Liquid Fluoride Thorium Reactor

mheslep

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

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

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

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

This.

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

mheslep

I don't follow. Under positive control an operator removes the fluid from the moderator area (graphite i believe?) and thus stops the reaction. If there's failure of control, the operator stops active cooling of the freeze plug (assuming that has not already happened), again the fluid leaves the moderator area and the reaction stops.

mheslep

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/

etudiant

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

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

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

The LFTR idea is that the U233 is controlled by gassifying the Pa233 stage, removing the breeding wait from the active reaction mass, and then returning it after it becomes U233 as the reactor needs it.

etudiant

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.

Astronuc

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).

http://www.webelements.com/protactinium/

The attraction of the Th-based fuel cycle is the lack of transuranic elements, although some quantity of U-235 or Pu-239 is required to initiate a Th-based system.

wizwom

Protactinium, not Plutonium. A LFTR never gets to any significant amount of Plutonium.
The chain is 232Th->233Th->233Pa->233U->fission
The 233Pa has an absorption cross section about 14 times that of 232Th, so you want to get it out of the way of neutrons if you can, and LFTR does exactly that as the molten salt passes through the flouridizer.

etudiant

Thank you very much, Astronuc and wizwom. Very helpful input.
That even the initial LFTR design prototype included a fairly capable fuel reconditioning element to remove undesirable fission products is entirely logical, but a new wrinkle to me.
It is certainly not a much discussed feature of this class of designs.

zapperzero

I dunno. I harp on it every chance I get. "A reprocessing plant near every power station! La Hague in your own back yard!" etc etc

mheslep

As far as I know most of the reticence about reprocessing comes about from the fact that Plutonium processing goes on with U235 fuel cycles. That's not an issue with a Thorium fuel cycle.