and (n, gamma) reactions on Th-232 (when averaged over the neutron energy distribution in the reactor). For our designed test reactor, we obtain an abundance of U-232/U-233 of about 6E-4, which is enough in 1.5 hours to kill any worker trying surreptitiously to assemble a bomb. Even if enough martyrs (about a thousand) could be found to attempt such a task, any timing electronics for the bomb would be destroyed by the gamma rays, and the escaping gamma rays would also be easily detectable even when transported in large shipping containers. No rogue group would attempt to build a U-233 weapon in a fashion where no nuclear-capable nation has ever succeeded.

[@puncheex2]

On your waste problem: Uranium solid fuel waste contains raw uranium-238 (up to 95% of the mass) which acted as an inert filler in the fuel, only present because of the difficulty in separating it from U-235, trans-actinides created by uranium catching a neutron and NOT fissioning (U-238 isn't very fissionable at thermal energies, so contributes most to this; this leads to breeding plutonium-239, but with other heavy radioactive nuclei as well), and fission products, light nuclei of about 50 different species that results from splitting the fissionables. The fuel is spent because most of the fissionable U-235 has been converted into FPs, many of which absorb neutrons more or less readily and thus poison the reaction. Plus some zirconium, which is the rod cladding because it doesn't absorb neutrons, and thus is also generally inert in the process. The FPs are by far the most dangerous fraction: most of them beta/gamma decay with rather short half-lives and thus high intensity. The worst of the lot overall is cs-137, with a 30 year half-life, and amounting to about 5% of all FPs by mass. Actinides follow in second place mainly because they are relatively rare as compared to the FPs, but their long half-lives mean they linger on at low intensity. The uranium-238 is hardly worth worrying about, as long as it is physically under control. With the LFTR, the liquid fuel is cycled through a chemical apparatus which adds more fuel (U-233 salt which was cooked from thorium salts in the blanket) and also extracts the palladium-233 (one of the trans-actinides, see above) and the fission products. These FPs don't have the trans-actinides or the uranium in them, and 95% of them have half-lives in the 30 year range, so if left to decay will do so to almost no radioactivity in 300 years, and their total mass is about 2% the mass of current spent fuel. The actinides are recycled, to eventually be fissioned as fuel when they catch enough neutrons to become fissionable. You can't do that in a solid fuel reactor; the actinides have to be removed, willy-nilly, when the FPs become too high. Extracting the FPs also makes the fuel much less able to cause problems with decay eat, which is what destroyed Fukushima. So, you ave a reactor that produces only 2% the mass of waste, which decays in 300 years rather than 10s of thousands. As compared to current solid fuel reactors, much less to coal power-plant waste, you could make the barrels out of gold and still save money on the storage, not to speak of the environment.

[@fshu1708]

puncheex2, thanks again for additional expert comments. I would add only three remarks. First, in our design (which has now received patents in the USA, China, Japan, Russia, Germany, France, England, and Sweden), we put the blanket salt that contains the ThF4 breedstock into a big pool that obviates the need to chemically extract Pa-233 (protactinium-233). This eliminates the proliferation concern that such extraction might produce weapons-grade U-233. Second, you are absolutely right about the importance of extracting fission products from the fuel salt. Above-unity breeding with a thermal or epithermal neutron spectrum depends critically on this extraction. Extraction of fission products is, in my mind, the best justification for making the fuel salt a liquid in the first place. As you point out, online extraction of as many fission products as possible (not just Xe-135) also gives the side benefit of easier handing of decay heat should there be a nuclear accident. Thus, designs of molten salt reactors that do not have the extraction function, of which there are now quite a few, are missing the two strongest reasons for making the fuel have a mobile state in which there has already been a "meltdown." Third, once the fission products are extracted, which have a much smaller volume (and mass) than the "spent fuel" of LWRs that is mostly untransformed U-238, I personally believe that one should move to a system of active management of such fission products, rather than their geological burial. As long as we advocate the need to spend money and effort to sequester nuclear "waste," there will be critics of the practice, or, worse, practitioners who do not undertake the job in a responsible manner. To promote a rational approach, we must ultimately make it worthwhile economically for people to turn such "wastes" into resources. When radioactivity was first discovered by pioneers of impeccable social conscience, such as Lord Rutherford and Madame Curie, it was regarded, correctly in my estimation, as a great boon to the advancement of human knowledge. Of course, now that we know also of its dangers, such powerful substances should be treated with proper respect and precaution. But given the scientific, medical, and technological benefits possible from such substances, we should not let fear-mongers drive the agenda concerning the management of such nuclear resources.

[@LeviFullerThe]

So it's just insanely expensive to transform thorium into a usable fuel, correct? And also difficult to store the energy?

