Liquid Fluoride Thorium Reactor

[edpell]

jkiel, yes to all you said.

We will have to wait for the new prototype throium MSR reactors from either China or Flibe Energy in the US.

Stanley, yes we have to filtered out the protactinium before it gets hit by a neutron and heads down the plutonium pathway.

 

[mheslep]

... Fortunately, the very high precision part of the facility, the turbines, only see clean steam from the secondary heat exchanger,

Steam is not necessary, or even two phase systems. Helium for example is a candidate.

 

[lftrsuk]

To avoid the 'knowing look' aimed at conspiracy theorists, IMHO, LFTR advocates should always quote Weinberg's explanation of why MSR research funding ceased, when asked the question - "If they're so good, why don't we have wall-to-wall LFTRs supplying all of our energy?"

He wrote: ... Why didn't the molten-salt system, so elegant and so well thought-out, prevail? I've already given the political reason: that the fast breeder arrived first and was therefore able to consolidate its political position within the AEC. But there was another, more technical reason. The molten-salt technology is entirely different from the technology of any other reactor. To the inexperienced, molten-salt technology is daunting. This certainly seemed to be Milton Shaw's attitude toward molten salts-and he after all was director of reactor development at the AEC during the molten-salt development. Perhaps the moral to be drawn is that a technology that differs too much from an existing technology has not one hurdle to overcome-to demonstrate its feasibility-but another even greater one-to convince influential individuals and organizations who are intellectually and emotionally attached to a different technology that they should adopt the new path. This, the molten-salt system could not do. It was a successful technology that was dropped because it was too different from the main lines of reactor development. But if weaknesses in other systems are eventually revealed, I hope that in a second nuclear era, the molten-salt technology will be resurrected ...

Post Fukushima, we merry band of LFTR advocates hope the 'second nuclear era' is to hand.

 

[mheslep]

Small (kW) LFTRs would be prohibitively expensive, and basically, small fissile systems would be prohibitively expensive from a commercial standpoint due to the security/safety and liability issues. Fissile material is classified as special nuclear material, and there are necessarily stiff regulations regarding control of SNM. Also, compact cores are much more sensitive with respect to control...

While I can see how all these problems apply to a theoretical small PWR, why must this be necessarily so for a LFTR? A small LFTR plant could be installed entirely below ground level, running a Rankine cycle on, say, helium requiring no fill water. The risk of a steam or hydrogen explosion even in the event of a catastrophic accident (e.g. earthquake, flood) could be pushed, it seems to me, to nearly non-existent. While the reactor itself would always contain fissionable materials, there need be almost no transfer of fissionable materials in/out of the plant after start-up, the plant being supplied by fertile materials, a point that must surely reduce security concerns.

 

[mheslep]

... Our experience with large volumes of radioactive molten salts is not huge, but it clearly indicates that everything gets eaten away, valves, pumps, heat exchangers, containers and measuring instruments. ...

[my highlight]

Can anyone comment on or point to data illustrating this claim? Edit: My understanding is that the fluoride salts proposed for LFTR (LiF-BeF2-UF4-ThF4) are chemically very stable, and that the alloy http://en.wikipedia.org/wiki/Hastelloy" (nickel,molybdenum, chromium) should hold up very well in its presence.

 

[Astronuc]

While I can see how all these problems apply to a theoretical small PWR, why must this be necessarily so for a LFTR? A small LFTR plant could be installed entirely below ground level, running a Rankine cycle on, say, helium requiring no fill water. The risk of a steam or hydrogen explosion even in the event of a catastrophic accident (e.g. earthquake, flood) could be pushed, it seems to me, to nearly non-existent. While the reactor itself would always contain fissionable materials, there need be almost no transfer of fissionable materials in/out of the plant after start-up.

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.

From a safety standpoint, it would be better to use a gas dynamic (Brayton) cycle for thermal to mechanical conversion. A steam (Rankine) cycle has the risk of corrosion of the primary heat exchanger and water/fluoride interaction.

