[ RadSafe ] Re: Spent Fuel and Decay - Dangerous for Millions of Years?

[Leo M. Lowe]

Hello,

Further to the on-going discussions on the decay of spent fuel, the 
National Waste Management Organization (NWMO), the Canadian 
organization responsible for advising on how Canada should manage 
it's spent nuclear fuel, gives a graph of the decay of the 
radioactivity of CANDU (natural uranium) fuel and a table of the dose 
rate around a spent CANDU fuel bundle (see Table A3-3 in NWMO final 
report available at http://www.nwmo.ca/ )

At 500 years of decay, the dose rate at 0.3 m distance from the 
bundle is 0.82 mSv/h.  Therefore, as has been pointed out by others, 
a worker could spend up to  7 working days (56 hours) next to the 
bundle and still not exceed the 50 mSv/y occupational dose limit for 
exposure in a single year.   While this is certainly not recommended, 
and the fuel is still quite "hot', this puts the oft-heard statements 
about the spent fuel being dangerous for millions of years in a 
different perspective.

Regards,
Leo Lowe
__________________________________________________________________________________________

Hi Mike,

_______________________________________________________________
Does anybody know of any calculations as to how long it takes spent fuel
to decay to the activity level it had before going into the reactor?  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.
Still I would be interested in knowing what the numbers look like.

From: "Brennan, Mike  (DOH)" <Mike.Brennan at DOH.WA.GOV>
____________________________________________________________


The World Nuclear Association at http://www.world-nuclear.org/info/inf04.html
shows a graph of the decay of the radioactivity of spent 
fuel.  Apparently, the total activity of the spent fuel is less than 
the activity of the original ore before 10,000 years of decay.  An 
OECD NEA 1996 report (Radioactive Waste Management in Perspective) is 
cited as the source of the graph.

A similar graph was in an issue of Nuclear News a few years ago, but 
I can't place the exact volume/date at the moment.

Regards,

Leo M. Lowe, Ph.D., P.Phys.

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