Energy From Thorium Discussion Forum

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PostPosted: Jun 25, 2016 1:52 pm 
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Joined: Nov 14, 2013 7:47 pm
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Location: Iowa, USA
Tim Meyer wrote:
If your proposal is sound, wouldn't the ORNL group had used it?


Of course not. That is because what I propose cannot produce highly depleted lithium on its own. Or, at a minimum, that using neutron bombardment alone is not likely to be economical even though it may be possible.

What I propose is using the neutron bombardment of lithium as the beginning and/or ending process in the production of high purity Li-7 salts. The problem is that high purity Li-7 is very expensive and having too much Li-6 contamination can prevent a MSR from maintaining a reaction. I propose taking advantage of the neutron absorption ability of Li-6 to provide shielding, and therefore getting some Li-7 by-product essentially for free. Alternatively or along with that I propose doing as Dr. LeBlanc has stated as a possibility is to use fuel enriched higher than needed for normal operation as a starter fuel to avoid the need and expense of exceedingly pure Li-7.

If neutron bombardment is used to assist in the depletion of lithium then it may prove more economical than using other means alone. I do not know if it will make Li-7 production more economical but I merely propose that it is possible and I'm discussing how that might be done.

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Disclaimer: I am an engineer but not a nuclear engineer, mechanical engineer, chemical engineer, or industrial engineer. My education included electrical, computer, and software engineering.


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PostPosted: Jun 25, 2016 4:57 pm 
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Joined: Jun 15, 2016 8:17 am
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Kurt Sellner wrote:
Mikeaustria wrote:
First, for the quantities and concentrations needed of the U233 vice the molten FLIBE, what might be the statistics of the dilution reactivity of, say, 1% 6Li/99% 7Li against U233?


I believe many of your questions can be answered by reading this blog entry from our host:
http://energyfromthorium.com/2006/07/11 ... from-them/

I'll try to summarize. The apparent "size" of a nucleus to a moving neutron is measured in a unit called a "barn". Lithium-6 has a cross section of about 940 barns, and thorium-232 has a cross section of about 7 barns. This means a neutron is many times more likely to be captured by Li-6 in the event of a collision than thorium. I'm not quite sure how the math works out on calculating the probabilities exactly but the ratio of Li-6 to Li-7 must be very very small or there will not be enough neutrons to sustain a reaction. Since the number of neutrons produced from a U-233 fission is less than 2.5 that means there are not many neutrons to spare. One of the neutrons produced replaces the one consumed in the fission, one is needed to breed more fuel, and what ever is left is the margin that can be spared to Li-6 capture, loss to the environment, or whatever. As this margin is very small the chemical composition of the fuel must be precise.

Mikeaustria wrote:
Boosting 7Li to, say, 99% vice 99.995% should cost a lot less and make the economics a better fit.

99% purity simply will not work. That is why we are discussing the economy of purity that is 99.99% pure vs. 99.99999% pure. The number of neutrons produced in a fission differs on the element being fissioned. U-233 produces less than 2.5 neutrons per fission but U-235 produces more. I'm not sure how many more but I do not believe it is greater than 3. This means that one can consume some small amount of impurity of LI-6 by starting the LFTR reactor with U-235 instead of U-233. This may make the economics more favorable but at the cost of obtaining high purity U-235 instead of high purity Li-6. Much of the cost of high purity U-235 is because of the regulation of high purity U-235. The differing regulation of U-235 and U-233 is because U-235 is useful for producing weapons while U-233 is not.

Mikeaustria wrote:
Is there a ready market for Tritium in industry, medicine, or other users?

Yes, tritium is used in making nuclear weapons which is why it is a highly regulated material. Tritium is used in small quantities for things like glow in the dark paint for things like watches, the tritium is mixed with a fluorescent material in the paint. I do not believe it has much use in medicine as it is a radiation hazard to anything living.

Mikeaustria wrote:
Could sequestered ' solid state ' forms of Tritium (various metal hydrides perhaps?) be used to safely power an array of RTGs to produce salable electricity for a few years as the Tritium decays?

I'm certain that using tritium in RTGs is technically feasible but there are much better materials for RTGs. Explaining all the pros and cons of tritium and other RTG materials would be a lengthy explanation. Essentially since we have access to much better RTG material the usefulness of tritium in RTGs is zero.

Mikeaustria wrote:
As 7Li is slightly heavier than 6Li, is there any adverse effect on salt viscosity or flow properties at the LFTR temperatures?

Not really. So much of the mass of the salt would be in the uranium and/or thorium that any reductions in mass from using Li-6 vs. Li7 would be lost in a rounding error.


Thank you Kurt! Your answers really helped me with understanding some of the difficulties. I didn't realize that 6Li was such a 'neutron-hog' at 940 barns, some 134 times the Thorium cross section. Yikes! That, plus the Tritium production hazard would certainly seem to make it a no-go. I can see why it would be so inefficient to have even a small amount of it hanging around. So, for my next 'dummies' question, I understand that 7LiF is the preferred salt. How much of a 'drain' on the system would subbing NaF be, or running straight BeF2? I realize that other alkalais, such as KF, would pose a probable disaster if neutron capture resulted in 40K production and its attendant gamma. Is there a Thorium or Uranium solubility issue, perhaps? I understand that there is also a likely problem with other group II metal fluorides, such as CaF2 or MgF2 with possibly high melting points or corrosion-inducing properties. Once again, please pardon my ramble. Just trying to get my head around some of these issues and get a 'feel' for the way forward on the exceptionally promising technology of LFTR!

As an aside, no disrespect intended, has anyone indeed produced a book for lay-people along the lines of a 'LFTR for Dummies'? If something like that could be written and promoted properly to the general public at large (who have been totally frightened out of their wits by blatant misinformation at the mere mention of 'nuclear' ) I would think it would be a great 'plus' to the LFTR story. If no one has started such a project, maybe I will take a crack at it. After all, I totally qualify on the ' dummies' side of the equation! :)
Mike


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