In an LFTR, the fission products are transported out of the core and deposited in a processing system. Those will accumulate over the lifetime of the core. I would expect the fission products to be placed in a stable form, which usually implies an oxide, as opposed to fluoride. The waste volume increases with size of the core.

I understand from someone familiar with MSR, that it nearly had a [possibly uncontrolled] criticality (or supercriticality) event, as well as having problems with plugging of piping. I haven't independently verified that however.

 

[mheslep]

From a safety standpoint, it would be better to use a gas dynamic (Brayton) cycle for thermal to mechanical conversion. A steam (Rankine) cycle has the risk of corrosion of the primary heat exchanger and water/fluoride interaction.

Yes, sorry, I meant a Brayton gas cycle of course when talking about helium as medium, thus no water, no steam, etc.

 

[Astronuc]

The LFTR is intriguing, but it's not trivial as some seem to make it.

One challenge to scaling it up is the radial power distribution. In a conventional reactor, one tailors the enrichment radially (different enrichments in different fuel rod) in the assembly, as well as different average enrichments in different groups of assemblies. In addition, one uses batches or groups of assemblies, such that the beginning of a cycle (BOC) each batch has different exposure of 1, 2, and perhaps 3 or 4 cycles, in addition to having a fresh or feed batch. In addition, current LWR fuel rods use lower enrichment blankets at the top and bottom of the axial fuel stack in order to reflect neutrons back into the core. The outer 6 to 8 inches of core, axially and radially, have high flux gradients and significant neutron leakage from the core - if they are highly enriched. So conventional cycle designs put high burnup, low power assemblies in the outer rows of the core.

If the reactor is more or less radially chemically homogenous, then one will find significant peaking toward the center of the core. This would be a disadvantage for a large LFTR.

The other key aspect is getting the heat (and fission products) out of the core. Does the plan call for pre- or post-processing (of fission product removal) before passing the liquid fuel to the primary heat exchanger. If pre-processing is the strategy, then the processing system is operating at high temperature (with potential material degradation issues) and then likely, a cooler fluid is passed to the heat exchanger with lower efficiency the result. On the other hand, if post-processing is the strategy, then the hotter fluid (with fission products) is passed to the primary heat exchanger (great for thermal efficiency), but some fission products may precipitate in the heat exchanger, and some fission products like Cd, Te, and a few others, can cause liquid metal embrittlement and/or corrosion.

 

[mheslep]

The LFTR is intriguing, but it's not trivial as some seem to make it.

One challenge to scaling it up is the radial power distribution. In a conventional reactor, one tailors the enrichment radially (different enrichments in different fuel rod) in the assembly, as well as different average enrichments in different groups of assemblies. In addition, one uses batches or groups of assemblies, such that the beginning of a cycle (BOC) each batch has different exposure of 1, 2, and perhaps 3 or 4 cycles, in addition to having a fresh or feed batch. In addition, current LWR fuel rods use lower enrichment blankets at the top and bottom of the axial fuel stack in order to reflect neutrons back into the core. The outer 6 to 8 inches of core, axially and radially, have high flux gradients and significant neutron leakage from the core - if they are highly enriched. So conventional cycle designs put high burnup, low power assemblies in the outer rows of the core.

If the reactor is more or less radially chemically homogenous, then one will find significant peaking toward the center of the core. This would be a disadvantage for a large LFTR. [...]

I think the above represents thinking still in terms of a solid core, pressurized reactor and not a liquid one as in a LFTR where the molten salt is continually in motion. The concern for temperature in a low (or no) pressure reactor, it seems to me, would mainly be the at the outside wall. Edit: I'm reminded that molten salt has a strong negative temp. coefficient which would work against heat gradients.