[@RawScience]

Levi Fuller , here is Dr. Shu's thoughtful response to your question: Answer to Levi Fuller vi Dr. Frank Shu: : The bottom line answer to your question, is: No, turning thorium into a useful fuel in MSBRs is not insanely expensive. Done well, it is a business opportunity to produce cheap energy that answers three questions often directed against nuclear power: Q1: What to do with LWR spent fuel? A1: Extract the Pu-239 and burn it in molten salt reactors, with the fission products much easier to dispose than the plutonium Q2: How to prevent the nuclear proliferation possible with ever larger stockpiles of Pu-239 building up in the world? A2: Stop producing Pu-239 that is relatively easy to separate chemically from U-238 (the major component of LWR spent fuel). Start producing U-233 that cannot be easily separated from even-neutron forms of uranium, especially the U-232 that is automatically created in well-designed MSBRs that makes weaponization practically impossible. Q3: How to provide sustainability when the world reserves of U-235, the preferred fuel for LWRs, is so limited? A3: Transform to using the Th-232/U-233 fuel cycle, where there is enough proven reserves of Th-232 to power the world’s primary energy needs for many centuries until fusion becomes the virtually inexhaustible energy choice of the long-term future. (See the discussion following Turner Davis’s question.) Below is the scientific reasoning behind the answers provided above. Thorium is a waste product of the refining of rare earths; therefore, by itself, thorium, which has few industrial applications, is very cheap. Thorium also has only one naturally occurring isotope, Th-232, so there are no expenses associated with isotope separation. To convert Th-232 to a useful fuel involves simply exposing it to a flux of neutrons from a nuclear reactor, with breeder reactors being those that have the largest excess of neutrons after accounting for the neutrons needed to sustain the fission chain reaction. When Th-232 captures a neutron, it becomes Th-233, which is unstable via two beta decays to become U-233, which is a fissile form of uranium (even number of protons and odd number of neutrons). U-233 is the preferred fuel used in molten salt reactors. Molten salt breeder reactors (MSBRs) are those where there is enough U-233 created from the Th-232 not only to resupply the U-233 burned in the reactor under discussion, but there is some left over to start new MSBRs. Since U-233 is radioactive, but decays slowly with a half-life of 159,000 years, there is no difficulty in storing and shipping it on the time scales that humans need to use the thorium fuel cycle to halt climate change. Note that Th-232 is only the breedstock (a fertile not a fissile) for making the real fuel, which is U-233. One needs neutrons to make U-233 from Th-233, but if U-233 is the fuel source that produces neutrons, and there is no naturally occurring U-233, how does one get the “nuclear fire” started? The answer is that one needs “kindling,” for which there are two possible candidates: (1) U-235 which is the only fissile (but rare) form of uranium that is naturally occurring, and (2) Pu-239 that is ubiquitously present in light water reactor (LWR) spent fuel and is normally considered the difficult part of LWR nuclear “waste.” In fact, the Pu-239 in LWR spent fuel is not a waste, but a resource. If one is allowed to extract it from LWR spent fuel (in a manner that does not separate it from other actinides), then it can be used as “kindling” in a molten salt reactor (the same one that we will later use as MSBRs that run on the thorium cycle) to drive a fission chain reaction where the excess neutrons from the fission of Pu-239 are used to irradiate a blanket salt that contains Th-232 to make the desired initial supply of U-233. Fortuitously, the Pu-239 stockpiled worldwide as stored spent fuel from a half-century of LWR usage is enough to start up a fleet of thorium MSBRs that can renewably supply the primary energy needs of the entire world by 2050 when one accounts for the extra breeding possible with MSBRs. Since many countries that have stored LWR spent fuel would pay one to take the spent fuel off their hands, obtaining “kindling” in the form of high-level nuclear waste would be a source of revenue and not an expense (if one does not wantonly burn up this “waste” without properly using the excess neutrons that are in Pu-239 to make the much cleaner fuel U-233).

[@236Mars]

Great video. Great idea. An evolutionary and revolutionary idea it is. Thank you Dr. Frank Shu. Has a live demonstration of this process been conducted so far?

[@fshu1708]

Joel Mwaura Thank you for your comment. The non-proliferation treaty (NPT) prevents Taiwan from chemically processing nuclear materials. Although our group can and do carry out feasibility demonstrations of many of the principles behind two-fluid molten-salt breeder reactors, we avoid experiments that would violate the legal restrictions of the NPT. Thus, we cannot build a full working prototype in Taiwan, but as of December 1, 2015, I have resigned my position at Academia Sinica in Taiwan to form a company in the United States where we will pursue the idea of a test reactor.

[@powerofanime1]

Hello Dr. Shu, have you gotten in contact with Kirk Sorenson? He's had much the same idea and I think you two working together could be even more effective in the pursuit of MSBR and LFTR viability.

[@Bobby-fj8mk]

Great video & answers to questions. How does the cleaning process work? What stops the delicate graphite reactor from crumbling to pieces?

[@fshu1708]

