[ RadSafe ] RE : Spent fuel and decay

[Jim Hardeman]

Mike --
 
Good observation. As I recall, the primary driver for long-term isolation of spent fuel is dose through the groundwater pathway. I'd have to go back and look not only at 10 CFR 63 but EPA's standards, but I believe EPA wants groundwater concentrations of radionuclides to be less than their Maximum Contaminant Level (MCL) over the entire period over which consequences are assessed -- I think that's now up to 100,000 years.
 
I don't know much about the geology of Washington State, but if your geology is anywhere like Georgia's, you'll find quite a few places that exceed MCL for naturally-occuring radionuclides (Ra-226, Ra-228, uranium being the ones currently regulated -- plus total alpha activity (excluding uranium) and total beta activity) . While the MCL for beta-emitters is based on dose (i.e. 4 mrem/yr assuming 2 liter per day consumption rate), the actual dose from Ra-226, Ra-228 and uranium at MCL concentrations will be substantially greater. The MCL for uranium, for example, is in ug/l (30 ug/l as I recall) rather than pCi/l. We've noticed that the U-234 / U-238 ratio in groundwater isn't constant, and that the dose associated with a concentration of 30 ug/l can vary considerably based on the U-234 / U-238 ratio.
 
What I'm getting at here is that the regulatory framework for spent fuel actually regulates risk through the groundwater pathway at a value far less than the risk associated with groundwater that might be associated with natural uranium deposits.
 
My $0.02 worth --
 
>From the piney woods of northern Michigan (on vacation)
 
 
 
Jim Hardeman, Manager
Environmental Radiation Program
Environmental Protection Division
Georgia Department of Natural Resources
4220 International Parkway, Suite 100
Atlanta, GA 30354
(404) 362-2675
Fax: (404) 362-2653
Personal fax: (678) 692-6939
E-mail: Jim_Hardeman at dnr.state.ga.us 


>>> "Brennan, Mike  (DOH)" <Mike.Brennan at DOH.WA.GOV> 7/26/2007 17:49 >>>
>From the information I have and the interesting site Leo linked to, I
agree that driver for activity in the long run is Pu.  I just ran a
spread sheet looking at the U235-U238-Pu239 ratios and the effect on
total activity, and it is clear that once the fission fragments have
decayed away enough (a couple hundred years, more or less) the total
activity is mostly driven by how much U238 was changed into Pu239, and
how much of the Pu239 was in turn destroyed by fission.  While these
numbers would all depend on the original make-up of the fuel and its
history with a reactor, it seems reasonable to say that the fuel would
reach its original activity after around 10,000 years (fairly wide band
"around"), and that it would be low enough activity to handle without
special precautions several thousand years before then.

All in all, while spent fuel presents non-trivial challenges for
storage/disposal, the claims that it must be kept isolated for 100,000
(or 1,000,000, as I heard recently) years is clearly not based on actual
risk.

   

-----Original Message-----
From: radsafe-bounces at radlab.nl [mailto:radsafe-bounces at radlab.nl] On
Behalf Of Pete_Bailey at fpl.com 
Sent: Thursday, July 26, 2007 10:32 AM
To: radsafe at radlab.nl 
Subject: [ RadSafe ] RE : Spent fuel and decay


> ... how long it takes spent fuel to decay to the Aactivity level it 
> had before going into the reactor?

If by 'activity level', you mean 'curies', one very very long time.

> I realize it is highly dependant on factors such as level of 
> enrichment, amount of burn-up, activation of cladding, etc, and 
> breeding of Pu.

Level of enrichment & 'burn-up'(MWT/MTU) drive the length of time.
Most activation products (active...of clad) fizz out in  a decade or
so...
The breeding of Pu....it ain't there to
start with (for all intent and purposes), and has mega-year half-life.
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