[ RadSafe ] RE : Spent fuel and decay

[Brennan, Mike (DOH)]

>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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