Register to reply

Liquid Fluoride Thorium Reactor

by gcarlin
Tags: fluoride, liquid, reactor, thorium
Share this thread:
etudiant
#73
Nov17-11, 06:38 AM
PF Gold
P: 866
Quote Quote by zapperzero View Post
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
#74
Nov17-11, 07:33 AM
P: 1,045
Quote Quote by etudiant View Post
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
#75
Nov17-11, 08:35 AM
PF Gold
P: 866
Quote Quote by zapperzero View Post
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
#76
Nov17-11, 10:28 AM
PF Gold
P: 3,098
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
#77
Nov17-11, 10:30 AM
PF Gold
P: 3,098
Quote Quote by zapperzero View Post
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
#78
Nov18-11, 02:52 AM
P: 1,045
Quote Quote by mheslep View Post
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
#79
Nov18-11, 06:25 AM
Admin
Astronuc's Avatar
P: 21,909
Quote Quote by mheslep View Post
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
#80
Nov18-11, 08:53 AM
PF Gold
P: 3,098
Quote Quote by zapperzero View Post
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
#81
Nov18-11, 10:11 AM
P: 1,045
Quote Quote by mheslep View Post
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
#82
Feb26-12, 08:31 AM
P: 71
Quote Quote by Astronuc View Post
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
#83
Feb26-12, 11:07 AM
PF Gold
P: 866
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
#84
Feb27-12, 07:30 AM
P: 1,045
Quote Quote by etudiant View Post
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
#85
Feb27-12, 08:05 AM
PF Gold
P: 866
Quote Quote by zapperzero View Post
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
#86
Feb27-12, 09:44 AM
PF Gold
P: 3,098
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
#87
Feb27-12, 10:06 AM
PF Gold
P: 866
Quote Quote by mheslep View Post
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
#88
Feb27-12, 10:28 AM
PF Gold
P: 3,098
Quote Quote by etudiant View Post
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
#89
Feb27-12, 10:56 AM
PF Gold
P: 866
Quote Quote by mheslep View Post
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
#90
Feb27-12, 11:04 AM
P: 1,045
Quote Quote by etudiant View Post
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.


Register to reply

Related Discussions
Chlorine and fluoride filters General Discussion 2
Polyvinylidene Fluoride (PVDF) Chemistry 1
Liquid-fuel molten salt reactor? Nuclear Engineering 4
Thorium Reactor Nuclear Engineering 3
Fluoride in drinking water General Discussion 14