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PostPosted: Apr 28, 2011 2:28 pm 
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Hi,

This is my first post on this forum so I hope I have the right forum section. I've been interested in the LFTR concept and know some of the basics so when people are suggesting future power sources on Internet forums I always suggest LFTR. I replied in a forum on an article trashing wind power here (typical of the anti-green energy pro-gas fracking website) that happens to mention LFTR I got a reply to a post and need help addressing the issues raised.

I'd like a reply to Andydaws post which goes like this:
Quote:
A thorium-u233 makes roughly the same quantities of waste as a "classic" u 238-Pu239/240 cycle - thorium protagonists have a nasty habit of failing to compare like with like - they compare a "once through" cycle for uranium fuelled plant with a recycling-based approach for the thorium plant.

I'll declare my hand - I think the Thorium-U233 cycle has potential - however, it's being chronically oversold by the enthusiasts, mostly relying on a pretty poor understanding of the issues by those they're selling to. It's one of several potential routes to a closed cycle for nuclear fuel, but by no means the most promising.

Let's be clear - making a thorium cycle work is hard, even if you use a throrium cycle in "conventional" (and that's stretching the normal use of the word) fast reactors. More usually, the enthusiasts go further - they argue for a cycle based on thermal breeding in a molten-salt system.

The neutron economy of such a system is utterly marginal - a 1% variation in the ability to extract fission products from the salt makes the difference between it producing surplus fuel, and needing continual top-ups.

Worse, making a molten salt system works requires, a large scale and complex chemical processing plant to be added on to a reactor. A Molten Salt Breeder Reactor (MSBR) won't work if you let just a few percent of the siffion products (like Xenon) stay in the fuel - you need 95% plus efficient extraction on every circuit of the fuel. Worse, you HAVE to get out 90% or more of the intermediate between thorium and uranium on every cycle - protactinium. And to get that out will involve delights like passing 800C flouride-uranium salts through a column of molten bismuth, then extracting the protactinium frojm the bismuth somehow.

I wish the people who latch onto this, like ducklings following a rolling ball, would bother to think through the engineering needed for such a system. Here's one for the enthusiasts - once the uranium is "bred" in the fuel/salt mixture (and leaving aside the delights of managing two such circuits, one for the fuel, and one for the breeder blanket), you have to get it out. And to do that, you have to bubble flourine - that well known non-reactive and benign gas - through that same 800C molten uranium salt, then capture the resulting uranium hexaflouride. "Hex" is not only "hot" both thermally, and radiologically, but it's venomously corrosive - the separation membranes in enrichment plants have to be made of pure (99.9% plus) nickel to withstand it, and even then last only a few years.

And yes, it has virtues - thorium abundance, and potentially, it can be "drained down" in an accident. But that still means you have to remove decay heat from a couple of thousand tonnes of fuel mixture (more than in a conventional reactor), and have secure cooling and storage for the chemical plant and fission product inventory.

As I said, it might have potential - but compared to something like a lead-cooled fast reactor, which have already been built in considerable numbers - the Soviets used them to power the "Alfa" class subs - doesn it look like an obvious route? Hardly...

He is someone who seems to know a lot about energy in general and is pro-nuclear on a lot of other posts that I have seem from him so I want to address the issues he raises and nail some of the negativity about LFTRs.

Now I know the Xenon should bubble out but is close to 100% extracted this way?
Where are we on Protactinium extraction - is the sort of approach outlined really the likely one taken?
How do you passively cool such a large amount of fuel if it is dumped out of the reactor?

Hope you guys can help me to come up with a good reply.

--
bamalam


Last edited by bamalam on Apr 29, 2011 11:58 am, edited 1 time in total.

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PostPosted: Apr 28, 2011 4:58 pm 
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Hi, and welcome! I'll try to answer some of the points, I'm sure others will be along shortly.

