Re: Yucca Mountain article

[daleboyce@charter.net]

After posting that the FP activity was reduced to that of the mined Uranium after 160 years, couldn't quickly find the reference.  I'll keep looking, but it is probably in a different book than I thought it was in.  In any case I pulled out my copy of Kessler "Nuclear Fission Reactors" Springer Verlag 1983, and using data from it for 3.2% enrichment fuel and 33,000 MWd/te burn up at discharge.  I calcluated it would take 566 years for the sum of Sr90 and Cs137 activities to equal the activity of the U238 in the fuel rod. So the rods will still be toasty after 160 years. However, it takes far more mined uranium to make a fuel rod than what is in the rod at the end, and mined uranium actiivty was what was mentioned in the paper.  If I find this factor, or the reference I was looking for I will post it.



As a preface to the rest of this post (I'm sorry it is getting long) for those of you not familiar with the decay of fission products, most fission products have half lives of of less than a year and many are extremely short-lived.  If memory serves the activity from a bomb drops off as a power law in time avtivity is proportional to T^-1.1 or T^ -1.2. This is a result of having many different isotopes with the whole gamut of half-lives.  A reactor has a different behaviour since the shorter lived isotopes are saturated early during the 3 years or so the fuel is in a reactor.  The isotopes with months to a few years will have grown in to similar activities as the shorter lived ones.  At discharge the activities will begin to drop sharply, but not as sharply as that from a bomb. FWIW this is why it is many years before decommissioning begins for a closed plant.  Ten or fifteen years buys you a lot, but after that it doesn't cool much in a reasonable time frame.



Further reading of the paper Tosh posted I find some serious errors. Example: after 300 years the fission product activities go down a factor of 1000 not a factor of 10.    The factor of 1000 per 300 years assumes that all of the year or less products have decayed before you start the clock. If you include the shorter lived activities the decrease is much greater than a factor of 1000 in 300 years.  Sr90 and Cs137 are the main contributors after a few tens of years.  I ignored I-129 as its >10,000,000 year half life and less than 1% fission yield means at the end of a fuel cycle its activity will be about that of the Uranium in the fuel.  Its main importance comes in when you try to recycle the fuel.



There are fission products and activation products made in a reactor with lives intermediate to the one year and 30 year fission products, but their activities at discharge are less than the Sr90 and Cs137 activities, and therefore will never surpass them.



If someone wants to argue Ni63 feel free. 



Still to say that the stuff is much safer sitting inside yucca mountain than in a dry cask in a parking lot is valid is an understatement of the nth degree.  I just hope that the Yucca Mountain plutonium mine is a workable mine when we need the fuel in the future.



Dale