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 Post subject: Is HEU really forbidden?
PostPosted: Sep 15, 2010 3:00 pm 
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It would be nice to use HEU for startup, and for David's DMSR which would not quite breed. If HEU is really forbidden, then we will also have trouble with extracted U-233. We could ship some U-233 with U232 "contamination" to a secure spot, have it blended with U-235, and then have it returned. Remember the arcs_n_sparks quote:

arcs_n_sparks wrote:
As some may know, the easiest way to move nuclear material is to wrap explosives around it and call it a weapon. Totally different set of rules....

Would 80% HEU be significantly less dangerous than bomb grade which is usually >90%?


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PostPosted: Sep 15, 2010 3:11 pm 
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According to this Wiki article, the first atomic bomb was 80% U255:

http://en.wikipedia.org/wiki/Enriched_uranium

So, although there is an inverse relationship between the degree of enrichment and the amount of uranium required to make a bomb, it would seem that it is practical to make a bomb with uranium enriched to somewhat less than 80% U235.


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PostPosted: Sep 15, 2010 9:18 pm 
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80% HEU would certainly be weapons usable, just need a few more kg of it. The limit is 20% U235 or 12% U233 or a weighted average thereof. In reactor use though, the other isotopes like U236 and U234 should mean it could be possible to go a little above these levels which I think the ORNL studies on a DSMR breeder assumed to help it break even.

I believe the logic of 20% or 12% enrichment is that while you could actually still get some sort of explosion, that the energy released by fission would actually be less than the explosives need to set it off. As well, the thing would need an enormous weight of uranium.

David L.


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PostPosted: Sep 15, 2010 9:51 pm 
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It's funny how we never describe a tank full of gasoline by its equivalent in napalm, but HEU gets that kind of treatment.

Fissile is demonized. Rod Adams is making me think more and more that this is no accident.


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PostPosted: Sep 17, 2010 12:24 am 
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David wrote:
...I believe the logic of 20% [U235] or 12% [U233] enrichment is that while you could actually still get some sort of explosion, that the energy released by fission would actually be less than the explosives need to set it off. ...

David, please (re?)read ORNL/TM-13517 ("Definition of Weapons-Usable U-233", by Forsberg & Hopper, 1998).

From what I can tell, the sole benefit of dilution with U238 is that it pushes up the critical mass, presumably to impractically high values (e.g. 750 Kg & 42 cm diameter, for a bare sphere of U235 20%). Note that this may be a case of antiquated missile-centric thinking. It is not clear that this would be impractical for a truck bomb. And the yield could be impressive, even if only a tiny fraction of the material fissions.

So any company that thinks they are going to fuel a hot-tub sized fast-neutron modular nuke with 3 tons of metallic 20% U235 (i.e. a direct use material) is clearly living in a fantasy world (unless maybe they pre-irradiated at the factory, so the fuel was self-protecting). They could never afford the required security.

For the low yield issue, perhaps you were thinking of reactor grade Pu? The neutron-emitting isotopes cause premature ignition, which disassembles the device before maximum compressions. But even this effect may have limited value nowadays: 1-10% of normal yield is still 100-1000 tons.

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Last edited by Nathan2go on Sep 17, 2010 12:45 am, edited 1 time in total.

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PostPosted: Sep 17, 2010 12:39 am 
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Kirk Sorensen wrote:
...Fissile is demonized. ...

Or maybe just very high value, hence the need for high security.

I heard a new story on the radio the other day about some country that still had an all-cash economy. People there would even buy houses with cash. And the couldn't figure out why they were having a big problem of people getting robbed as they left the bank.

High value targets attract crime.


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PostPosted: Sep 17, 2010 2:37 am 
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I have always maintained that a weapon is as much intention as technology. Withholding technology of power production (or even 'Dual Use') for the sake of avoiding misuse is counterproductive. Everything from torches to fertilizer to passenger planes have been misused as weapons.
Fissile nuclear material, on the other hand can convert a million tons of mined uranium lying as enrichment tailings or SNF besides thorium to an inexhaustible stock of nuclear fuel. What is required are breeder reactors. Thermal thorium reactors like the LFTR are hopeful breeders but fast breeders reactors are a reality.


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PostPosted: Sep 17, 2010 5:32 am 
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Kirk Sorensen wrote:
It's funny how we never describe a tank full of gasoline by its equivalent in napalm, but HEU gets that kind of treatment.

Fissile is demonized. Rod Adams is making me think more and more that this is no accident.


Rod certainly changed my mind on a lot of things. That to which you refer is just one of them.


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PostPosted: Sep 17, 2010 8:10 am 
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Nathan2go wrote:
So any company that thinks they are going to fuel a hot-tub sized fast-neutron modular nuke with 3 tons of metallic 20% U235 (i.e. a direct use material) is clearly living in a fantasy world (unless maybe they pre-irradiated at the factory, so the fuel was self-protecting). They could never afford the required security.


There are no inordinate security requirements for uranium enriched to <20% U235.


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PostPosted: Sep 17, 2010 10:17 am 
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Nathan2go wrote:
David wrote:
...I believe the logic of 20% [U235] or 12% [U233] enrichment is that while you could actually still get some sort of explosion, that the energy released by fission would actually be less than the explosives need to set it off. ...

David, please (re?)read ORNL/TM-13517 ("Definition of Weapons-Usable U-233", by Forsberg & Hopper, 1998).