Sir Bobby, excellent questions to which my answers are given in detail below. Cleaning the salt: To complete the breeding cycle, there are two salts that require cleaning. The fuel salt requires the removal of fission products that would absorb neutrons needed to maintain the chain reaction, with an excess of neutrons from each fission of U-233 that goes toward the conversion of Th-232 into Th-233. The last species becomes U-233 after two beta-decays. The most important fission product to remove is Xe-135, which has a huge cross section (2 million times greater than normal) for capturing neutrons. The isotopes of Kr are also abundantly represented in fission products and are important to remove for the same reason. Fortunately, both are noble gases that will bubble out of the liquid salt if we sparge a carrier noble gas like helium into the pump bowl. Next, the fissile U-233 needs to be reclaimed before further cleaning of the fuel salt. Sparging fluorine gas, F2, into small samples of fuel salt will convert UF4 into UF6. While UF4 is a liquid at temperatures characteristic of the fuel salt, UF6 is a gas and will, if encouraged, bubble out of the molten salt. This step also removes compounds of volatile fission products like dangerous I-131 with an 8-hr half-life . To separate the UF6 from the other volatile species, the vapor is passed through a powder of NaF-BeF2, which will adsorb the UF6 while letting the other gases through to be bottled and stored as radioactive nuclear waste. By then passing hydrogen gas, H2, though the powder, the trapped UF6 is converted back to UF4 with the release of 2 molecules of HF gas for each molecule of UF4 or UF6. The NaF-BeF2-UF4 can be put back into the reactor core as clean fuel salt, while the 2HF can be converted by electrolysis to F2 + H2. The recycled F2 can be used to convert a new batch of UF4 to UF6 that is trapped in fresh NaF-BeF2, while the recycled H2 can be used to convert the UF6 trapped in NaF-eF2-UF6 into a new batch of cleaned fuel salt, NaF-BeF2-UF4. Where does the fresh NaF-BeF2 come from? The small samples of fuel salt into which we sparge fluorine gas, F2, has not only NaF-BeF2, but also non-volatile fission products, some of which are extremely radioactive. The high radioactivity will steadily try to increase the temperature of the fuel salt. If we pump down the pressure above the contaminated fuel salt, we can vacuum-distill the fuel salt so that NaF-Be2 boils away as a gas once the temperature gets above 1000 C. This vacuum distillation produces gaseous NaF-BeF2, which will become a liquid and then a solid as it cools down. The pure solid NaF-BeF2 is what is made into a powder to capture the UF6 in the previous paragraph. The non-volatile fission products left behind need to be divided into very small parcels, so that their large surface-to-volume ratios allow them to be cooled, and then converted from fluoride forms into safer oxide or silicate forms. We put the parcels of solid oxide or silicate fission products into cold storage for up to five years, before fusing them with non-radioactive glasses to be buried for, say, three hundred years until the radioactivity decays to safe background levels. Blanket salt When we write UF4 or UF6 in the above, the U we are assuming is mostly U-233 (with a little accompanying U-232). Natural uranium is mostly U-238 with a little U-235 mixed in. Where do we get the U-233? The answer is from the blanket salt, which is either NaF-BeF2-ThF4 or, more simply, NaF-ThF4, with natural thorium being almost pure Th-232. When Th-232 is irradiated by neutrons generated in the reactor core in excess of what is needed to maintain the chain reaction, the Th-232 can capture a neutron and become Th-233. After two beta decays, the Th-233 turns into U-233. In the blanket salt, the U-233 is in the form of UF4. To separate the UF4 from the rest of the blanket salt, we remove a small batch of the blanket salt from the pool for off-line processing. The processing consists of sparging F2 into the small sample, whiich converts the UF4 into UF6 that bubbles out of the blanket salt. Unlike uranium, thorium does not have any valence state higher than +4, so the thorium stays as ThF4 and remains as a liquid in the cleaned blanket, salt which can be put back into the pool. As before, a powder of NaF-BeF2 can capture the UF6, with the combination turned into fuel salt by the methods described earlier. Indeed, to prevent the cleaned fuel salt from dropping in U-233 concentration, we should add the UF6 extracted from the blanket salt as a supplement to the UF6 that comes from cleaning the fuel salt to make up whatever was lost by fission reactions in the core. In this way, the reactor becomes a breeder, which is self-sustaining in its fuel requirements as long as we have enough thorium in the blanket salt. Strength of Graphite Your other question concerns how to prevent the "delicate" graphite reactor from crumbling into pieces? It is true that the graphite in pencils is delicate and will crumble into pieces if one removes it from its wooden or metal casing. However, reactor-grade graphite is made from much larger coherent pieces and has much greater physical integrity. Such graphite is composed of a stack of 2-D sheets (called graphene), which are very strong in the lateral directions, hundreds of times stronger than structural steel of the same weight. They are not very strong in the direction perpendicular to the 2-D sheet; indeed, sheets of graphene can be lifted away from other sheets by scotch tape (the method used by the team that won the Nobel Prize in Physics in 2010). But if one compresses the reactor-grade graphite in the perpendicular direction, rather than try to pull it apart -- as is case with the modular construction shown in our video -- then the configuration is very strong, and can easily withstand the conditions present in a molten salt reactor. Indeed, a peculiarity of graphite, in contrast to metals, is that its mechanical strength increases with increasing temperature. Well above the temperature where even the most refractory metals melt or vaporize, reactor-grade graphite remains an intact strong solid. In particular, the mechanical macroscopic strength of graphene in the lateral direction translates on a microscopic level to a resistance against chemical attack by all elements other than extremely reactive oxygen. The reason is that each carbon atom in graphene connects to three other carbons by a strong double bond, known to chemists as a sigma bond. It is very difficult for any other atom to insert itself between any two carbons held together by a sigma double-bond. The only way for even an oxygen atom to attack a sheet of graphene is at the edges, where by definition the carbon atoms do not have other carbons with which they are bonded on the empty side of the edge. This is why reactor-grade graphite burns very slowly, even if you apply a blowtorch to it (as we and others have done as demonstration tests). As soon as you remove the torch, the flames go out because the graphite can burn only at the edges. The corrosion resistance of graphite means that it is an ideal material with which to build nuclear reactors, where fission products are generated with chemical positions that span all the columns (but not all the rows) of the periodic table. The only problem is that even reactor-grade graphite is porous, and one must be able to seal the pores if one wants to build a two-fluid molten-salt reactor, where the molten fuel salt and blanket salt do not mix inside the reactor. This is a problem we solved by extending techniques discussed by the Oak Ridge National Lab in the 1960s. We subsequently realized that the sealing is easier if we constructed the reactor out of carbon-fiber-reinforced-carbon (CFRC) tubes rather than blocks of reactor-grade graphite. (SpaceX seems to have subsequently made the same discovery in its construction of carbon-fiber tanks to hold liquid rocket fuel.) Carbon fibers are basically yarns made of long strands of graphene. If one rolls up a sheet of graphene into a long cylinder, the result is called a carbon nanotube, which is one of the strongest materials ever constructed by humans. If one folds and connects the ends of a short carbon nanotube in the third dimension, so that there are no edges anywhere, the result is a buckyball or fullerence, which is the strongest 3-D structure per unit mass in nature, except for diamonds, where each carbon is double-bonded with, not three, but four other carbons. Thus, contrary to everyday experience, carbon-based materials, when they are made properly, are the least likely in nature to "crumble into pieces."