 

[Astronuc]

I think the above represents thinking still in terms of a solid core, pressurized reactor and not a liquid one as in a LFTR where the molten salt is continually in motion. The concern for temperature in a low (or no) pressure reactor, it seems to me, would mainly be the at the outside wall. Edit: I'm reminded that molten salt has a strong negative temp. coefficient which would work against heat gradients.

Well, there is ρgh, and the ΔP around the circuit.

Strong negative temperature coefficient of what?

A relatively high thermal conductivity minimizes a thermal gradient.

 

[mheslep]

Well, there is ρgh, and the ΔP around the circuit.

Yes, but never approaching anything like the ~2200 PSI of the primary loop in a PWR.

Strong negative temperature coefficient of what?

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

 

[etudiant]

I seem to remember reading that the LFTR prototype had one excursion that could have become quite serious, but do not have any details.
There has been a fair amount of discussion about the need to monitor the the fuel composition, as the reaction is rather more dynamic, with thorium getting transmuted to U 233, in a fluid matrix.
Is there a good reference on these topics, hopefully somewhat more concise than the rather weighty tome mentioned above?

 

[Astronuc]

I seem to remember reading that the LFTR prototype had one excursion that could have become quite serious, but do not have any details.
There has been a fair amount of discussion about the need to monitor the the fuel composition, as the reaction is rather more dynamic, with thorium getting transmuted to U 233, in a fluid matrix.
Is there a good reference on these topics, hopefully somewhat more concise than the rather weighty tome mentioned above?

Possibly here - http://www.gen-4.org/Technology/systems/msr.htm

The Molten Salt Reactor (MSR) system produces fission power from a molten salt fuel circulating in a fast or epithermal-spectrum reactor and contains an integrated fuel cycle.

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.

 

[Stanley514]

"If they're so good, why don't we have wall-to-wall LFTRs supplying all of our energy?"

I`ve read that necessity to reprocess constantly fuel chemically right on site
could possibly make it uneconomical.

 

[mheslep]

I seem to remember reading that the LFTR prototype had one excursion that could have become quite serious, but do not have any details.
There has been a fair amount of discussion about the need to monitor the the fuel composition, as the reaction is rather more dynamic, with thorium getting transmuted to U 233, in a fluid matrix.
Is there a good reference on these topics, hopefully somewhat more concise than the rather weighty tome mentioned above?

The aging Fluid Fueled Rectors is the bible of the LFTR crowd. I am unaware of anything more concise.

 

[mheslep]

...

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.

Link:
http://lpsc.in2p3.fr/gpr/gpr/publis-rsf/Article-NuclScienceEng49-07.pdf

 

[Astronuc]

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

Link:
http://lpsc.in2p3.fr/gpr/gpr/publis-rsf/Article-NuclScienceEng49-07.pdf

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.

Nevertheless, because reprocessing and partitioning of actinides (and transuranics if U-235 is used in the early stage of operation) and fission products is necessary, then this makes a small core rather uneconomical. Instead, LFTRs seem to be limited to nuclear plant operation. It would seem feasible to do a modular system with a common processing facility for the fission products.

Interestingly, the modern (Gen-IV) MSR designs seem to favor no graphite.

 

[mheslep]

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.
...

Thanks for looking. I'm interested in exploring the details.

Yes, the graphite moderator coeff. is positive (+1.6e-5), as is the fertile fuel coeff (+2e-5), but http://energyfromthorium.com/2006/08/20/comparing-the-temperature-coefficients-of-two-fluid-and-one-fluid-lfrs/" from the ORNL work, the fuel salt coeff is -8e-5 in the best case LFTR design, giving an overall -4e-5. I have not dug into the French paper enough yet to determine the difference in viewpoint, but I believe the LFTR "two fluid" approach, i.e. separate fertile and fuel streams, is the one yielding the large negative coeff. of reactivity.

Another more expensive option would be to use heavy water as the moderator.

 

[Astronuc]

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.

 

[mheslep]

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.

