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PostPosted: Sep 22, 2010 5:49 pm 
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Here is the current DOE safeguards table, which may help some of the questions related to uranium and various isotopes.

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PostPosted: Sep 22, 2010 6:17 pm 
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By a bomb hazard I'm guessing you mean an accident causing the fuel to go supercritical and then explode.

U232 won't help with a bomb hazard.
A bomb would mean reactivity excursion in the extreme.

Normal operations in a reactor is with a reactivity of 1. It can vary slightly above 1 when the reactor is increasing its power output and slightly below 1 when the reactor is decreasing its power output.

If the reactivity gets significant above one you have a problem.

This could happen if:
a) you get the fuel into a more optimal geometry. For example, if you had a long thin tube like David's and somehow a spill could rearrange the fuel into a sphere then the reactivity would go up (since leakage would go down). In some early fast reactors if the fuel melted and ended up at the bottom of the core reactivity would increase. In a LFTR as long as the drain system was reasonably designed I can't conceive of a way to get the molten salt to form a nice sphere.

b) add moderation. If the moderation is suddenly increased then the reactivity can spike. This happened at Chernobyl when the graphite tip of the control rod was lowered into the reactor. If the LFTR is a thermal reactor then there isn't a reasonable risk of adding moderation. If the LFTR is a fast reactor then one will need to be careful in the selection of materials in the reactor core area to be sure that they can't serve as a moderator.

c) you remove neutron absorbers from the fuel (or the fuel from neutron absorbers). For example, if a reactor has been idling for several hours (after running full bore for more than a day) it will build up 135Xe as 135I decays. One could remove all control rods and boron10 to increase reactivity to overcome the poisoning effects. As the 135Xe gets burned off the reactivity will increase. Caution over this is the reason why there is a timeout in LWRs. Once they have been idling for several hours you aren't allowed to power them up until the 135Xe decays back down to manageable levels. In the context of a LFTR I'm thinking this means care when doing processing that any uranium extraction steps not risk getting a large lump of isolated uranium someplace. Second, it means being certain we don't precipitate fissile out of the salt (by adding oxygen).

Certainly reactivity excursions represent the worst possible accidents and in design we must take great care that they are not possible. But so far as I know, any real design reactivity excursions must be prevented even in the most implausible accident scenarios.

A negative thermal coefficient of reactivity is the solid way of avoiding reactivity excursion problems. If somehow you get the reactivity to be too high the temperature rises and the reactivity decreases.


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PostPosted: Sep 22, 2010 7:47 pm 
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Lars wrote:
Jaro,
Our inventory of tritium inside the core is pretty small.....

I get the impression that my post is being misinterpreted.
There was NO intent to argue that an MSR that uses 20% LEU and contains some tritium, could explode like a small A-bomb. That is simply ridiculous, for a variety of reasons.
Rather, the post was addressed to the question of proliferation potential of 20% LEU when fabricated into a configuration optimized for a bomb: solid metal, minimum of voids, maximum possible multiplication factor held in check by absorber rods, etc., etc.

Thanks to Axil for posting the KIWI-TNT test report.
It reminded me that my memory of the reactor design was faulty: For reasons of compactness, the NERVA reactors were designed with rotating drum control rods, rather than the usual pull-push type typical of land-based reactors.
So instead of rapidly expelling the rods from the core, to induce the explosion, the control drums were quickly rotated into the least-absorbing configuration -- the report makes a brief note of that, if you read carefully.....


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PostPosted: Sep 22, 2010 8:51 pm 
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Lars wrote:
By a bomb hazard I'm guessing you mean an accident causing the fuel to go supercritical and then explode.


NO! NO! NO! (Sorry for shouting.) I mean a proliferation hazard, and even more important, a political hazard. We fight two double standards here. First is the fiction that nuclear anything is uniquely more dangerous than many commonly accepted hazards. Bhopal killed more people than Chernobyl. Second, the military is allowed to possess working nuclear bombs, and have even dropped a few by accident in Spain, Greenland, and off the coast of Georgia, but the nuclear power industry is not allowed to possess any bomb material. I am hoping to be able to start thorium reactors on 90% U-235. Maybe we will have to have it denatured by the military with U-232 before we get our hands on it. If we can't do that, then using a thorium blanket from which we extract U-233 will be forbidden.