[@Bobby-fj8mk]

Thank you Dr. Frank Shu for that very detailed reply. The cleaning process is very complicated & the graphite construction is not easy either.

[@fshu1708]

Sir Bobby, "complicated" is in the eye of the beholder; however, I agree with your basic point that 2F-MSBRs are not for novices. Given that current fission-reactor technology has no solution to the problem of high-level nuclear waste, the proposed steps are relatively simple and involve no processes that are not already practiced by industry (e.g., fluoride volatility or sparging gases into liquids). Likewise, carbon-fiber composites are already used to build airplanes like the Boeing 787 (Dreamliner) to save on weight without sacrificing strength. Sealing the graphite-based materials against intrusion by liquids or gases is the only novel step in its use for nuclear reactors.

[@Bobby-fj8mk]

Thanks Dr Frank Shu, I am the moderator on Ozpolitic for the Environment sub forum. I started a thread on Thorium & included your remarks here: www.ozpolitic.com/forum/YaBB.pl?num=1519823686/0#8

[@yacobus2206]

I blame the uranium lobby for blocking continued funding in the early 1970s; one of those historic missed opportunities.

[@rawan95B]

Amazing video! may I ask whats the density of W-UO2-ThO2?

[@fshu1708]

Raybee95, by W-UO2-ThO2, do you mean a mixture of uranium dioxide and thorium dioxide? As a solid, UO2 has a density of about 11 g/cm3; ThO2, 10 g/cm3. As liquids, the values would be somewhat less. However, the melting point (m.p.) of UO2 is very high, about 2895 Celsius; ThO2, even higher. The high melting temperatures make uranium and thorium in the form of UO2 or ThO2 desirable for reactors that use solid elements, but they are a distinct disadvantage for molten salt reactors (MSRs). Mixtures of solids can have lower m.p.'s than either substance used alone; ratios that give a minimum m.p. are called eutectics. Recent studies by Bohler at al. (2014) using lasers to do the melting find a eutectic ratio of UO2/ThO2 = 95%/5% that has a m.p. of 2814 Celsius, which is only of slight help. To be of practical use in MSRs, the m.p. of salt mixtures should be below 700 Celsius, and preferably 400 C to 500 C, or below, if possible. As I explained in answer to jarmo valitalo, the preferred combinations involve fluoride mixtures; UF4 and/or ThF4 in combination with LiF, NaF, and/or BeF2. (It is also possible to use chloride salts if one wants an un-moderated neutron spectrum, which is the MSR design that Terrapower is pursuing.) I hope I've answered your question. Thanks for joining the forum!

[@rawan95B]

Yes, thank you !!!

To clarify, you can make UO2 flow at 2000C, you don't need to get something to its melting point to make it flow.

[@blueisnotgreen7258]

exactly how does adding a neutron to thorium make it a "lighter version of uranium" ?

[@fshu1708]

