Link:Astronuc said:...
Perhaps - Mathieu L., et al., (2009), Possible Configurations for the TMSR and advantages of the Fast Non Moderated Version, Nuclear Science and Engineering 161, pp. 78-89.
mheslep said:Yes, but never approaching anything like the ~2200 PSI of the primary loop in a PWR.
Reactivity. As the salt density falls with increasing temperature, reactivity falls: (1/k) dk/dT ~= -3.8 X 10 -5 / °F
See pg 640 here:
http://www.energyfromthorium.com/pdf/FFR_chap14.pdf
If you are inclined there's more here:
http://www.energyfromthorium.com/pdf/FFR_part2.pdf
From Fluid Fuel Reactors, Lane, McPherson, Maslan, 1958 Forward by Weinberg
Thanks for the links. I'll have to dig into them.mheslep said:Link:
http://lpsc.in2p3.fr/gpr/gpr/publis-rsf/Article-NuclScienceEng49-07.pdf
Thanks for looking. I'm interested in exploring the details.Astronuc said:Thanks for the links. I'll have to dig into them.
After a cursory review, I have to mention a note of caution on the moderator temperature coefficient - it is core design specific and depends on whether the moderation is within the salt or solid, e.g., graphite. Moderation in the salt would be accomplished by Li (enriched in Li-7, depleted in Li-6) and Be, and the moderation coefficient would be more negative than if moderation were primarily in the graphite. It is also important where the moderation occurs, e.g., throughout the core, or within the blankets, radial and axial (upper and lower cores).
I believe the first citation references a Be (in BeF2) moderated system, so it would be more negative. The second citation Mathieu L., et al., indicates the earlier MSR had a positive moderator coefficient, which I believe is related to the lack of moderation (Be) in the fuel-coolant salt mix. It also indicates that reprocessing and extraction of fission products was uneconomical. On the other hand, these were areas for improvement.
...
Yes I noticed, but have no idea why polygonal shapes are preferred.Astronuc said:There are several papers on Thorium based fuel and fuel cycle in an upcoming meeting, but they are more conventional (not salt-bassed) fuel, but in thermal, epithermal and fast reactors. So lots of folks are taking Th-based fuel cycles seriously.
The VVER type fuel assembly/core system (hexagonal or triangular lattice) is apparently good for the Th-fuel cycle.
The triangle pitch makes for a tighter lattice. I'd have to check the fuel to moderator ratio, but I believe it's less than a square lattice on a unit cell basis. I believe the hexagonal arrangement of assemblies produces less leakage.mheslep said:Yes I noticed, but have no idea why polygonal shapes are preferred.
http://itheo.org/thorium-energy-conference-2011The ThEC11 Program is full of exciting topics and speakers. Some of the highlights can be found below. To view or download each speakers presentation in PDF-format, please click on any of the titles below.
Thanks.Astronuc said:FYI - Highlights of the Thorium Energy Conference - ThEC11
http://itheo.org/thorium-energy-conference-2011
Accelerated systems bascially allow for a sub-critical core, so reactivity transients are much less likely if not virtually impossible. The fast fission is more symmetric so there are less volatile/gas fission products produced.mheslep said:I also examined the accelerator based reactor report.
http://www.itheo.org/sites/default/files/pdf/Report%20from%20the%20DOE%20ADS%20White%20Paper%20Working%20Group%20-%20Stuart%20Henderson%20-%20Fermilab%20-%20ThEC11.pdf
If I understand correctly, the proposed advantages for a driven reactor would be i) the ability instantaneously stop the fission reaction and ii) the ability to burn other fuels besides uranium and thorium. However, the problem demonstrated at Fukushima was decay heat not uncontrolled fission, and there is no shortage of thorium and U238 that can be burned in other reactors like a LFTR.
As I understand it the MSRE at ORNL used only U liquid salts and they never got to the point of converting Th. Hence it could be said MSRE provided much information on molten salt designs, but that is large stretch from a LFTR, and all of the associated reprocessing.lftrsuk said:In the days of slide-rules, plastic models, manual machine tools and welding, the go-ahead for the Molten Salt Reactor Experiment (MSRE) was given to Alvin Weinberg, at the Oak Ridge National Laboratoty (ORNL) in 1960. In 1965, the reactor was switched on and ran until 1969. A cadre of nuclear physicists spent much more time agonising over similar levels of minutiae than is being spent in this thread, but in the end they had to go ahead and build the thing. What was demonstrated to be a sound, working Molten Salt Reactor was 75% of what a prototype LFTR needs to be...
mheslep said:As I understand it the MSRE at ORNL used only U liquid salts and they never got to the point of converting Th. Hence it could be said MSRE provided much information on molten salt designs, but that is large stretch from a LFTR, and all of the associated reprocessing.
lftrsuk said:Flibe Energy or one of the other new-start companies will have LFTR hardware operating within 5 years.
Astronuc said:The VVER type fuel assembly/core system (hexagonal or triangular lattice) is apparently good for the Th-fuel cycle.
