Liquid Fluoride Thorium Reactor

[mheslep]

I don't follow. Under positive control an operator removes the fluid from the moderator area (graphite i believe?) and thus stops the reaction. If there's failure of control, the operator stops active cooling of the freeze plug (assuming that has not already happened), again the fluid leaves the moderator area and the reaction stops.

 

[mheslep]

Thank you for the clarification.
Does this mean that the reaction only stops once the molten salt vaporizes?
Or is there a negative trend as the temperature of the salt rises?

Is there a solid reference which discusses these issues in the context of a review of operational considerations for a MSTR?

From the original Oak Ridge MSR work, Fluid Fueled Reactors:
As the salt density falls with increasing temperature, reactivity falls: (1/k) dk/dT ~= -3.8 X 10^-5 / °F
See pg 640-642 here:
http://www.energyfromthorium.com/pdf/FFR_chap14.pdf
If you are inclined there's more here:
http://energyfromthorium.com/pdf/

 

[etudiant]

From the original Oak Ridge MSR work, Fluid Fueled Reactors:
As the salt density falls with increasing temperature, reactivity falls: (1/k) dk/dT ~= -3.8 X 10^-5 / °F
See pg 640-642 here:
http://www.energyfromthorium.com/pdf/FFR_chap14.pdf
If you are inclined there's more here:
http://energyfromthorium.com/pdf/

Hi mheslep,
Thank you for the information and the very helpful references.
The reports, while very informative, are unfortunately more focused on feasibility and economics than on divergences from expected operations. As these are somewhat science advocacy documents, that is not surprising.
As an uninformed observer, it does worry me that the reactivity merely falls with density, because the nuclear reactions are so much faster than any change in density could be. It suggests that local excursions are not ruled out, even if the negative coefficient does preclude a Chernobyl type factor of 1000 power surge.

 

[mheslep]

Hi mheslep,
Thank you for the information and the very helpful references.
The reports, while very informative, are unfortunately more focused on feasibility and economics than on divergences from expected operations. As these are somewhat science advocacy documents, that is not surprising.
As an uninformed observer, it does worry me that the reactivity merely falls with density, because the nuclear reactions are so much faster than any change in density could be. It suggests that local excursions are not ruled out, even if the negative coefficient does preclude a Chernobyl type factor of 1000 power surge.

Could you illustrate by showing how such an excursion is ruled out with a traditional pressure water solid fueled reactor? Clearly control rods insertion is also not instantaneous.

 

[etudiant]

Could you illustrate by showing how such an excursion is ruled out with a traditional pressure water solid fueled reactor? Clearly control rods insertion is also not instantaneous.

Am no expert, but afaik, in conventional reactors, the fuel is in fixed arrays, so the evolution of the nucleides can be allowed for.
In a large pool of thorium fluoride gradually transmuting to U233, it seems at least possible for gradients to form with potentially quite different fuel concentrations and compositions.
I would like to have some idea of how the system would react to such changes in nuclear geometry.
Given that we have had bad experiences with interrupted cooling flows (Fermi reactor most notably) it is reasonable to consider the effect of loss of mixing in the MSTR beforehand. After all, when there is a lot of nuclear material in a small volume, as is the case for the MSTR, belt and suspenders engineering must be the minimum requirement.

 

[wizwom]

Am no expert, but afaik, in conventional reactors, the fuel is in fixed arrays, so the evolution of the nucleides can be allowed for.
In a large pool of thorium fluoride gradually transmuting to U233, it seems at least possible for gradients to form with potentially quite different fuel concentrations and compositions.
I would like to have some idea of how the system would react to such changes in nuclear geometry.
Given that we have had bad experiences with interrupted cooling flows (Fermi reactor most notably) it is reasonable to consider the effect of loss of mixing in the MSTR beforehand. After all, when there is a lot of nuclear material in a small volume, as is the case for the MSTR, belt and suspenders engineering must be the minimum requirement.

The LFTR idea is that the U233 is controlled by gassifying the Pa233 stage, removing the breeding wait from the active reaction mass, and then returning it after it becomes U233 as the reactor needs it.