From what I can tell, the sole benefit of dilution with U238 is that it pushes up the critical mass, presumably to impractically high values (e.g. 750 Kg & 42 cm diameter, for a bare sphere of U235 20%). Note that this may be a case of antiquated missile-centric thinking. It is not clear that this would be impractical for a truck bomb. And the yield could be impressive, even if only a tiny fraction of the material fissions.

So any company that thinks they are going to fuel a hot-tub sized fast-neutron modular nuke with 3 tons of metallic 20% U235 (i.e. a direct use material) is clearly living in a fantasy world (unless maybe they pre-irradiated at the factory, so the fuel was self-protecting). They could never afford the required security.

For the low yield issue, perhaps you were thinking of reactor grade Pu? The neutron-emitting isotopes cause premature ignition, which disassembles the device before maximum compressions. But even this effect may have limited value nowadays: 1-10% of normal yield is still 100-1000 tons.



Yes I have read ORNL TM 13517 and skimmed it again before my first post. I can not recall where I read the comment about yield being roughly the same as the explosives needed at the 20% cutoff. Until I can find the source this of course is not a reliable statement but I also think you are grossly underestimating the difficulty of designing a weapon using 20% U235. It is certainly not as simple as being just too heavy for a missile launch system.

As well, the effect on critical mass are exponential in nature so if there are any lingering worries of 20% U235, almost any system calling for 20% use (research reactors, fast breeder startups, DMSR) can lower this, to say 15% for which the critical mass would be numerous tonnes and orders of magnitude more difficult to fabricate anything that would have any significant yield.

Anyone out there with more expertise care to chime in?

David L.


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PostPosted: Sep 20, 2010 2:02 am 
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Nathan2go wrote:
David wrote:
...I believe the logic of 20% [U235] or 12% [U233] enrichment is that while you could actually still get some sort of explosion, that the energy released by fission would actually be less than the explosives need to set it off. ...

David, please (re?)read ORNL/TM-13517 ("Definition of Weapons-Usable U-233", by Forsberg & Hopper, 1998).

From what I can tell, the sole benefit of dilution with U238 is that it pushes up the critical mass, presumably to impractically high values (e.g. 750 Kg & 42 cm diameter, for a bare sphere of U235 20%). Note that this may be a case of antiquated missile-centric thinking. It is not clear that this would be impractical for a truck bomb. And the yield could be impressive, even if only a tiny fraction of the material fissions.

So any company that thinks they are going to fuel a hot-tub sized fast-neutron modular nuke with 3 tons of metallic 20% U235 (i.e. a direct use material) is clearly living in a fantasy world (unless maybe they pre-irradiated at the factory, so the fuel was self-protecting). They could never afford the required security.

For the low yield issue, perhaps you were thinking of reactor grade Pu? The neutron-emitting isotopes cause premature ignition, which disassembles the device before maximum compressions. But even this effect may have limited value nowadays: 1-10% of normal yield is still 100-1000 tons.


But if 750 KG of uranium were required to achieve criticality, bringing the segments together quickly and accurately enough to get a real explosion would be exceedingly difficult. Exactly what would happen I don't know, but I'd guess that there would be a flash of radiation followed by having a pool of molten uranium burning through the floor of the truck and landing on the pavement below. Probably a few people nearby would be killed, but it would probably be less effective than an ammonium nitrate and Diesel fuel bomb. Or perhaps the chemical explosion used to bring the uranium segments together would do more damage than the nuclear fission.


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PostPosted: Sep 20, 2010 5:56 am 
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Below is a photo of the explosion of the KIWI reactor, a small graphite-moderated type, used for research in rocket propulsion (the NERVA program).

The explosion was triggered by rapidly expelling neutron absorbing control rods from the core (pieces of white-hot graphite are seen flying above the fireball).

If you did the same thing with a large mass of uranium metal, enriched to 20%, the fast spectrum could easily yield an explosion a thousand times bigger, or more (assuming a neutron source is provided).
Addition of tritium could boost that by another factor of a hundred: The fusion reaction rate typically becomes significant at 20 to 30 megakelvins. This temperature is reached at very low efficiencies, when less than 1% of the fissile material has fissioned (corresponding to a yield in the range of hundreds of tons of TNT).

Image


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PostPosted: Sep 20, 2010 10:12 am 
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Jaro,
Our inventory of tritium inside the core is pretty small. Assuming the high end production of 2400 curies per day we generate 90 grams per GWe-yr. If we remove the tritium on average w/in a minute of its creation then the inventory in the core is <0.2 milligrams.

In addition, we will likely have a bottle of the stuff being collected. This would be in the off-gas processing area. A day's worth of collection would be around 0.25 grams.

Finally, we would have an off-gas storage area where we might have as much as 12 years worth of tritium or 1 kilogram worth. (Maybe not depends on the value of the tritium, the proliferation concerns about isolated tritium, and the pain of transporting it - none of which I have investigated). I presume the off-gas storage area can be outside the reactor building.

I am thinking that concerns about tritium boosting any explosion are only relevant for the tritium that is actually inside the core and that the amounts that are inside the core are so small that it isn't a concern at all. Am I off in this thinking?


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PostPosted: Sep 20, 2010 8:00 pm 
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KIWI TNT test and explosion

This test was produced by disconnecting the coolant system from the reactor.

Image



Here is the test report.

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PostPosted: Sep 22, 2010 5:24 pm 
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So U-235 above 20% is a bomb hazard. Would denaturing it with U-232 make it acceptable? If not, then how are we going to handle U-233?


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