Energy From Thorium Discussion Forum

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PostPosted: Aug 18, 2008 11:39 am 
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Hello

I have been reading this site avidly. I am clear that Kirk favors Liquid Chloride reactors for transmuting atomic "waste". He favors Liquid Fluoride for large scale production of nuclear POWER (correction: originally said "waste").

Can someone explain to me, in laymans terms, why this is so? I don't have a background in nuclear physics but am very interested in the subject.

Is there a reason why we can't just put Plutonium or other long lived materials in a Fluoride reactor? What would drive the decision between building a plant of one type or the other? And in an ideal world would we want to have plants of both types?

PS. Understanding these difference are critical. When talking to anti-nuclear people who are concerned about nuclear waste, they might be willing to consider a Chloride reactor if it meant that the nuclear waste issue would be drastically reduced. For people concerned with global warming they might be interested in the Fluoride reactor as it means we could, possibly, retire coal plants and reduce greenhouse gases. In other words, what to say depends greatly on the audience.

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Last edited by georgefc3 on Aug 18, 2008 4:18 pm, edited 1 time in total.

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PostPosted: Aug 18, 2008 3:40 pm 
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georgefc3 wrote:
I have been reading this site avidly. I am clear that Kirk favors Liquid Chloride reactors for transmuting atomic "waste". He favors Liquid Fluoride for large scale production of nuclear waste.


He favors Liquid Fluoride for large scale production of nuclear Power.


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PostPosted: Aug 18, 2008 3:42 pm 
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Liquid Chloride reactors are being looked at to burn EXISTING nuclear waste.

Liquid Fluoride reactors can drastically reduce FUTURE nuclear waste production as compared to LWR's.


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PostPosted: Aug 18, 2008 4:20 pm 
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Great

I updated my post - I said waste when I meant power as you indicated.

Thanks for responding!!!

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PostPosted: Aug 18, 2008 5:00 pm 
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georgefc3 wrote:
Hello

I have been reading this site avidly. I am clear that Kirk favors Liquid Chloride reactors for transmuting atomic "waste". He favors Liquid Fluoride for large scale production of nuclear POWER (correction: originally said "waste").

Can someone explain to me, in laymans terms, why this is so? I don't have a background in nuclear physics but am very interested in the subject.

Is there a reason why we can't just put Plutonium or other long lived materials in a Fluoride reactor? What would drive the decision between building a plant of one type or the other? And in an ideal world would we want to have plants of both types?


We cant just burn plutonium in a liquid fluoride reactor for several reasons. First because plutonium is less soluable in fluoride salts like FLiBe than chloride salts, so unless you keep the plutonium concentration low you run into problems (not sure if it plates out or what, but any condensation of plutonium goo inside the reactor would be bad for reactivity control.) The other is that plutoniums thermal fission cross section isn't that much higher than its capture cross section (when compared to U233), so you run into problems with neutron economy in a thermal reactor with plutonium as the primary fuel. You can add a little plutonium and other actinides to a fluoride reactor, but you just cant run the whole reactor on transuranics.

Chloride reactors are excellent for burning transurancs however. More actinides are more soluable in chloride salts, and it offers the hardest neutron spectrum you can get in a critical reactor that I know of. The breeding ratio is especially high so you have plenty of neutrons to spare for whatever purpose you want. Transuranics at these neutron energys just break apart and so you waste far fewer neutrons.

The reason why we prefer thermal fluoride reactors to fast reactors are several however. First because of the very high breeding ratio chloride reactors represent a political problem in that they are very good for producing weapons material, where fluoride reactors have a barely break even breeding ratio. If you try to make weapons material from a fluoride thermal reactor you run out of fuel to run the reactor.

Another reason we prefer fluoride reactors is that they are much more mature. We've prototyped them, we have very detailed understanding of what structural materials are compatable with fluorides, where with chlorides we dont necissarily have a mature understanding of how to deal with many of the structural issues like corrosion of different structural materials at high temperatures.

And the final reason we prefer fluorides to chlorides is thermal reactors are inherently safer than fast reactors. I have no doubt that a well engineered liquid chloride reactor could be far safer than light water reactors, but one of the sticking points of fast reactors is they have a very low delayed neutron component. The delayed neutron fraction in reactors is the percentage of neutrons that are produced several minutes after an atom fissions by one of the daughter products. At thermal energies this percentage is rather large, and this means that the reactivity of the reactor swings one way or the other rather slowly. At fast neutron energies the atoms are just smashed apart and the delayed neutron component is low, so the reactivity can swing very fast one way or the other. This means if you have a poorly designed fast reactor, you can have a criticality excursion happen before you can SCRAM, and then you have to figure out how to dump several tens to hundreds of gigawatts fast before it shuts down.

As a note, I dont have a degree in nuclear engineering either, so one of the more experienced members of the forum can feel free to correct me, but these are the issues as I understand them.


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PostPosted: Aug 18, 2008 6:01 pm 
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The previous post assumes the choice is between thermal fluoride and fast chloride reactors. One can also build semi-fast fluoride reactors that have some of the advantages and disadvantages of each. The French group is following this line (they call it a non-moderated reactor). Similarly, David on this forum, advocates a non-moderated reactor.

