Liquid Fluoride Thorium Reactor

mheslep

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

lftrsuk

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/te...#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.



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

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

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

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

etudiant

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

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

zapperzero

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

From the website: 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

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

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

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

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

etudiant

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?