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 Post subject: Re: Spent fuel activity
PostPosted: Mar 09, 2015 2:58 am 
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The data I got from this source:

http://www.princeton.edu/sgs/publicatio ... t-2011.pdf

figure 1.4 and then I simply extrapolated out to 600 years. I also used this source:

http://web.ornl.gov/info/reports/1979/3445600507280.pdf

You can compare surface A with surface C dose rates and see that's about a factor of 20 lower @ 1 meter compared to surface dose. Your source would still be several hundred mSv/h.

Perhaps your source is for a large number of fuel assemblies in a thin waste container? Even so it sounds way too high. Are there secondary sources of gammas that build up over time?


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 Post subject: Re: Spent fuel activity
PostPosted: Mar 09, 2015 4:20 am 
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Thanks for the references - I will have a look at those.

Cyril R wrote:
Perhaps your source is for a large number of fuel assemblies in a thin waste container? Even so it sounds way too high. Are there secondary sources of gammas that build up over time?

The context of the NWMO data is for analysis of radiolytic production of reactive radicals from water infiltration into the storage canisters, potentially accelerating corrosion over time.
So yes, I think they would have picked the worst possible location in a storage canister for their surface dose rate estimates.
Still, how much worse can it be ? ....a factor of 2 or 3 at most ?
Exactly what sort of geometry are we envisaging for the hypothetical encounter with SNF, 500 years from now ?


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 Post subject: Re: Spent fuel activity
PostPosted: Mar 09, 2015 5:45 am 
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Well I was thinking of a narrative where someone in the future encounters the spent fuel assemblies and decides to have a look at them. This person would not eat the spent fuel but may want to check out what's what. How old they'd have to be for this person to be safe.

Or maybe if we want to put them in a museum, how long before that could be done safely using realistic dose for damages (not LNT sillyness).

The goal being of course to show that the dreaded spent nuclear fuel does not take a million years to be safe to be around to.

Do you know why your graph goes up in gamma dose beyond 11 Sv/h contact dose at 500 years to maybe a 15+ Sv/h at millions of years? Is this some actinide decaying into a gamma emitting actinide (isomer?)?


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 Post subject: Re: Spent fuel activity
PostPosted: Mar 09, 2015 5:52 am 
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Quote:
Still, how much worse can it be ? ....a factor of 2 or 3 at most ?


Yes that's what it should be if you have a few tens of assemblies in a can rather than one freestanding. But your graph shows 11 Sv/h contact dose at 500 years which should be around 500 mSv/h @ 1 meter. Whereas my extrapolation gets around 0.1 mSv/h for a single assembly at this point. I can imagine it being off by a factor of 5 or 10 because of my extrapolation and because of the single assembly vs a bunch. But that still leaves two orders of magnitudes unaccounted for.

Since your graph shows a rising dose after 500 years I think there may be some gamma emitter decaying in what is causing this. If so then the spent fuel is not as safe as I am claiming. I hope you have some answers Jaro.


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 Post subject: Re: Spent fuel activity
PostPosted: Mar 09, 2015 7:44 am 
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Cyril,

The ordinate in Jaro's graph is in Gy per year, not Gy/h


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 Post subject: Re: Spent fuel activity
PostPosted: Mar 09, 2015 7:59 am 
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djw1 wrote:
Cyril,

The ordinate in Jaro's graph is in Gy per year, not Gy/h


Great catch! That explains the discrepancy just fine.

The 11 Sv/a is then 1.26 mSv/h contact dose which is consistent with my blatant extrapolation of slightly below 0.1 mSv/h @ 1 meter distance considering the apparent factor of 20 or so between contact dose and 1 meter distance.


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 Post subject: Re: Spent fuel activity
PostPosted: Mar 09, 2015 1:58 pm 
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Cyril R wrote:
Do you know why your graph goes up in gamma dose beyond 11 Sv/h contact dose at 500 years to maybe a 15+ Sv/h at millions of years? Is this some actinide decaying into a gamma emitting actinide (isomer?)?

That is curious.
Maybe spontaneous fission of Pu242 ? (alpha decay of Cm246 would keep up the supply)
Another possibility might be Cm247 alpha decaying to Pu243, which then beta decays to Am243, which fissions spontaneously.


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 Post subject: Re: Spent fuel activity
PostPosted: Mar 10, 2015 12:34 am 
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Cyril R wrote:
Ok you guys, here's a first graphical attempt at the risk or "what do do when you encounter a spent nuclear fuel assembly!". Can you guess the legend?

Something to replace the question mark with:
Image


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 Post subject: Re: Spent fuel activity
PostPosted: Mar 10, 2015 4:28 am 
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Well that sign should be on the spent fuel at all times. Or not. We can simply assume people of the future are wise enough to not eat metal fuel tubes with ceramic contents. If the people of the future are not wise enough to do that they will kill themselves by eating a toxic ore or rock, or lead strips they find in a junkjard, sooner or later and the discussion is moot.


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 Post subject: Re: Spent fuel activity
PostPosted: Mar 10, 2015 12:09 pm 
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Do I need that sign on my solar panels?


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 Post subject: Re: Spent fuel activity
PostPosted: Mar 10, 2015 12:56 pm 
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alexterrell wrote:
Do I need that sign on my solar panels?


Absolutely, a solar panel contains more than enough toxins to kill you. And they're the type that are still toxic a billion years from now.


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 Post subject: Re: Spent fuel activity
PostPosted: May 15, 2015 6:41 am 
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Cyril, do you have the reference for the graph in the original post (toxicity vs time, log/log)? It looks inconsistent with others I have seen, for example this IAEA Bulletin on P23 has a similar plot for total waste toxicity, and untreated spent fuel doesn't cross the natural U equivalent line until 250,000 years. What DOES cross at ~12,000 years, like the total in your plot, is their 'line 2', which is what you get with PUREX recycling, taking most of the Pu out and fissioning it, letting only the minor actinides go to the waste stream. This would also explain why the Pu toxicity in the plot is so low.


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 Post subject: Re: Spent fuel activity
PostPosted: May 15, 2015 7:25 am 
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So, how long do you have to wait until reprocessing becomes far less problematic because you don't have something that is destroying your solvent, trying to boil itself and giving anyone who goes near it fatal doses?

100 years? 200? 300?


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 Post subject: Re: Spent fuel activity
PostPosted: May 16, 2015 2:07 am 
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In the absence of a firm decision on closed cucle, the progress on pyro-processing research is slow.
http://www5vip.inl.gov/technicalpublica ... 411188.pdf
Maybe the Russians are further along.
You could remove the metallic cladding mechanically and reuse it as a radioactive metal.
Chlorinating the ceramics after wet grinding it could volatilize all the actinides along with some fission products. Fractional distillation could separate them.
Uranium could be separated from other actinides as hexaflouride which condenses at a lower temperature.
Fission products could be clad in recovered metal for deep borehole disposal. Actinides could be recycled in MSR or fast reactors. Perhaps it will take Russians to take the risks.


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 Post subject: Re: Spent fuel activity
PostPosted: May 16, 2015 5:01 am 
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As far as I know the only people with a properly functioning industrial MOX plant are the French.

Interestingly, as I understand it the primary reason that MOX fabrication costs so much is because of radiation from Americium ingrowth from the decay of 241Pu.
Plutonium from spent fuel that has been aged for ~100+yrs will have almost no 241Pu left - which means once it has been seperated via PUREX or similar it should not produce significant quantities of Americium.


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