Liquid Fluoride Thorium Reactor

[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.

 

[mheslep]

I did find this TED video interesting from lftrsuk' links above, by a couple of MIT nuke eng's on they are calling the "WaMSR", a molten salt designed to burn spent fuel (UOx?) instead of thorium. Good idea from a political / marketing stand point as it plays on the desire to get rid of nuclear waste.

https://www.youtube.com/watch?v=AAFWeIp8JT0

They address one of the advantages discussed above in this thread: the Zirc Alloy metal cladding used in solid fuel reactors has a short life (4 years tops) which forces replacement and limits burnup, increasing the waste stream. Ok, great. But in an MSR, at some place the critical portion of the salt still has to be contained by some solid material (graphite?), that solid material will undergo a high flux and over time have to be replaced. Is this not moving the problem from one place to another? Perhaps the advantage of MS over solid Zirc rods is that, while the graphite (?) moderator might require replacement, the liquid fuel does not and can continue burn up? Can such a moderator be replaced without replacing essentially the entire reactor vessel?

 

[zapperzero]

Can such a moderator be replaced without replacing essentially the entire reactor vessel?

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]

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/

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.

There is perhaps confusion between liquid fluoride salt cooling and sodium cooling.
The latter does indeed tend to explode on contact with water and does burn or at least oxidize very rapidly, with lots of heat, on contact with air, but molten fluorine salts don't.
Separately, the Soviets did deploy lead bismuth cooled reactors on a nuclear submarines, but found them to be a maintenance headache.

 

[zapperzero]

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.

You've worked with uranium tetrafluoride?? Fine. I must have been imagining things. My apologies to one and all.

 

[etudiant]

You've worked with uranium tetrafluoride?? Fine. I must have been imagining things. My apologies to one and all.

I stand corrected.
Uranium tetrafluoride is indeed nasty stuff, unlike the more stable fluoride salts I've had dealings with.

 

[mheslep]

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

 

[mheslep]

Yes you can. Although, if you fancy fishing stuff out that soup, you're a braver person than I am...

Can you elaborate as to how? To my knowledge it was never attempted on the MSR reactor back in the 1960s.

 

[zapperzero]

Can you elaborate as to how? To my knowledge it was never attempted on the MSR reactor back in the 1960s.

You can build handles or notches into the moderator blocks and move them around with a crane, like they do now with fuel elements. It's relatively easy, mechanically speaking, because you know exactly where they are and you can use sonar if you don't. But what to do with them after you've lifted them out? What if the crane breaks or jams, midway through?

In designs where fuel circulates through channels dug in the moderator, it's "a bit" more complicated.

I don't think graphite would be used, pyrolitic carbon more likely, ideally coated in something that is less porous (although it may get electroplated all by itself, I don't know).

 

[Astronuc]

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

From the website:

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.

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.

 

[mheslep]

You can build handles or notches into the moderator blocks and move them around with a crane, like they do now with fuel elements. ...

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]

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.

Why not? You could have a bucket of molten fluoride salts which keeps hot via fission, with a heat exchanger loop (FLiBe maybe?) running through and pylons made of carbon bricks stacked on top of each other for moderation. You need an inert atmosphere on top, but other than that, what's to keep you from also hanging a crane above the bucket and wrapping the whole package up in a concrete biological shield, like some demonic chocolate egg?

 

[wizwom]

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.

The criticality of a molten salt reactor is controlled by varying the concentration of fissile to moderator, that is, tweaking the k-infinity of the reactor, rather than the control mechanism of a solid element reactor, where you tweak the probability of non-leakage.

Looking at ORNL's report (By L.G. Alexander), though, they are currently steering toward a system where the moderator is separate from the salt; this, of course, is a poor choice. If one uses MgF2 salt as the moderator (about on par with water in moderation) one could do a wholly homogeneous reactor.

To breed, per Lietzke & Stoughton 1957, atom ratios of 17 Mg per Th and 105 F per Th (inclusive) would be needed. This would be a molar ratio of 12.3 MgF2 to 1 UF4.

The scalability issue is that any molten fuel means you are pumping subcritical fissile fuel through your heat exchangers. But if you want to design for higher power, you need larger heat exchangers. The size of each heat exchanger is limited by the need to remain highly subcritical even at your expected highest breeding level. Similarly for pipe size. So, I would imagine a gigawatt range LFTR to have a large number of ~30 cm pipes going to rather small heat exchangers (once-through would be fine, since you don't need to worry about the possibility of over-heating the primary loop). Whether you use the heat exchangers as a NSSS or a brayton cycle heater is immaterial (although a closed Brayton is a definite necessity, there will be fission occurring in all the piping for the molten salt, and thus activation of everything within about a foot of the fluid).

 

[etudiant]

Does the LFTR stability depend on the size of the fuel pool?
It seems logical that a gigawatt unit would be swimming pool sized, so the temperature and the concentration of the fuel might vary materially depending on where in the pool the measurements are taken, even if the fuel is getting pumped past heat exchangers. That seems difficult to control accurately. Is this a concern?
More generally, it is clear after Fukushima that simply meeting a 'design basis' spec is not enough, it is important to have a sense of the possible consequences for a beyond spec accident.
In the case of the various national breeder programs, the accidents that discouraged their proponents were fortunately not catastrophic. The LFTR proponents would enhance their case if they would subject their designs to very critical scrutiny, so that the public gets confidence that hostile eyes have not found cause for alarm.

 

[zapperzero]

Does the LFTR stability depend on the size of the fuel pool?

Size and geometry. Temperature also matters, indeed, and so does the homogeneity of the mix, which is by no means guaranteed.

 

[etudiant]

Size and geometry. Temperature also matters, indeed, and so does the homogeneity of the mix, which is by no means guaranteed.

Thank you for this feedback.
Is it possible to expand on this issue a bit more?
It seems, afaik, a large pool of a 1000*C mixture of thorium fluoride, with substantial amounts of uranium and other transmutation products, where reaction speeds are muted if the temperature rises too much.
Clearly drain plugs are not going to work fast, so preventing excursions, a core requirement, must rely on the thermal effects on reaction rates.
How well proven is that for a range of radionucleide mixtures? Is there a risk of the salt getting vaporized in an excursion?

 

[mheslep]

so preventing excursions, a core requirement, must rely on the thermal effects on reaction rates.

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.

 

[etudiant]

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.

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]

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?

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]

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).

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?

 

[zapperzero]

Or is there a negative trend as the temperature of the salt rises?

This.

Of course this doesn't address the issue of how to actually stop the reaction if you feel like it.