 

[etudiant]

The LFTR idea is that the U233 is controlled by gassifying the Pt233 stage, removing the breeding wait from the active reaction mass, and then returning it after it becomes U233 as the reactor needs it.

You are suggesting the LFTR design envisages bubbling up Plutonium vapor for recycling after it decays back to U233?
This is news to me.
Imho, it does not seem a good idea.

 

[Astronuc]

You are suggesting the LFTR design envisages bubbling up Plutonium vapor for recycling after it decays back to U233?
This is news to me.
Imho, it does not seem a good idea.

Higher order fluorides, UF₆, are volatile. In the gaseous diffusion and centrifuge enrichment processes, UF6 gas is used as a carrier from which U(235)F₆ is separated from U(238)F₆. Similarly, different fluorides have different stability domains and volatilies, so one tailors the process to favor a particular element. One would take advantage of differences between PaF₄/PaF₅ and UF₄ (Boiling point: 1417°C) / UF₆ (Boiling point: 56.5°C).

The element is a dangerous toxic material and requires precautions similar to those used when handling plutonium. Protactinium is one of the rarest and most expensive naturally occurring elements.

http://www.webelements.com/protactinium/

The attraction of the Th-based fuel cycle is the lack of transuranic elements, although some quantity of U-235 or Pu-239 is required to initiate a Th-based system.

 

[wizwom]

You are suggesting the LFTR design envisages bubbling up Plutonium vapor for recycling after it decays back to U233?
This is news to me.
Imho, it does not seem a good idea.

Protactinium, not Plutonium. A LFTR never gets to any significant amount of Plutonium.
The chain is 232Th->233Th->233Pa->233U->fission
The 233Pa has an absorption cross section about 14 times that of 232Th, so you want to get it out of the way of neutrons if you can, and LFTR does exactly that as the molten salt passes through the flouridizer.

 

[etudiant]

Thank you very much, Astronuc and wizwom. Very helpful input.
That even the initial LFTR design prototype included a fairly capable fuel reconditioning element to remove undesirable fission products is entirely logical, but a new wrinkle to me.
It is certainly not a much discussed feature of this class of designs.

 

[zapperzero]

It is certainly not a much discussed feature of this class of designs.

I dunno. I harp on it every chance I get. "A reprocessing plant near every power station! La Hague in your own back yard!" etc etc

 

[mheslep]

As far as I know most of the reticence about reprocessing comes about from the fact that Plutonium processing goes on with U235 fuel cycles. That's not an issue with a Thorium fuel cycle.

 

[wizwom]

I dunno. I harp on it every chance I get. "A reprocessing plant near every power station! La Hague in your own back yard!" etc etc

Except its not "a reprocessing plant" - its an integral part of the reactor, and it never ships fuel out, and, in fact, should have trivial amounts of waste flow (just the fission products, about 1 gram per MWd).

And on the plus side: no "spent fuel" to store. No refueling shutdowns.

 

[zapperzero]

Except its not "a reprocessing plant" - its an integral part of the reactor, and it never ships fuel out, and, in fact, should have trivial amounts of waste flow (just the fission products, about 1 gram per MWd).

And on the plus side: no "spent fuel" to store. No refueling shutdowns.

Oh it is a reprocessing plant, only it's co-located with the reactor and is integral to its functioning, unlike current reprocessing plants.

Are you including gasses in your gram/MWd?

 

[mheslep]

1 MW-day/gram is ~100% burn up of a fissionable fuel, so yes, IF 100% is achievable w/ a LFTR, 1 gram includes the mass of all fission products. Natural thorium is ~100% Th232, the fertile isotope.

By contrast, a traditional 5% LEU reactor would produce the same mass of fission products per unit energy, but 20X the waste mass at 100% burnup. However, as I understand the current process, largely because of fission product poisoning solid fuel reactors typically achieve on the order of 10% burnup, adding another 10X of waste mass (not fission product) per unit energy. So we might expect a LFTR to produce 200X less waste than a traditional LEU solid fuel reactor.