Blueis Notgreen, You've asked a deep question that requires a long exposition for complete clarity. Natural uranium has an atomic nucleus that possesses 92 protons. The heavier and more abundant isotope has 146 neutrons, while the lighter and less abundant isotope has 143 neutrons. Thus, the former is called U--238 because 92+146 = 238, while the latter is called U-235 because 92+143 = 235. In an atomic nucleus that is in its lowest energy state, protons pair with other protons that have opposing spins. The same is true of neutrons. Possessing an odd number of neutrons, U-235 is "fissile" because one of the 143 neutrons must be unpaired. Adding a neutron, no matter how slow is its incoming speed (or low is its incident energy), with an opposite spin to the unpaired neutron in U-235, releases enough extra energy as sometimes to cause the nucleus to vibrate apart and fission, releasing a great amount of nuclear energy in the process. In contrast, adding a slow neutron to U-238 does not cause it to fission because U-238 has no unpaired neutron for the incoming neutron to pair with. Thus, as first explained by Niels Bohr, U-238 is not "fissile," but "fertile." It is fertile because absorbing an extra neutron without vibrating apart causes U-238 to become U-239, which does have an unpaired neutron. However, U-239 has 92 protons and 147 neutrons, which are too many neutrons compared to protons. As it turns out, U-239 is then unstable to two of the neutrons transforming successively into protons (via what are called "beta" decays), with the new nucleus possessing 94 protons and 145 neutrons. Nuclei with 94 protons are called plutonium, so this particular isotope is called Pu-239. Having an odd number, 145, of neutrons, Pu-239 is also fissile, and it can fission when it absorbs either a slow or a fast neutron, releasing a lot of nuclear energy in the process, as well as 2 or 3 neutrons that start out fast. One of the 2 or 3 neutrons is needed to sustain a chain reaction, the excess above 1 is available to convert U-238 into U-239, leading ultimately to Pu-239. If more Pu-239 is created than destroyed in the chain reaction, we have a breeder reactor that runs on the U-238/Pu-239 fuel cycle. Let us now consider thorium, an element that has 90 protons. The only naturally occurring isotope of thorium is Th-232 with 90 protons and 142 neutrons. With an even number of neutrons, Th-232 is not fissile. but fertile. In a nuclear reactor, there are a lot neutrons flying around, usually with many more that are slow than are fast if moderators like hydrogen or enough carbon are around to slow down neutrons. Nuclei are much more likely to absorb a neutron if it is slow than if it is fast. If Th-232 absorbs a slow neutron, it will become Th-233 without fissioning. However, Th-233 with 90 protons and 143 neutrons has too many neutrons compared to protons, so two of the neutrons will successively beta decay to become protons. The first beta decay produces protactinium-233, or Pa-233, with 91 protons and 142 neutrons. The second beta decay transforms Pa-233 into U-233 with 92 protons and 141 neutrons. U-233 is the "lighter version of uranium" in the video because it is less heavy than either U-235 or U-238, which are the two naturally occurring forms of uranium. In any case, U-233 with 92 protons and 141 neutrons is a fissile. If it captures a slow or a fast neutron, it can fission. Reactors can be built that surround a core of fissioning U-233 with a Th-232 blanket that can capture the excess neutrons from the chain reaction in the core. Such reactors are called "thorium breeder reactors," if the rate of making U-233 in the blanket exceeds the rate at which U-233 is being destroyed in the core by the fission chain reaction. Breeder reactors that use the Th-232/U-233 fuel cycle can operate with the neutrons being either mostly slow or mostly fast depending on the presence or absence of moderators. Because the fission reactions are easier to initiate with slow neutrons than with fast neutrons, the amount of U-233 required in the core to have a critical mass capable of sustaining a chain reaction is much less for a slow breeder than for a fast breeder. For this reason, most advocates for thorium breeder reactors, like Kirk Sorensen and myself, prefer slow thorium breeders possible with molten fluoride salts that have graphite moderators to fast breeders possible with molten chloride salts that use no moderators. Reactors that surround a core of fissioning Pu-239 with a U-238 blanket are also possible breeder reactors. However, unlike the case with U-233, the excess neutrons above that needed to sustain a chain reaction of Pu-239 is less than unity, on average, when the fission event is initiated by a slow neutron. As a consequence, slow breeders are impossible for reactors that operate on a U-238/Pu-239 fuel cycle. Such breeders can only be "fast" and require large critical-masses of Pu-239 in the core. It then takes a longer time to breed the fuel to support a large fleet of fast breeder reactors than to support a large fleet of slow breeder reactors. This time difference accounts for one of the reasons why supporters of an expanded new generation of nuclear reactors often back thorium breeders over fast breeders, even though the latter are further researched than the former. Climate change mitigation would be served by supporting either line of development.

[@blueisnotgreen7258]

i guess i should have expected an answer like that on a youtube video about thorium reactors ...cuz i think the guy in the beginning who said that was just oversimplifying an analogy or something. also i dont debate that thorium reactors work, or that they are better in many ways than a typical fission reactor IN THEIR HIGHLY DEVELOPED STATES. But thorium reactor technology is very much in its nascent stage with only really 1 operational thorium reactor (in India.) The demands and costs to give thorium reactors the same technological progress as fission plants (which have been developing for 70 years) would be too great to justify the switch. At present typical fission reactors are better than Th reactors in a lot of ways like overall safety. If someone wanted to build a thorium reactor what is stopping them? but does thorium get hot enough to melt steel beams? THAT is the question.

[@blueisnotgreen7258]

im referring to gen 5 as i live in the US - im guessing they are mostly gen V in the us. i realize some have older ones and infrastructures that make it hard to upgrade/build new ones but as i understand the difference between IV and V are not great compared to gen I and II. its exponential returns on improved technology. basically saying that later generations are all pretty good and the shortcomings of gen III and IV reactors are at least studied and able to be compensated for to a degree.

[@blueisnotgreen7258]

but really - what is stopping a person from building a reactor? there are wealthy science lovers whod back such a reactor if the return on investment was high enough and some whod do it purely for the advancement of the technology. id rather put my money into the stellarators and tokamaks, wouldn't you? the pollution, cost-effectiveness and safety risks associated with a working fusion reactor seem far more worthy of the money we spend on garbage like national defense...once again....from the US here :/

[@puncheex2]

Th-232 + n-1 --> Th-233 Th-233 --(beta decay, half-life 22 minutes)--> Pa-233 Pa-233 --(beta decay, half-life 27 days)-->U-232 Taa Daa.

[@robertbalu8001]

the breeder is only a minor problem - you can always use external source, like conventional reactor or fast spectrum (U-235) molten salt...