Andydaws wrote:
A thorium-u233 makes roughly the same quantities of waste as a "classic" u 238-Pu239/240 cycle...
If he means fission product waste, this is true. However, while the fission products are very nasty on a short to medium time scale, they only need to be isolated from the environment for ~500 years. This is a long time by normal standards, but it is not a major engineering challenge. There are many structures that old, built with primitive techniques and still intact. The long term nuclear waste problem is the transuranium elements. These are the materials that require isolation for hundreds of thousands of years, and drive the requirements for geological storage. The U-238/Pu-239 cycle uses Pu by the tonne, and inevitably makes substantial quantities of the other actinides. The Th-232/U-233 starts six mass units lower, and isn't using a transuranic as the main fuel. Transuranic content is only a few % of the level in a U/Pu cycle reactor. That is still enough to be a problem, and require substantial effort to keep them out of the waste as much as possible, but we start from a better place.

Andydaws wrote:
......they argue for a cycle based on thermal breeding in a molten-salt system.

The neutron economy of such a system is utterly marginal - a 1% variation in the ability to extract fission products from the salt makes the difference between it producing surplus fuel, and needing continual top-ups.....
More like 5% than 1%. We don't really need high breeding gain, there is plenty of uranium to start all the reactors we need, provided that they then run on thorium. Conversion ratio of barely over 1 is fine. That gives enough margin to use simpler reprocessing than was proposed for the MSBR design in the 60's

Andydaws wrote:
..... you HAVE to get out 90% or more of the intermediate between thorium and uranium on every cycle - protactinium.....
No, if you aren't desperate for breeding gain, there is enough margin to allow this step to be omitted.

Andydaws wrote:
.....once the uranium is "bred" in the fuel/salt mixture (and leaving aside the delights of managing two such circuits, one for the fuel, and one for the breeder blanket), you have to get it out. And to do that, you have to bubble flourine - that well known non-reactive and benign gas - through that same 800C molten uranium salt, then capture the resulting uranium hexaflouride. "Hex" is not only "hot" both thermally, and radiologically, but it's venomously corrosive .....
Aside from perhaps not being necessary anyway, the required processes were all demonstrated at ORNL 40+ years ago. When they tried to get the same process to work with plutonium instead of uranium, they did have serious problems, but for U it is doable.

Andydaws wrote:
......And yes, it has virtues - thorium abundance, and potentially, it can be "drained down" in an accident. But that still means you have to remove decay heat from a couple of thousand tonnes of fuel mixture.....
Too many zeros. More like 100 tonnes of fuel mix per 1 gigawatt plant, a bit less than conventional reactors. All reactors have the same decay heat problem, but it is easier in MSRs because the fuel will move (convect) passively, so no local hot spots.


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PostPosted: Apr 28, 2011 5:39 pm 
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Luke wrote:

Andydaws wrote:
.....once the uranium is "bred" in the fuel/salt mixture (and leaving aside the delights of managing two such circuits, one for the fuel, and one for the breeder blanket), you have to get it out. And to do that, you have to bubble flourine - that well known non-reactive and benign gas - through that same 800C molten uranium salt, then capture the resulting uranium hexaflouride. "Hex" is not only "hot" both thermally, and radiologically, but it's venomously corrosive .....
Aside from perhaps not being necessary anyway, the required processes were all demonstrated at ORNL 40+ years ago. When they tried to get the same process to work with plutonium instead of uranium, they did have serious problems, but for U it is doable.


Note that MSRE did demonstrate the fluornators, but there is still work to do. Certainly it is possible to use pure nickel and this is what Rez in the Czech Republic is using for their flame fluorination of uranium. Two approaches were proposed in the MSBR program for the fluorinators, a falling-drop type and a frozen wall type. The frozen wall fluorinator is probably the preferred approach. The use of frozen walls for corrosion protection is much more mature now than it was back in the 1960s when ORNL was working on the MSBR. I don't recall the fluorinator design for the MSRE design, but certainly it was not designed for long-term operation. Therefore, technology development work on fluorinator technology is needed.

Let me add that, while having a negative tone, the comments by this person are not far off. There is a significant amount of technology development required to bridge the gap from the ORNL experimental reactor to a production capability. Let me just remind you all that an MSR has never operated with thorium and the full chemical processing has not been demonstrated at other than small scales. So, while the fundamental concept has been demonstrated and the concept hold promise and significant potential, there are remaining technical challenges.


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PostPosted: Apr 29, 2011 11:18 am 
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Thanks for the help. I've posted some response to the issues raised in the forum I was referring to using the information supplied here.