A cloride based reactor would definitely have a harder (faster) spectrum than any fluoride based design concept I know of. Definitely, the harder the spectrum the higher the probability that a neutron capture will result in a fission. So a cloride reactor would be more neutron efficient at burning the minor actinides (and for that matter everything else too).

It will take a significant investment to work out all the details for using cloride salts in nuclear reactors. Whether and when we should invest in working out all the details with cloride salts for nuclear reactors is an open question. I think the consensus is that we should do fluoride first.

The french believe you can put Pu (and actually all the material heavier than U in the spent nuclear fuel) directly into a fluoride reactor to run it. The thought is to start with spent nuclear fuel transuranics (TRU) and breed U233 for running the reactor. After running the reactor on U233 for 80 years or so the heavy atoms do get burned up down to a small residual level.

There appears to be many ways to approach this problem.


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PostPosted: Aug 19, 2008 10:01 am 
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After 80 years? How much waste are we talking about? I mean, can, say, a chloride run "LCTR" burn the entire waste stream of one..or more...LWRs?

David

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PostPosted: Aug 19, 2008 11:29 am 
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Dezakin

Thank you for your highly detailed response. To summarize:

* The Liquid Fluoride reactor is the preferred design for producing large amounts of POWER
* The Liquid Fluoride reactor is inherently safer than the Liquid Chloride reactor
* The Liquid Fluoride reactor has been built and is more of a known quantity than the Liquid Chloride reactor
* The Liquid Fluoride reactor does not have the ability to transmute large amounts of Plutonium and other transuranic elements (actinides)
* Liquid Chloride reactors are safer than the current generation of nuclear power plants but not as safe as the Liquid Fluoride reactor
* The safety issue can be mitigated through engineering. That is: a well designed and engineered reactor can make a big difference.
* Liquid Chloride reactors are in a more primitive stage of development.

Does this basically summarize what you just said??

Thanks!

George

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PostPosted: Aug 19, 2008 2:54 pm 
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Yeah, thats pretty much it.


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PostPosted: Aug 19, 2008 3:00 pm 
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dwalters wrote:
After 80 years? How much waste are we talking about? I mean, can, say, a chloride run "LCTR" burn the entire waste stream of one..or more...LWRs?

David

A liquid chloride incinerator would be able to consume roughly 1 tonne of transuranics per GWe year if you have it set up similar to a LFTR, roughly the annual output of 2 1GW LWRs from what I understand.


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PostPosted: Aug 19, 2008 3:44 pm 
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Could a Chloride reactor burn depleted Uranium (U-238) as well as Plutonium, etc? Would it be desirable for any reason to remove this from the mix?

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PostPosted: Aug 19, 2008 4:14 pm 
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georgefc3 wrote:
Could a Chloride reactor burn depleted Uranium (U-238) as well as Plutonium, etc? Would it be desirable for any reason to remove this from the mix?

A liquid chloride reactor could only burn depleted uranium by turning it into plutonium first. They would make excellent breeders and also be very good at turning depleted uranium into high quality Pu-239. I cant see any reason why you would work hard to incinerate depleted uranium, and frankly theres too much of it to burn anyways.


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PostPosted: Aug 19, 2008 4:28 pm 
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dezakin wrote:
georgefc3 wrote:
Could a Chloride reactor burn depleted Uranium (U-238) as well as Plutonium, etc? Would it be desirable for any reason to remove this from the mix?

A liquid chloride reactor could only burn depleted uranium by turning it into plutonium first. They would make excellent breeders and also be very good at turning depleted uranium into high quality Pu-239. I cant see any reason why you would work hard to incinerate depleted uranium, and frankly theres too much of it to burn anyways.


My thought was that a Chloride reactor would be very expensive to build. If we ran out of Plutonium the reactor might have to be shut down for a lack of fuel.

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PostPosted: Aug 19, 2008 5:19 pm 
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With hundreds of LWRs globally and 50 years worth of spent fuel, a liquid chloride reactor is in no danger of running into a fuel shortage.


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PostPosted: Aug 19, 2008 5:46 pm 
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An LWR produces around 260kG of transuranics per GWe-yr.

Any reactor takes around 1000 kG (1 t) of fissile material for 1 GWe-yr (more or less depending on the efficiency of the generator). Roughly speaking a reactor running on transuranics can consume the output of four LWR's - if that is the main goal. Chloride reactors will do better at quickly consuming all the transuranics whereas a fluoride one will take a longish time running on clean U233 to finally finish consuming the higher actinides. This is a direct result of the softer neutron spectrum.

So one could build chloride reactors to do cleanup duty jointly with LWR's and use fluoride reactors to generate volumes of electricity. Or start fluoride reactors using transuranics and then run them on U233 to cleanup (it takes ALOT of LWR transuranics to startup 50-60 GWe-yrs worth for each fluoride reactor). It is a trade of development money (for the chloride reactor) versus patience (80 years to get the cleanup done - this means we transfer the salt from one aging flouride reactor to a new one since the targeted lifetime is around 60 years).

A very complicated trade.

If we do create a chloride reactor we need to be sure it does a good job of converting fissile transuranics to fissile U233. The shortage is not of fertile material but rather fissile material to startup with.


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