[@fshu1708]

Robert Balu, I apologize for being slow to respond to your comment, but the holiday season kept me busy with travel and other engagements. Your comment is insightful and deserves a full reply. As your comment indicates. because of favorable neutron regeneration numbers at low incident neutron energies, reactors running on the Th-232/U-233 fuel cycle are possible as thermal breeders, whereas breeder reactors running on the U-238/Pu-239 fuel cycle are only possible if they have a fast neutron spectrum. Reactors that burn U-235 cannot be breeders at any incident neutron energy for two reasons: (a) because there is no naturally occurring fertile U-234 that could absorb a neutron (together the emission of gamma ray) and turn into fissile U-235 capable of absorbing another neutron and fissioning into daughter nuclei together with the emission of 2 or 3 neutrons that can either sustain a chain reaction and or be used to transform a fertile into a fissile; (b) because U-235 has a relatively high probability of capturing a neutron and using the excess energy to emit a gamma ray to become U-236 relative to absorbing a neutron and using the excess energy to vibrate apart into two fission products, with the release of 2 or 3 neutrons. U-236 has such a small cross-section for absorbing neutrons that it is a virtual dead-end and can be considered to be neither a fissile nor a fertile. Thus, although the extra neutrons released by the fission of U-235 can be used to convert Th-232 in a blanket to make fissile U-233, reactors burning U-235 are either pure burners or converters, and not breeders. Converting Th-232 into U-233 in a blanket surrounding a core that burns U-235 is an excellent use of the natural uranium in the world's reserves of high-grade uranium ores. However, the process is not sustainable for the long term if the U-233 so produced is not ultimately used by a reactor that makes more U-233 in its own blanket containing Th-232 than the core burns. With only reactors that burn more U-233 than they breed in their Th-232 blankets, the world would steadily use up all the available U-235 (or Pu-239) in converter reactors to supplement the shortfall from Th-232/U-233 fuel cycle. Thus, a breeder reactor that make more fuel than it burns to sustain the chain reaction in the core is the holy grail of sustainable nuclear energy, with the understanding that to get a large fleet of breeders on the Th-232/U-233 fuel cycle started, one probably has to use a small fleet of U-235 burning converter reactors. With long-term sustainability for hundreds to thousands of years as the goal, one can choose, in principle between thermal spectrum breeders (e.g., molten fluoride reactors) or fast-spectrum breeders (e.g., molten chloride reactors). Unfortunately, fission cross-sections are hundreds of times larger for thermal neutrons than they are for fast neutrons. As a consequence, for the same power output, the mass of fuel needed in the core of the reactor to reach criticality is typically a hundred or more times greater in a fast spectrum breeder than a thermal spectrum breeder. This basic fact implies that there is simply not enough time left in this century to grow a fleet of fast-breeder reactors using the U-238/Pu-239 fuel cycle that can address climate change unless one uses a small fleet of Pu-239 burners to convert Th-232 into U-233 for a larger fleet of Th-232/U-233 thermal (or epithermal) breeders. As a precaution against unforeseen difficulties, it would be wise to have designs for molten-salt reactors (MSRs) that (1) as waste burners can burn the Pu-239 and minor actinides from the spent fuel (high-level nuclear "waste") from conventional reactors; (2) as converter reactors can burn high-assay lightly enriched uranium (19.75% U-235 and 80.25% U-238) in the core to convert Th-232 in a blanket to U-233; and (3) as breeder reactors can burn U-233 in the core while breeding more U-233 from Th-232 in a blanket than it burns in the core. Our research group is working on all three reactor types that share as a common platform the two-fluid design with a core and a pool.

[@robinhyperlord9053]

Could Thorium fusion beat Deterium fusion and Hydrogen fusion?

[@fshu1708]

