Liquid Fluoride Thorium Reactor


by gcarlin
Tags: fluoride, liquid, reactor, thorium
mheslep
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#91
Feb27-12, 11:39 AM
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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
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#92
Feb27-12, 11:42 AM
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Quote Quote by etudiant View Post
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
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#93
Feb27-12, 12:19 PM
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Quote Quote by mheslep View Post
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
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#94
Feb27-12, 12:42 PM
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Quote Quote by etudiant View Post
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
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#95
Feb27-12, 02:46 PM
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Quote Quote by mheslep View Post
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
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#96
Feb27-12, 11:41 PM
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Quote Quote by etudiant View Post
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
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#97
Feb28-12, 12:19 AM
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Quote Quote by wizwom View Post
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
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#98
Feb28-12, 04:57 AM
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Quote Quote by etudiant View Post
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: 1417C) / UF6 (Boiling point: 56.5C).

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
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#99
Feb28-12, 06:51 AM
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Quote Quote by etudiant View Post
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
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#100
Feb28-12, 07:41 AM
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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
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#101
Feb28-12, 08:34 AM
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Quote Quote by etudiant View Post
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
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#102
Feb28-12, 10:58 AM
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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
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#103
Mar5-12, 12:19 AM
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Quote Quote by zapperzero View Post
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
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#104
Mar5-12, 05:17 AM
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Quote Quote by wizwom View Post
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
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#105
Mar5-12, 09:42 AM
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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
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#106
Mar12-12, 11:58 AM
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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_...horium_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
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#107
Mar12-12, 05:32 PM
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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
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#108
Mar12-12, 09:06 PM
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Quote Quote by Stanley514 View Post
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_...horium_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.


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