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PostPosted: Sep 22, 2010 10:16 pm 
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Sorry for my misunderstanding.

Dr. Peterson was part of a panel that set up a rubic for evaluating the proliferation resistance. It is available on the web. Here is its title.
"Evaluation Methodology for Proliferation Resistance and Physical Protection of Generation IV Nuclear Energy Systems"


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PostPosted: Sep 24, 2010 3:38 pm 
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Lars: Thank you for the reference, but I don't see it. Can you be more specific.

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PostPosted: Sep 24, 2010 3:48 pm 
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I simply put the title into google search and it was the first hit.

But here it is attached (I hope).


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PostPosted: Sep 24, 2010 10:17 pm 
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Thank you for your patience.


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PostPosted: Sep 24, 2010 11:53 pm 
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No problem. I wasn't patient enough to finish reading the document though :( Not as fun to think about as the reactor.


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PostPosted: Sep 29, 2010 4:10 pm 
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Lars wrote:
No problem. I wasn't patient enough to finish reading the document though :(


I skimmed it. It looks like a good start to a many manyear effort to come up with realistic risk assessments of proliferation. Nothing in this thread gives me any assurance that either HEU denatured by U-232 or U-233 denatured by U-232 will be politically acceptable. So I am now very much leaning to David's DMSR. It is not a breeder, but will burn uranium significantly more efficiently than LWR's, so it would be a substantial improvement. IMHO, it has the least political risk of all the MSR's.


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PostPosted: Sep 29, 2010 9:42 pm 
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Both the PB-AHTR and the Indian CHTR will use pure thorium fuel cycle TRISO fuel. What is the problem?

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PostPosted: Sep 30, 2010 1:43 am 
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pstudier wrote:
Nothing in this thread gives me any assurance that either HEU denatured by U-232 or U-233 denatured by U-232 will be politically acceptable. So I am now very much leaning to David's DMSR. It is not a breeder, but will burn uranium significantly more efficiently than LWR's, so it would be a substantial improvement. IMHO, it has the least political risk of all the MSR's.

I believe Dr. Peterson has stated the properly handled even Pa isolation could be acceptable as long as it is part of the hot cell. Remember that an LWR produces almost pure PU239 initially. Between these two observations I'm inclined to think that if we can define the reactor core as controlled boundary then we could have HEU in the core. Note that "politically acceptable" is an ill-defined term. Many would say that new nuclear power itself was politically unacceptable. Though I'd agree that we need to make an effort to reduce the proliferation risks of a reactor design I don't think this should be the primary goal. Rather in order of importance:
1) safety (something I think every reactor built in the US will achieve). Spending significant sums to increase safety beyond Gen IV requirements would be misplaced priorities in my opinion.
2) cost
3) waste
4) proliferation risk

If one values minimizing waste over proliferation risk then you might be reluctant to put 100 tonnes of graphite inside the reactor and thus avoid the largest flow of waste (around 3 tonnes / GWe-yr).


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PostPosted: Sep 30, 2010 6:03 am 
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Lars wrote:
I'm inclined to think that if we can define the reactor core as controlled boundary then we could have HEU in the core.

Many millions of dollars are currently being spent to convert even tiny little research reactors from HEU to LEU -- there is NO chance whatsoever that any civilian power plant would be allowed to start up on HEU.
But it might be possible to start up on LEU and over time develop HEU in-situ.
I wouldn't bet my money on it though: newcomers are unlikely to get any special breaks, when regulations are constantly being ratcheted back.


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PostPosted: Sep 30, 2010 9:22 am 
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I agree on the startup with HEU.
On evolution in the core to HEU I'm not sure - at least for trusted nations where waste is a primary concern. In Europe, the concern over proliferation seems to be considerably lower and the concern over waste higher than in the US.


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PostPosted: Sep 30, 2010 10:56 am 
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Axil wrote:
Both the PB-AHTR and the Indian CHTR will use pure thorium fuel cycle TRISO fuel. What is the problem?

Not sure what you mean Axil -- PB-AHTR uses 10% LEU.


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