Robin, fusion releases energy when two light nuclei like hydrogen or deuterium collide with each other at high speeds because they are at a very high temperature (100 million K or more). At such high temperatures, the electrons of all atoms are completely stripped away from their nuclei. The high speeds of the freely moving positively charged nuclei allow them to overcome their mutual electric repulsion so that they can come close enough for strong nuclear forces of short range to bind them into a heavier atomic nucleus, releasing nuclear binding energy in the process. (This fundamental fact of basic physics is the downfall of claims that "cold fusion" can be achieved with apparatuses that operate at ordinary temperatures and pressures.) No material walls can withstand temperatures of 100 million K. Strong magnetic fields are needed to confine the plasma to prevent their fast moving charged particles from striking the metal walls of the fusion reactor. Unfortunately, present-day schemes using deuterium (hydrogen-2) and tritium (hydrogen-3) will release neutrons in the fusion process, H-2 + H-3 --> He-4 + n, and the neutrons being electrically neutral cannot be deflected by the magnetic fields and will fly to the walls of the reactor. These neutrons are a small fraction of the totality of particles, so the heat produced by their absorption by the metal walls is not a problem, but the neutrons will make the walls radioactive over time. Fortunately, the release of neutrons is not a fundamental aspect of the fusion process. For example, fusion with deuterium and helium-3 releases a proton instead of a neutron, H-2 + He-3 --> He-4 + p. Protons (the nuclei of ordinary hydrogen, H-1) are positively charged, and can be kept away from the walls, in principle. by the magnetic fields of the reactor. However, ionized He-3 has a charge of +2, and overcoming their electric repulsion requires even higher temperatures, around a billion K. Moreover, unlike tritium, which is made abundantly in certain types of fission reactors, He-3 is very rare on Earth, so getting enough of it for use as an energy source will require mining objects like the Moon or asteroids that have been accumulating He-3 from the solar wind for aeons. Heating the plasma to high temperatures and maintaining the magnetic fields against dissipation also require a lot of energy. Right now, plasma physicists are trying to prove that it is possible to get more energy out than one puts in. Apart from these practical considerations, the most challenging problem with fusion reactors today is that complex fluctuations of the collective plasma can cause a breakdown of the confining magnetic configurations so that the fusion reactions can be sustained for only a few minutes per day. It will be some time before fusion reactors become practical power plants. Nevertheless, if we deal wisely with climate change in the interim, fusion is probably the future of energy. Fission releases energy when heavy nuclei like uranium or plutonium absorb a neutron and splits apart into two smaller fragments that emit more neutrons. The most important task of the released neutrons is to sustain a chain reaction when the reactor has a critical mass of the uranium or plutonium. Thus, the release and transport of neutrons are fundamental features of the fission process. Because neutrons can penetrate atomic nuclei without repulsion, fission reactions can occur at any temperature, including room temperature (as occurs in special reactors dedicated to making plutonium for weapons production). Controlling the rate of reaction is relatively easy. In conventional reactors, neutron absorbing rods are inserted or withdrawn as required to decrease or increase the rate of reaction. In molten salt reactors, control rods are not even needed because the salt will expand out of or contract into the core automatically if the reactions run too fast or too slow, releasing too much or too little energy. In such reactors designed to be breeders where one wants to avoid the waste of any neutrons, neutron-absorbing rods are only used for emergency shutdown. The release of energy will raise the temperature of a power reactor above ambient values, but not so high (say, much above 1000 K) as to threaten the integrity of metal containers (or, in the case of our design for molten salt reactors, the graphite-based equipment). Energy is released in the fission process because in very large atomic nuclei, the long-range repulsion of the positive protons prefers the nucleus to separate into two fragments despite the attractive but short-range nuclear forces that attempt to keep the collection of neutrons and protons together. This interplay of electromagnetic and nuclear forces (with the weak nuclear force able to change neutrons into protons and vice versa keeping a balance between protons and neutrons in an atomic nucleus) explains the strange dichotomy that it is energetically favorable for light elements to fuse to form heavier elements, while it is also energetically favorable for very heavy elements to fission into two lighter elements, with the bottom of the valley of nuclear energy space occupied by iron, specifically Fe-56. For technical reasons explained elsewhere in this discussion forum, only very heavy nuclei with an even number of protons and odd number of neutrons, called fissiles, like U-233, U-235, or Pu-239 will undergo fission after absorbing a slowly moving neutron. Thorium has only 1 natural isotope, Th-232 (90 protons and 142 neutrons), so it is not fissile. It is quite abundant in the crust of the Earth, and is a byproduct of rare Earth mining that produces materials for the electronics industry and for the turbines used in wind power. Th-232 can be transformed into Th-233 by the absorption of a neutron, with Th-233 containing too many neutrons (143) compared to protons (90), so that the weak nuclear force will cause two of the neutrons to turn into protons, a process called beta decay because it is accompanied by the emission of two electrons (and two antineutrinos). The Th-233 then becomes U-233 (92 protons, 141 neutrons), which is a fissile. In fact, U-233 is the best fissile because when it absorbs a neutron, it will fission about 90% of the time, while 10% of the time it will become U-234, which is not fissile. In contrast, U-235 (the only naturally occurring fissile, upon absorbing a neutron, will fission only about 80% of the time, while 20% of the time it will become U-236, which is also not fissile. Superficially, Pu-239 looks the best because when it fissions it can release close to 3 neutrons per fission event (even more in so-called fast reactors), better than the average of roughly 2.5 neutrons per fission event of U-233 and U-235 when they split into fission fragments, Since 1 neutron is required to sustain the chain reaction, 2 excess neutrons are available from Pu-239 fission for turning U-238 into U-239, which undergoes 2 beta decays to become more Pu-239. This sounds great for breeding U-238 (which is the predominant form of natural uranium, but non-fissile) into Pu-239, but unfortunately, when Pu-239 absorbs a slow neutron, 60% of the time it will fission, but 40% of the time, it produces Pu-240, which is non-fissile. One has then "wasted" a neutron, If one goes through the math, accounting for other non-fissiies, like fission products, that can parasitically absorb neutrons, one discovers that the U-238/Pu-239 fuel cycle can produce greater than unity breeding (i.e., more Pu-239 produced than burned in the reactor) only in reactors that do not slow down ("moderate") the fast neutrons that typically emerge from the fission process. U-235 cannot breed at all -- in part because it has a relatively low neutron yield per fission event coupled with a poor branching ratio into capture to produce U-36 instead of fission, and also because there is little U-234 occurring naturally (as an alpha decay product of U-238) to act as a "fertile" to breed into fissile U-235. Only the Th-232/U-233 fuel cycle has the advantage to give breeder reactors that operate by slowing down neutrons to where the so-called cross-sections are much larger, compared to the fast neutrons that merge from a fission event, for capturing neutrons that produce another fission event. This circumstance makes the critical mass required to achieve a chain reaction much smaller than in unmoderated (or "fast") reactors. In my opinion, the Achilles heel of the fast reactor approach is there being not be enough time left before climate change overwhelms us for fast reactors to produce critical masses of fuel to start the fleet of power plants needed to decarbonize the economy. To bring a close to this reply, your question raises the interesting issue whether it is possible to couple fusion and fission through the process of the emission and absorption of neutrons. This idea is the motivation behind proposals for a fusion-fission hybrid, which is based on the concept that fusion with deuterium-tritium is neutron rich (if one has a good way to produce tritium from Li-6), while fission in the Th-232/U-233 fuel cycle is neutron poor. In other words, the deuterium-tritium fusion reaction produces neutrons that one doesn't need, or even want, while the Th-232/U-233 transformation is begging for neutrons to achieve greater then unity breeding (something that Oak Ridge National Lab talked about, but never proved or even attempted in its experiments with molten salt reactors). Our research group thinks ORNL's idea will work, but our theoretical calculations do show that the task is not easy, even in graphite-based systems that are very clean in their neutron economy. It may be worthwhile to consider fission-fusion hybrid schemes if achieving greater than unity breeding does proves to be a show-stopper for two-fluid molten-salt breeder reactors.

