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PostPosted: May 10, 2011 5:52 pm 
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Joined: Apr 29, 2011 10:08 pm
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in my inexorable quest for a backyard LFTR, i overlooked an important factor.
U233 fission results in some pretty nasty fission products. half lives of up to a million years, and almost 15% of all reactions in the fuel salt will result in these.

the question is, why is none of this mentioned in the MSRE report, or in proposed fuel cycle break downs for lftr?
i suspect that the current calculations for fission products only really apply to solid fuel reactors. i'm aware that parasitic neutron absorptions are greatly reduced in the fluoride design for instance.

or is the 15% by mass fission products simply assumed in these reports?


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PostPosted: May 10, 2011 6:41 pm 
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Joined: Nov 30, 2006 3:30 pm
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Million year half lives mean that they're essentially harmless. The shorter the half-life the more you have to worry about it. Til it's gone.


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PostPosted: May 10, 2011 11:04 pm 
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JAK wrote:
in my inexorable quest for a backyard LFTR, i overlooked an important factor.

The one that has hundreds of thousands of dollars in fissile material alone? Unless your backyard is producing power in the tens of megawatts range, I'm going to assume that you're incredibly ignorant or you're trolling. Maybe you want a nuclear powered automobile also. If you want to start a company that produces power from nuclear reactors for isolated communities, it might make sense.

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U233 fission results in some pretty nasty fission products. half lives of up to a million years, and almost 15% of all reactions in the fuel salt will result in these.

Meaningless in terms of radioactivity and exposure risk. Great if you want to drum up fear. Radioactivity is inversely proportional to half life and directly proportional to decay energy.

http://en.wikipedia.org/wiki/Long-lived_fission_product

Quote:
the question is, why is none of this mentioned in the MSRE report, or in proposed fuel cycle break downs for lftr?
i suspect that the current calculations for fission products only really apply to solid fuel reactors. i'm aware that parasitic neutron absorptions are greatly reduced in the fluoride design for instance.

Parasitic neutrons by fission products sometimes lead to shorter half lived fission products. Xe135 in normal reactors eats up neutrons becoming stable Xe136, while in a LFTR it is isolated and decays into Cs135 with a long half life of 2.3 million years. The radioactivity of Cs135 is nearly meaningless given its low activity, a million times smaller than Cs137. In a thousand years, when the radioactivity of the spent fuel is lower than the original ore, I suspect the danger these isotopes pose will not be high on the list of worries.


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PostPosted: May 11, 2011 12:11 am 
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Joined: Apr 29, 2011 10:08 pm
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that would indeed explain it. the radioactivity of the fission products is negligible so it isn't much of a concern.

Quote:
The one that has hundreds of thousands of dollars in fissile material alone? Unless your backyard is producing power in the tens of megawatts range, I'm going to assume that you're incredibly ignorant or you're trolling. Maybe you want a nuclear powered automobile also. If you want to start a company that produces power from nuclear reactors for isolated communities, it might make sense.


guess you haven't seen my other thread, hu?
i'm very seriously designing a 90 kilowatt thermal reactor. given how incredibly energetic U233 moderated fission is, i could probably get away with a very small amount of it, and a very small core. it all depends on what the calculations say.


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