If they are nuclear professionals I doubt they say things like ORNL's "Molten Salt Reactor was 75% of what a prototype LFTR"lftrsuk said:This thread, with all of the pros and cons of LFTRs is way behind the times. The queries raised have been mulled over by nuclear professionals and conclusions reached on 'Energy from Thorium' and several other websites...
zapperzero said:From drawing board to criticality in five years? Right.
mheslep said:If they are nuclear professionals I doubt they say things like ORNL's "Molten Salt Reactor was 75% of what a prototype LFTR"
mheslep said:I do not see where Flibe Energy makes any five year claims, nor any connection at all with the US Military.
mheslep said:Can such a moderator be replaced without replacing essentially the entire reactor vessel?
zapperzero said:Yes you can. Although, if you fancy fishing stuff out that soup, you're a braver person than I am.
LFTR is, to my mind, a profoundly stupid, dangerous idea and so's any other liquid fluoride salts based scheme. You have a highly corrosive coolant that explodes if it comes in contact with water and burns if it comes in contact with air. Pair that with a burnable moderator. Now imagine what a large-break LOCA looks like.
I could only envision this being safe if it was built on the far side of the moon or something like that, a friendly place that's very cold by default and has no oxygen or water around.
And all that money and brainpower is beng thrown down the drain because lead-moderated, lead-cooled is Not Invented Here. EDIT: and by "here" I mean in the US.
Here, have a peek at the near future.
http://myrrha.sckcen.be/
etudiant said:This is not correct.
Liquid fluoride salts are essentially inert in air. I worked with them.
There is not enough reactivity in oxygen or nitrogen to displace the fluorine from the salt.
zapperzero said:You've worked with uranium tetrafluoride?? Fine. I must have been imagining things. My apologies to one and all.
Can you elaborate as to how? To my knowledge it was never attempted on the MSR reactor back in the 1960s.zapperzero said:Yes you can. Although, if you fancy fishing stuff out that soup, you're a braver person than I am...
mheslep said:Can you elaborate as to how? To my knowledge it was never attempted on the MSR reactor back in the 1960s.
From the website:mheslep said:Though UF4 is toxic, neither the molten salt proposed for the reactor or UF4 alone is explosive in contact with air or water.
http://ibilabs.com/UF4-MSDS.htm
I've been in conversion shops where UF6 is hydrolized to UO2F2 at about 100 C. It also reacts with steam, which is the basis of the 'dry conversion' process. As far as I know, Th fluoride behaves similarly.UF4 can be readily converted to either uranium metal or uranium oxide. UF4 is less stable than the uranium oxides and produces hydrofluoric acid in reaction with water; it is thus a less favorable form for long-term disposal. The bulk density of UF4 varies from about 2.0 g/cm3 to about 4.5 g/cm3 depending on the production process and the properties of the starting uranium compounds.
Chemical Properties
Uranium tetrafluoride (UF4) reacts slowly with moisture at ambient temperature, forming UO2 and HF, which are very corrosive.
That assumes some kind of open top reactor, i.e. solid fuel and water cooled. I don't see how that can be done with a molten salt reactor.zapperzero said:You can build handles or notches into the moderator blocks and move them around with a crane, like they do now with fuel elements. ...
mheslep said:That assumes some kind of open top reactor, i.e. solid fuel and water cooled. I don't see how that can be done with a molten salt reactor.
Astronuc said:Small cores require higher enrichments, and that's problematic with respect to control/kinetics. The shielding would becomes disproportionately large for small cores. I believe that an LFTR is even more complicated because of the need for a feed and bleed system, which is outside the core, and the need to deposit the fission products in some stable form.
etudiant said:Does the LFTR stability depend on the size of the fuel pool?
zapperzero said:Size and geometry. Temperature also matters, indeed, and so does the homogeneity of the mix, which is by no means guaranteed.
Which clearly they do, right? A substantial expansion of the fluid from heat, much less a vaporization, would cause the area to drop below critical. For the salt to boil, an area would have to somehow rise ~971degC above the freeze plug.so preventing excursions, a core requirement, must rely on the thermal effects on reaction rates.
mheslep said:Which clearly they do, right? A substantial expansion of the fluid from heat, much less a vaporization, would cause the area to drop below critical. For the salt to boil, an area would have to somehow rise ~971degC above the freeze plug.
The freeze plug would not be instantaneous, but the coefficient of expansion of the liquid salt is ~instantaneous, and so in turn is the reaction rate which is based on density (negatively).etudiant said:That is the question.
It is not clear to me that a large volume of molten salt would respond quickly to an overtemperature.
Certainly a freeze plug mechanism will take several seconds to work even in a small reactor.
That is an eternity in terms of reaction time.
So the issue is what are the faster acting self limiting elements of the fuel mix and how does this translate to operational management. Is there a risk of prompt excursions in this system?
mheslep said:The freeze plug would not be instantaneous, but the coefficient of expansion of the liquid salt is ~instantaneous, and so in turn is the reaction rate which is based on density (negatively).
etudiant said:Or is there a negative trend as the temperature of the salt rises?
From the original Oak Ridge MSR work, Fluid Fueled Reactors:etudiant said: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?
mheslep said: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/
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 said: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 said: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 said: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 said: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.
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).etudiant said: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.
http://www.webelements.com/protactinium/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.
Protactinium, not Plutonium. A LFTR never gets to any significant amount of Plutonium.etudiant said: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.