 

[Stanley514]

Any energy source is finite (see elemental thermodynamics). Thorium will easily last for tens of thousands of years, consume long lasting TRU waste, and produce on-demand energy without emissions of any air pollution or green house gases. I'd say that is good enough.

I have some doubts on tens of thousand years.Sounds too good to be true.
Wikipedia gives us info that total extractable world Thorium reserves are estimated at 1 million 600 thousands of tons. http://en.wikipedia.org/wiki/ThoriumIf we divide this number per 7 billions of modern human on Earth inhabitants,we receive weight less than 200 grams per person.
Are they going to tell that if Thorium will be main and primary energy source for humans it will last more than one generation?I have doubts on it...

One more problem: In currently proposed designs of LFTR they suppose to use Liquid FLiBe salt http://en.wikipedia.org/wiki/Liquid_fluoride_thorium_reactorSo it will require tons of Lithium and Berillium per reactor.Berillium is even much rare than Thorium.And needed for many critical apps.Neither Thorium or Berillium are present in salt water.

 

[mheslep]

If completely burned in a reactor, 200 grams of Th would yield 10 kW of thermal power, the US average power use per capita, for ~53 years. There is a great deal of Th mass in the oceans, not counted in those land based reserve figures. I see a source show 10pg/ml Th in ocean water, or 13.4 million tonnes total.The only material that would be consumed in such a reactor is the Th. Other supporting materials can be reused.

 

[etudiant]

I have some doubts on tens of thousand years.Sounds too good to be true.
Wikipedia gives us info that total extractable world Thorium reserves are estimated at 1 million 600 thousands of tons. http://en.wikipedia.org/wiki/ThoriumIf we divide this number per 7 billions of modern human on Earth inhabitants,we receive weight less than 200 grams per person.
Are they going to tell that if Thorium will be main and primary energy source for humans it will last more than one generation?I have doubts on it...

One more problem: In currently proposed designs of LFTR they suppose to use Liquid FLiBe salt http://en.wikipedia.org/wiki/Liquid_fluoride_thorium_reactorSo it will require tons of Lithium and Berillium per reactor.Berillium is even much rare than Thorium.And needed for many critical apps.Neither Thorium or Berillium are present in salt water.

Thorium resources are likely to be vastly greater that currently estimated, as it has not been much in demand historically. Moreover, most of the current supply is afaik as a byproduct of rare Earth mining, where thorium is an unwanted contaminant. So our current resource estimate is really an estimate of waste product abundance.

Beryllium however is another matter. It is a pretty rare mineral in any of its forms, with no large resource anywhere afaik.

 

[ShotmanMaslo]

I have some doubts on tens of thousand years.Sounds too good to be true.
Wikipedia gives us info that total extractable world Thorium reserves are estimated at 1 million 600 thousands of tons. http://en.wikipedia.org/wiki/ThoriumIf we divide this number per 7 billions of modern human on Earth inhabitants,we receive weight less than 200 grams per person.
Are they going to tell that if Thorium will be main and primary energy source for humans it will last more than one generation?I have doubts on it...

One more problem: In currently proposed designs of LFTR they suppose to use Liquid FLiBe salt http://en.wikipedia.org/wiki/Liquid_fluoride_thorium_reactorSo it will require tons of Lithium and Berillium per reactor.Berillium is even much rare than Thorium.And needed for many critical apps.Neither Thorium or Berillium are present in salt water.

There is far more than 1 600 000 tonnes of recoverable Th in the Earths crust. Those numbers refer only to estimated amount in presently known high quality reserves on dry land - easily accessible thorium mineral deposits recoverable at price below X. And considering we have not even seriously looked for thorium, it is probably a gross understatement of real world reserves.

And we can also use far lower quality reserves for LFTR, since thorium fuel price is negligible compared to the value of generated electricity and reactor costs. Even the method advocated by Weinberg - "burning the rocks" - extracting thorium from ordinary soil, has favorable EROEI (energy returned on energy invested), since thorium atom is so energy dense and LFTR uses 99% of the Th fuel, instead of 1% of uranium fuel as ordinary nuclear power plants.

