From: jac@ds8.scri.fsu.edu (Jim Carr)
Newsgroups: sci.physics
Subject: Re: Nuclear Waste Disposal
Date: 11 Dec 92 19:13:44 GMT
Reply-To: jac@ds8.scri.fsu.edu (Jim Carr)
Organization: SCRI, Florida State University

In article <11DEC199201405423@csa3.lbl.gov> sichase@csa3.lbl.gov (SCOTT I CHASE) writes:
>In article <11508@sun13.scri.fsu.edu>, jac@ds8.scri.fsu.edu (Jim Carr) writes:
>> 
>>The better ideas for transmuting dangerous elements center around 
>>nuclear beams, mainly protons.   ...
>>          ...                 The right combination of projectile, 
>>energy, and target could allow you to turn a dangerous long-lived 
>>material into a very radioactive but short-lived material that does 
>>not require long-term storage, or even into a stable and safe material.  
>
>I agree with everything you said, but this part is worth emphasizing, so
>I have reposted it.  In practice, accelerator transmutation is a trade-off.
>You end up with highly nasty short-lived products from mildly nasty long-lived
>input.  You reduce the lifetime from many thousands (or more) of years to 
>tens or hundreds of years, significantly reducing the storage problem.

And just in case anyone is learning anything from this ...

I noticed that I was not careful to point out the importance of what is 
called "specific activity" in the radiation business.  The specific 
activity is, simply put, the amount of radiation per unit mass.  Thus 
longer half-life means lower levels of "activity" because the same number 
of decays is spread over a longer time.  Most *storage* problems are 
associated with materials whose lifetimes are in the middle territory 
of thousands or tens of thousands of years.  Why? 

For shorter lifetimes, the material is extremely "hot" but will be gone 
in a short time.  Fuel rods may be stored for decades before being 
processed, so all the stuff with half-lives less than a year is gone 
to start with.  The remaining stuff with 100 year lifetimes can be 
diluted and processed for storage, and a confinement system good for, 
say, 1000 years is all you need.  Geology is fairly predictable on 
that time scale. 

For really long lifetimes, you have something that is not very "hot", 
and is basically the same as what we live with all the time -- stuff 
like thorium, which is everywhere.  Such waste could be put right 
back in the uranium mines, and the radiation level in the mines would 
be less than when you started. 

In the middle, the material is dangerously radioactive *and* lives 
a long time.  Thus it must be stored so it is confined for several 
millenia.  Accelerator-based transmutation offers a way to deal 
with part of this aspect of the disposal problem. 

>You still need a permanent nuclear waste storage facility, but you don't
>have to design it to survive for millenia.

In practice I suppose you do, but the risk analysis is much improved 
because the consequences of a design defect due to imperfect knowledge 
of future conditions on the site are reduced significantly. 

--
J. A. Carr                                    |  "The New Frontier of which I  
jac@gw.scri.fsu.edu                           |  speak is not a set of promises
Florida State University  B-186               |  -- it is a set of challenges."
Supercomputer Computations Research Institute |   John F. Kennedy (15 July 60)
Newsgroups: sci.physics