[@robinhyperlord9053]

@@fshu1708 Thanks.

[@fshu1708]

Robin, I forgot to mention that fusion-fission hybrid devices do exist -- they are called "hydrogen bombs." Hydrogen bombs consist of fission bombs (also called "atomic bombs") that are detonated, in modern weapons, to implode (inwardly compress) a core of lithium-deuteride, with the lithium being Li-6 that becomes H-3 (tritium) plus He-4 when the Li-6 absorbs a neutron. The "deuteride" part of the lithium-deuteride contributes deuterium. Under the tremendous heat of the implosion process, the tritium plus deuterium then fuse to release the fusion energy that we discussed earlier in this thread. Apart from the energy needed to compress the core to a superhot plasma, the fission bomb also provides the initial supply of neutrons to begin the release of tritium via Li-6 disintegration. By themselves, fission bombs have limited power because they involve the assemblage of two subcritical masses of U-235, or the implosion by ordinary chemical explosives of a subcritical mass of Pu-239 into a supercritical state, that then results in an outward explosion that characterizes all bombs. Because a critical mass of U-235 and Pu-239 have well-defined limits, atomic bombs have limited power, even though terrifying large by the standards of chemical explosives. With hydrogen bombs, there is no limit how much lithium-deuteride is packed into the core of the bomb to be imploded to trigger a runaway fusion reaction. Therefore, hydrogen bombs when they explode can have insanely amounts of power. However, just as President Eisenhower was sufficiently wise in the 1950's to initiate a program called "Atoms for peace" to turn atomic-bomb technology to nuclear power plants, perhaps some future American President will initiate a program that turns hydrogen-bomb technology to fusion-fission hybrid power plants that can supply the world with unlimited amounts of energy for peaceful purposes. Perhaps such power plants will even use thorium in the fission part of your original question.

[@ChristCenteredMinist]

I want one...

[@Soraviel]

that's amazing

[@tski3458]

So we gave the world the seeds of our own destruction when we really didn't have to after all. I hope this gets more traction within my lifetime. I am afraid that those in control who think we may need more pu-239 will not pay for this. Not in the US anyway. We humans are far to entrenched in that nuclear "suicidal glee" . we should have listened to Eisenhower. Thanks for the Video.

[@fshu1708]

T Manners, you might be interested to read "The Making of the Atomic Bomb" by Richard Rhodes. It's a masterful account, getting the science correctly behind the subject and capturing the personalities behind the work. The decision by the Allies during World War 2 to develop the atomic bomb before Germany was necessary from the perspective of the scientific community at the time. From a historical point of view, there are not many, even today, who would argue that Allied scientists made a mistake to put weapons production first and power reactors second in the race to win the war. The irony was that the war in Europe was won before the bombs were ready, and it then became a political decision to use the bombs on Japan to end the war in Asia. In his book, Rhodes also points out that during wars there are always patriotic scientists who will borrow discoveries made to benefit humankind to make weapons of mass destruction for the killing of one's opponents -- chemical fertilizer technology to produce toxic gases during World War 1, nuclear fission to manufacture atomic bombs during World War 2, and perhaps genetic engineering to promote deadly infectious diseases in World War 3. The solution to these social/antisocial tendencies is to ban war, not to ban chemical fertilizers that allow the feeding of billions of hungry people, or nuclear power plants that can help stop climate change, or new drugs that can save countless lives in developed and undeveloped parts of the world.

[@iosef3337]

2:00 "so this kind of reactor would never produce weapon grade plutonium" *facepalm*

[@iosef3337]

You can make plutonium as simple as making the graphite hollow, and with a robotic hand, puting and taking out U 238 (the abundant kind of uranium), waiting and then chemically separate the plutonium, thats it. (sorry for bad english)

[@puncheex2]

Sure you can, but what does it take to produce the quantities to make a weapon? Compare the physical size of Shu's reactor to that of the plutonium production reactors at Hanford. It would be a practical impossibility to do so clandestinely, as well. Such would have to be built in at the manufacturing stage where suitable inspection protocols would make it impossible.

[@fshu1708]

puncheex2, thanks for the great reply to iosel 333. The whole weapons proliferation concern is literally a diversion of the fact that practical bomb-making reactors are very different from reactors designed to generate nuclear power. It's a mistake to mix the two.

[@threejaguar]

Any bad guy that wants to toast U238 will just make a CANDU heavy water reactor. Using a power reactor for such work is stupid and pointless.

[@paulisham5669]

We hear talk after talk year after year but no progress and no current discussions.

[@michaelgreenfield9090]

And so nothing happens !! Politics, Politics !!!!

[@loktom4068]

Still talking? Is year 2117 enough time for talking?