There were threads about this on Energyfromthorium.com forum:
http://www.energyfromthorium.com/forum/viewtopic.php?f=2&t=3398
http://www.energyfromthorium.com/forum/viewtopic.php?f=2&t=3512
http://energyfromthorium.com/2006/04/29/how-much-thorium-would-it-take-to-power-the-whole-world/

 

[Stanley514]

There is a great deal of Th mass in the oceans, not counted in those land based reserve figures. I see a source show 10pg/ml Th in ocean water, or 13.4 million tonnes total.

Concentration of Thorium in seawater is negligibly small,something like
0.0000004 ppm.This is a million times more rare than Uranium.
http://mistupid.com/chemistry/seawatercomp.htm
I guess it would be no practically to retreive it, for sure.

It would be interesting what chemical elements beside Thorium could be used as a fertile
nuclear fuel.Theoretically any element which is havier than Iron could be used to get energy by fission.What about Tungsten?

It would be bigger success if they would manage to get energy from Boron.Such as in fusion reactions.There is 6 trillions of tons of Boron in seawater and it could be retrieved at competitive price already now.

 

[mheslep]

Concentration of Thorium in seawater is negligibly small,something like
0.0000004 ppm.This is a million times more rare than Uranium.
http://mistupid.com/chemistry/seawatercomp.htm
I guess it would be no practically to retreive it, for sure.

The Th concentration figure from my reference has a concentration 25X higher than yours in seawater, and Uranium at 4ppb in seawater is 300X higher than Thorium (my reference) in seawater. At that concentration (10pg/ml), 100k cubic meters (100e6 liters) of seawater are required to produce a gram of Th, which as we know produces 1MW-day of heat energy in a reactor. Is that practical? I dunno.

It would be interesting what chemical elements beside Thorium could be used as a fertile
nuclear fuel.Theoretically any element which is havier than Iron could be used to get energy by fission.What about Tungsten?

I don't think net energy is possible with any of the other natural elements besides the the traditional fertile isotopes of thorium and uranium (Th232, U234&238). The problem with using anything else is the process results in a net loss of neutrons. Unless I've missed something*, once all of the U and Th is gone, along with any transuranics made by U and Th, i.e. Pu, then net energy fission is done on this planet.

*I suppose there's always high Z fusion to build it all back up again, but so far that requires a supernova.

 

[mheslep]

I have some doubts on tens of thousand years.Sounds too good to be true.
Wikipedia gives us info that total extractable world Thorium reserves are estimated at 1 million 600 thousands of tons. http://en.wikipedia.org/wiki/ThoriumIf we divide this number per 7 billions of modern human on Earth inhabitants,we receive weight less than 200 grams per person.
...

oh, as others have pointed out, that figure refers to reserves, i.e. go to spot X,dig to depth Y, and it is likely that Z tons of Thorium will be found there. As to the total mass of Th on earth, Th is estimated to be 1.5e-5 of the total mass of the earth, ie 15 ppm, or 9 million billion tons.

 

[Stanley514]

Unless I've missed something*, once all of the U and Th is gone, along with any transuranics made by U and Th, i.e. Pu, then net energy fission is done on this planet.

It is said that one of major constituents of geothermal heat is Potassium-40.
Could we use this element as a fertile fuel somehow?

 

[mheslep]

The heat from P40 is decay heat, not nuclear fission.

 

[etudiant]

A simple search in Bing brings up an excellent summary from the World Nuclear Association here: http://www.eoearth.org/article/Thorium

The punch line in terms of the resource is in the summary of pros and cons:
' The main attractive features are:
• the possibility of utilising a very abundant resource which has hitherto been of so little interest that it has never been quantified properly,
• the production of power with few long-lived transuranic elements in the waste,
• reduced radioactive wastes generally. '

So we don't know how much thorium is to be found because we've never looked.
We do know it is several times more abundant than Uranium and a vastly better burn up fuel.
Surely that is enough to at least work the problem, even if the resource is not a solution for all time.