There was more in another post I missed from Andydaws about why people are so enthusiastic in supporting LFTR:
Quote:
..... Similarly, the argument that the reactor doesn't contain such a large inventory of fission products. True, in part - if it's used as a breeder the fission products MUST be removed, as neutron economiy is marginal at best. But, they're still there on site, needing highly secure storage and cooling, just like spent fuel ponds. And there's a whole additional set of vulnerabilities introduced by having the processing facilitites to remove them - having a large inventory of extremely reactive flourine, for example, or needing to process extremely hot (in both sense) fuel through other extremely hot materials like bismuth.

The worst habit, though, and one that many have fallen for, is the trick of comparing dissimilar fuel-cycles. They compare the waste generation and fuel utilisation of a molten salt design requiring immediate, on site reprocessing to remove poisons and nascent fuel, with LWRs running a "once trough" fuel cyccle. The appropriate comparison would be LWRs/breeders running a classic reprocessing cycle. And if you allow for the fact that "classic" reprocessing lets you allow 10 or 20 years of decay between taking the fuel from the reactor, versus having to do it straight away in an MSBR, which makes handling much easier.

MSBR and the thorium cycle are concepts with a lot of potential - but kidding yourself that thatey're some sort of deus ex machina is naive.

Further help needed with addressing some of the issues above needed please!!

However his points raise my own questions in relation to the fission products. I seem to remember somewhere a diagram where Kirk seems to show a small volume is needed in the commercial LFTR plant for this reprocessing. Is this true?
What sort of temporary store is needed for the high-level waste that immediately comes out of the reactor? Isn't it going to be in more concentrated high gamma/beta decay form than the spent fuel rods of a typical LWR reactor (which can be stuffed in simple cooling ponds for years and contain more than fission products)?

Links to docs on this would be useful. Thanks in advance.

--
bamalam


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PostPosted: Dec 17, 2011 12:22 am 
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Great work! I appreciated your effort and I'm so glad that you shared this to us. I will definitely look forward for your next post to learn more things from you.

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PostPosted: Dec 17, 2011 12:55 am 
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Quote:
..... Similarly, the argument that the reactor doesn't contain such a large inventory of fission products. True, in part - if it's used as a breeder the fission products MUST be removed, as neutron economy is marginal at best.

Removal of the off-gases has been proven and is comparatively easy. These dominate the neutron absorption by fission products. If your goal is breeding it is true that a thorium cycle is inferior to a 238U cycle in a fast reactor but if you are satisfied with isobreeding there is enough margin to allow choices to reduce costs (like skipping protactinium separation if the neutron spectrum is fastish).
Quote:
But, they're still there on site, needing highly secure storage and cooling, just like spent fuel ponds.

He has a point here. Something that Jaro and I do bring up from time to time. One difference is that the fluoride fission products are in a form that can be reliably stored at high temperature (600C) and it is much easier to dissipate heat from 600C than from 50C. But the heat is intense enough that we do have to insert various wait stages in the processing to allow the fission products to cool down.

Quote:
And there's a whole additional set of vulnerabilities introduced by having the processing facilitites to remove them - having a large inventory of extremely reactive flourine, for example, or needing to process extremely hot (in both sense) fuel through other extremely hot materials like bismuth.

I don't like bismuth in the processing. I'd prefer aluminum. But it is not at all clear that either will be present at the reactor site. The local processing may be limited to fluorination and vacuum distillation both for cost and security reasons.

Quote:
The worst habit, though, and one that many have fallen for, is the trick of comparing dissimilar fuel-cycles. They compare the waste generation and fuel utilisation of a molten salt design requiring immediate, on site reprocessing to remove poisons and nascent fuel, with LWRs running a "once trough" fuel cycle. The appropriate comparison would be LWRs/breeders running a classic reprocessing cycle. And if you allow for the fact that "classic" reprocessing lets you allow 10 or 20 years of decay between taking the fuel from the reactor, versus having to do it straight away in an MSBR, which makes handling much easier.

I think both comparisons are valid. You can compare against the status quo to say how much better LFTR is than our current practices (and the most likely future practices). You also should compare against alternative future technologies including IFR and TWR.

Quote:
I seem to remember somewhere a diagram where Kirk seems to show a small volume is needed in the commercial LFTR plant for this reprocessing. Is this true?