Yes I noticed, but have no idea why polygonal shapes are preferred.

 

[Astronuc]

Yes I noticed, but have no idea why polygonal shapes are preferred.

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.

 

[Astronuc]

FYI - Highlights of the Thorium Energy Conference - ThEC11

The 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.

http://itheo.org/thorium-energy-conference-2011

 

[mheslep]

FYI - Highlights of the Thorium Energy Conference - ThEC11

http://itheo.org/thorium-energy-conference-2011

Thanks.

I immediately went the LFTR report from Gehin of ORNL. As the original test reactor back in the 60's never did the fission product chemical removal step it seems to me that would be the major technical risk area. However, as it is a chemical step, I would think a prototype could be stood up that proves out most of the design with no radioactive isotopes, i.e. run the separator with U238 not 233, Cs133 not 137 and so on.

 

[mheslep]

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.

 

[Astronuc]

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.

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.

 

[lftrsuk]

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.

In these days of computer modelling and cad/cam we could have the first-of-a-kind LFTR up and running in 5 years and I feel confident that enough 'learning-curve' can be 'gone-through' to have a modular design ready for production 5 years after that.

I have no qualms in trying to campaign for UK manufacture of LFTRs and if any of you feel so inclined, make it happen here by voting on 38Degrees, the Campaigning Website and search for "UK manufacture of Liquid Fluoride Thorium Reactors".

Alternatively, sign the e-petition on the HM Government website. Google: “HM Government e-petition", put 'thorium' in the search and 'View' "Save £50 billion..."

 

[mheslep]

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...

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]

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.

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.

Flibe Energy or one of the other new-start companies will have LFTR hardware operating within 5 years and the technology will not even be on the radar of the UK Government. Neither the newly launched NRC or the NNL will have thorium on the agenda for consideration in the UK's nuclear future.

 

[zapperzero]

Flibe Energy or one of the other new-start companies will have LFTR hardware operating within 5 years.

From drawing board to criticality in five years? Right.

 

[Rive]

The VVER type fuel assembly/core system (hexagonal or triangular lattice) is apparently good for the Th-fuel cycle.

Some times ago I've seen a PDF about the irradiation tests of a mixed Th fuel, to be used in existing VVER reactors. It was from 2009, IIRC.

 

[mheslep]

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...

If they are nuclear professionals I doubt they say things like ORNL's "Molten Salt Reactor was 75% of what a prototype LFTR"

 

[lftrsuk]

From drawing board to criticality in five years? Right.

That's what Flibe Energy say (through the US Military). Try their website: http://flibe-energy.com/

 

[lftrsuk]

If they are nuclear professionals I doubt they say things like ORNL's "Molten Salt Reactor was 75% of what a prototype LFTR"

No, the 75% was me; a humble, but optimistic, manufacturing engineer (retired).

However, on Energy from Thorium forums: http://www.energyfromthorium.com/forum/ there is a General Discussion Forum, for the likes of me - but, supported by comments from nuclear professionals such as Professor Per Peterson and Dr David Le Blanc. Maybe for you and other contributors to this thread, the Forum: Fluoride Reactor Design will be more informative; it has 78 Threads running at the moment which has elicited 1873 Comments.

 

[mheslep]

I do not see where Flibe Energy makes any five year claims, nor any connection at all with the US Military.

 

[lftrsuk]

I do not see where Flibe Energy makes any five year claims, nor any connection at all with the US Military.

If you fish around you'll find it; Kirk Sorensen says it on one or more of his videos.

Also, have a look at this: http://www.orlygroup.com/secondary_revenue_streams.html

And this: http://atomicinsights.com/2011/11/tedx-new-england-nuclear-entrepreneurs-aiming-to-use-waste-for-fuel.html#comment-12784

Something's going to happen in the next 5 years - sad to say, it's not likely to be here in the UK as there are too many 'What Ifs' in the air, combined with zero experience and zero political vision.