 

[mheslep]

A simple search in Bing brings up an excellent summary from the World Nuclear Association here: http://www.eoearth.org/article/Thorium

The punch line in terms of the resource is in the summary of pros and cons:
' The main attractive features are:
• the possibility of utilising a very abundant resource which has hitherto been of so little interest that it has never been quantified properly,
• the production of power with few long-lived transuranic elements in the waste,
• reduced radioactive wastes generally. '

So we don't know how much thorium is to be found because we've never looked.
We do know it is several times more abundant than Uranium and a vastly better burn up fuel.
Surely that is enough to at least work the problem, even if the resource is not a solution for all time.

Those are the strong points of the fuel cycle, but I think they are secondary to the reactor fail-safe advantages gain by operating a molten salt reactor, finally providing a path to eliminate 300 atm pressurized water and all that goes with it.

 

[zapperzero]

Those are the strong points of the fuel cycle, but I think they are secondary to the reactor fail-safe advantages gain by operating a molten salt reactor, finally providing a path to eliminate 300 atm pressurized water and all that goes with it.

Correct me if I am wrong, but is there not a secondary cooling loop which uses water, in all MSR designs? How does this constitute "eliminating" it?

I don't think using thorium is a bad idea per se, it's just that I think mixing two un-proven technologies (MSR and HEU-initiated thorium cycle) is not so safe. The Indian approach of modifying the well-known and long-proven CANDU design (for all its flaws) seems to be lower risk. Better the devil we know.

 

[zapperzero]

I don't think net energy is possible with any of the other natural elements besides the the traditional fertile isotopes of thorium and uranium (Th232, U234&238). The problem with using anything else is the process results in a net loss of neutrons. Unless I've missed something*, once all of the U and Th is gone, along with any transuranics made by U and Th, i.e. Pu, then net energy fission is done on this planet.

Why? If you have a reasonable way to produce the required neutrons (such as fusion), you can split atoms all you like, for a net gain in energy. The reaction is not self-sustaining in bulk, is all.

 

[Astronuc]

Why? If you have a reasonable way to produce the required neutrons (such as fusion), you can split atoms all you like, for a net gain in energy. The reaction is not self-sustaining in bulk, is all.

Not for most nuclei.

The Russians have some data on fission of Rn(Z=86)-222, and the cross-section are quite low. One would more likely get an (n, n') or (n,#n) reaction, or some other spallation reaction. They also indicate no fission for Po isotopes, or the cross-sections are so low compared to other spallation reactions that one cannont measure any discernible fission event. Other countries don't have any data regarding fission of isotopes below Ra-223.

http://www.nndc.bnl.gov/sigma/getPlot.jsp?evalid=12956&mf=3&mt=18&nsub=10

See - σ(n,F) - at http://www.nndc.bnl.gov/chart/reCenter.jsp?z=83&n=126 - and select Zoom 5 to see readily fissionable isotopes (that is with thermal neutrons). The lightest is Ra-223 and that has very low cross-section.

For a closer look - http://www.nndc.bnl.gov/chart/reCenter.jsp?z=88&n=135 (Zoom 4) and make sure one picks σ(n,F) at the top bar.

 

[mheslep]

The question was not about the fission likelihood of elements other than U and Pu, but could lesser elements be built up to heavy through repetitive neutron capture and beta decay to arrive at U or Pu the way Thorium can be from a single neutron, i.e. breeding fissionable materials from fertile elements. Clearly this doesn't work in a fission reactor, in which case every neutron captured loses a potential ~200MeV.

I had not considered using the neutrons from a fusion reactor as ZZ suggests, but I see at least two problems with that approach: i) even in neutronic fusion, those neutrons are required to breed tritium in a net energy reactor, i.e. like fission a neutron wasted to build heavy elements wastes a potential 17MeV from making tritium. ii)I have no idea of the cross section and beta decay chain that might be required to breed, say Si into U, or if it is possible without regard to energy. I'd guess somewhere along the way there will no beta decay 'step up' available, only alpha to go down. But without researching the issue, IF the cross section decay chain was advantageous for neutron absorption all the way up to U and Th, I think we would see the production of those elements in stars like ours. We don't, short of heavy element fusion in novae.