The processing rate required - if you do not separate protactinium - is pretty slow so that you don't need a huge machine to do the job. The experimental vacuum distiller from MSRE is sufficient for a 1GWe reactor if it is run continuously. The processing plant is mostly small. I'm thinking it would fit into a living room with a double high ceiling. However, there are some large items including 1) storing the offgas for an hour or so to let it cool (almost 20MW!), 2) carbon filtration system to trap the Xe and Kr for decay periods, 3) the fission product holding tank.
I don't imagine the size of the fission product holding area is a cost driver. I'd imagine it is primarily dealing with the heat given off which should be pretty similar to an LWR. Check out ORNL 4541 from the pdf repository for a pretty good description of what ORNL planned.


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PostPosted: Dec 20, 2011 1:34 am 
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Bismuth is not an "extremely hot" molten metal. Bismuth is a low-melting material, mp = 270C, less than lead.

Also, handling molten fluorides is done every day, at any aluminum smelter. Sure, these are different materials, with different problems, most especially radioactivity, but this is a well-known tech.

And even if we don't get 100% isobreeding, so what? Topping off isn't really a game changer. It's just throwing in some HEU or Pu every now and then. We have recovered Pu from LWR spent fuel, and both Pu and HEU can be easily gotten from surplus bomb fissile. For each 1% we can't breed, that's just 10 kg fissile/yr we have to find elsewhere, far less than any LWR.


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PostPosted: Dec 20, 2011 5:03 am 
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Mike wrote:
And even if we don't get 100% isobreeding, so what? Topping off isn't really a game changer. It's just throwing in some HEU or Pu every now and then. We have recovered Pu from LWR spent fuel, and both Pu and HEU can be easily gotten from surplus bomb fissile.

That's where things can get awkward if you are committed to running a proliferation resistant fuel cycle. You can't ship separated Pu and HEU around as you need it, things are more complicated than that. If you haven't read up on the ORNL's DMSR design and it's fuel cycle I'd recommend that you take a look at ORNL-TM-7207 Concept Design of a DMSR.

At a high level these options sound quite workable, but as you get into some of the detail, the number of workable options shrinks. In the case of ORNL-TM-7207 they were proposing minimal processing on site, no separation on site for fissile material and shipments of fuel only ever as denatured U235 (as 20% U235, 80% U238).


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PostPosted: Dec 20, 2011 11:09 am 
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There's weapons grade plutonium and there's reactor grade plutonium. Shipping reactor grade plutonium is not a proliferation hazard. Even more so than shipping 20% LEU since with LEU you can still upgrade isotopically (not possible for Pu).


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PostPosted: Dec 20, 2011 12:29 pm 
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Cyril R wrote:
There's weapons grade plutonium and there's reactor grade plutonium. Shipping reactor grade plutonium is not a proliferation hazard. Even more so than shipping 20% LEU since with LEU you can still upgrade isotopically (not possible for Pu).
Re Pu this seems to be a classic area where there is a gap between handling restrictions and actual usefulness of the material for weapons purposes. As I understand it based on someone else's post, only 80%+ Pu238 is free from heavy duty fissile security arrangements. So while I agree that well burned reactor grade Pu is a poor and unattracive material for any would be bomb maker, the regs remain restrictive on shipping, etc, so separated Pu is probably not a good fuel in a logistics sense for topup fuel in a running reactor. There is also the public perception/opinion issues that come with separating and shipping separated Pu.


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PostPosted: Dec 21, 2011 6:58 am 
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Lindsay wrote:
Cyril R wrote:
There's weapons grade plutonium and there's reactor grade plutonium. Shipping reactor grade plutonium is not a proliferation hazard. Even more so than shipping 20% LEU since with LEU you can still upgrade isotopically (not possible for Pu).
Re Pu this seems to be a classic area where there is a gap between handling restrictions and actual usefulness of the material for weapons purposes. As I understand it based on someone else's post, only 80%+ Pu238 is free from heavy duty fissile security arrangements. So while I agree that well burned reactor grade Pu is a poor and unattracive material for any would be bomb maker, the regs remain restrictive on shipping, etc, so separated Pu is probably not a good fuel in a logistics sense for topup fuel in a running reactor. There is also the public perception/opinion issues that come with separating and shipping separated Pu.



There is a public perception issue because people don't know there are multiple isotopes of plutonium and some of these can't be used to make a weapon because they fission prematurely. In fact most people don't know what an isotope is, or what a spontenous fission is.

Where did you read that reactor grade Pu (>20% combined Pu240,238,242) needs high safeguards? It's much safer than shipping 20% LEU (which you can further enrich easily; most of the enrichment work has been done already).


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PostPosted: Dec 21, 2011 11:21 am 
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Cyril,

It is true that by IAEA rules, only Pu with more than 80% Pu238 is considered safe guarded and subject to less stringent rules. It doesn't matter if we agree or disagree, that just happens to be the current regulations. As for other mixes like spent fuel Pu, for every web entry that claims it impossible to make a weapon there is a counter one saying it possible. I think the only thing we can say with certainty is that for mixes with significant isotopes other than weapons grade +93% Pu239 it would be very difficult and take a very high level of technical savvy to make any explosion beyond a "fizzle" of just a small fraction of a kiloton (like at least one of North Korea's tests). And of course the more Pu238,240,242 the worse it is for a weapon but those that precisely know how difficult are certainly not going to tell us anytime soon.

David L.


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PostPosted: Dec 21, 2011 12:51 pm 
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Do you think shipping with high safeguards is prohibitive?
Seems like they are already doing such things since France, England, Russia, are doing reprocessing.
We could mix the Pu with thorium at the processing center. It is more difficult to separate thorium from Pu than uranium.
Why should shipping reactor grade plutonium mixed with thorium be any harder than shipping MOX?


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PostPosted: Dec 21, 2011 12:53 pm 
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The lowest quality plutonium ever to be used in a weapon was 85% Pu239. It didn't work very well but they used enough plutonium to get 20 kiloton yield. It was spent fuel from low burnup English gas cooled reactors. In regulatory terms, this is considered fuel grade plutonium, the intermediate category between weapons grade and reactor grade. Below 85% Pu239 the yield drops off very rapidly.

I found an interesting discussion on the issue.

http://www.fas.org/nuke/intro/nuke/O_9705.htm

It appears the IAEA is not that worried about Pu with less than 80% Pu239. It also looks like, meeting the safeguards is not such a difficult or costly problem. Many on this forum think its onerous and expensive, but like Lars I can't really imagine why. If you make hundreds of millions of dollars of electricity every year, cost of the safeguards would be very marginal, and the IAEA doesn't actively discourage production and seperation of reactor grade plutonium (just fuel grade and weapons grade plutonium).


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PostPosted: Dec 21, 2011 2:28 pm 
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Lars wrote:
Do you think shipping with high safeguards is prohibitive?
Seems like they are already doing such things since France, England, Russia, are doing reprocessing.
We could mix the Pu with thorium at the processing center. It is more difficult to separate thorium from Pu than uranium.
Why should shipping reactor grade plutonium mixed with thorium be any harder than shipping MOX?


No, I agree with you. Shipping LWR reactor grade Pu shouldn't be any more restrictive that shipping MOX. I was just commenting on the regulations and the lack of certainty regarding weapons use. As Cyril points out too, the only bomb people claim was "reactor" grade was when they just said anything below 93% was reactor grade and they used Magnox fuel elements with such a low burnup it was no lower than 85% Pu239 (I've heard 88% estimated). If that was hard I'm sure real spent fuel and more importantly anything we'd see in MSR (pure cycle or DMSR) would be even harder. We have to be careful not to claim its impossible though, we just don't know.

With respect to using spent fuel Pu, depending on what design we are using it in, we might even be using LWR spent fuel "ash" which is whatever is left after you use fluoride volatility (burn the powered spent fuel in a fluorine flame, uranium leaves as UF6). This "ash" of Pu, Am, Cm and fission products of stable fluorides would be much cheaper to get than trying to get Pu on its own (and also lets you get Am and Cm easily). Theoretically they can get Pu as PuF6 but it is a much more delicate operation. The "ash" won't be quite as reactive given the load of fission products tagging along but still an interesting fuel source for startup or top ups (and old CANDU fuel is excellent, far less fission products per kg of Pu, just have to burn a lot more to get your ash). Shipping will be both harder and easier. Harder since we have to shield the radiation but easier in that the proliferation resistance goes up even further.

David LeBlanc


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