From: zcbag@cnfd.pgh.wec.com (B. Alan Guthrie)
Newsgroups: sci.energy
Subject: Re: Proton Accelerator Reactors ?
Date: 8 Sep 1995 17:29:50 GMT
Organization: Westinghouse NMD
Lines: 174

In article <42orud$sd2@hermes.is.co.za> franks@vironix.co.za (Frank
Scrooby) writes:

>Many moons ago I read a report in one of the more technical American
>Science journals about an experimental fission reactor which would be
>built to experiment with the possibility of a safer form of fission.
>The theory was that you could take a metal with a common isotope of the
>same atomic mass of ordinary fuel rod material. You'd put sufficent of
>'fuel rods' of this material in a special reactor and bombard them with
>accelerated protons. If enough of these protons were absorbed by the
>'fuel' rods they would transmute in uranium or plutonium and begin to
>fizzle as normal reactor fuel. If the reactor gets to hot and needs to
>be shut down you simply which off your accelerator. No more additional
>protons and the fuel rods return to their orginal state and stop
>fizzling. 
>
>My questions are : Does this theory work ?
>                   Was this test reactor built?
>                   If so what were the results ?
>                   If so when will these reactors begin coming on line?
>                   Won't the transmuted 'fuel rods' still be as
>		    radioactive as plutonium in any case ?
>                   If this scheme works couldn't we accelerate our existing 
>                   plutonium waste to an atomic number around 110 and let it 
>                   fizzle out in the half second half life these heavy
>                   elements have ?
>
>Just Curious. 


   I believe that the proposal was to use neutrons from an accelerator to
   drive a sub-critical reactor.  A sub-critical reactor is one in which the
   fuel cannot sustain a chain reaction.  

   A proton-driven reactor doesn't make sense because the coloumbic repulsion
   between the accelerator protons and those in the target nuclei would
   preclude much transmutation.  Most of the protons would be scattered and
   would quickly lose so much energy that they would be unable to overcome
   the coloumbic repulsion ever.

   Back to the neutron-driven reactor - uranium when it fissions gives off
   two or three neutrons (as well as many other things).  A chain reaction
   is sustained when one of these neutrons is absorbed and causes another
   fission.  If the fuel is arranged such that the first fission causes only
   0.9999999999999999 fissions then the reactor is subcritical (just barely)
   and the chain reaction will die out, since each fission produces less
   than one other fission.

   The proposed idea is take a subcritical reactor (which is probably more
   like 0.95 to 0.99 in its neutron multiplication) and use an accelerator
   to supply neutrons to sustain the reaction.  Going back to my hypothetical
   reactor, let us suppose that the reactor has a multiplication factor
   of 0.95; that is, the neutrons resulting from 100 fissions will cause only
   95 fissions in the next "generation."  The accelerator would supply the
   additional five neutrons so that power level would be maintained at a
   constant level.

   First of several problems with this scheme - the accelerator will require
   a large amount of energy.  I don't have a number, but let us suppose
   that to produce 1000 MW of electricity (the typical size of a nuclear
   reactor), the accelerator will need, I dunno, 100 MW.  Then, to put
   1000 MW out to the customers, my plant really needs to generate 1100 MW.
   Since a fission will produce roughly 200 MeV/fission, regardless of whether
   the initiating neutron is from another fission event or from an accelerator,
   the reactor will need to burn the extra uranium, resulting in the generation
   of a large amount of fission products, which are typically radioactive and
   which comprise the lion's share of high-level nuclear waste.  So this
   scheme causes me to generate more nuclear waste.

   Secondly, what this scheme provides me in nuclear safety is that the
   reactor is not subject to various "reactivity" accidents.  Reactivity
   is a term which we nuclear engineers use to discuss the neutron
   multiplication of the core and the time behaviour of the reactor.  If
   the reactivity inserted into a reactor is too high, then the power
   level can increase rapidly and to excessive levels.  Typical
   reactivity accidents include control rods being rapidly withdrawn from
   the core and the dilution of neutron poisons dissolved in the reactor
   coolant.  These accidents are not difficult to protect against, and,
   as far as I know, have never been an actual problem at a light-water
   reactor power plant.  The Chernobyl accident and the SL-1 accident in
   Idaho in 1961 were reactivity accidents, so that matter is not
   entirely academic.  The idea behind the accelerator-driven reactor is
   when the accelerator is turned off, the chain reaction would be
   subcritical and the reactor power would quickly die away.

   The more important nuclear safety problem is how to deal with the decay
   heat generated by those fission products after the reactor is shutdown.
   One cannot control this heat as it comes from the radioactive decay of
   the material.  If cooling is lost to the core, then the core will overheat
   and eventually melt, as happened at Three-Mile Island.  The accelerator-
   driven reactor will still have this problem (and will actually have more
   decay heat to remove, since it will have more fission products).

   So the accelerator-driven reactor solves the problem of reactivity
   accidents (which isn't hard to solve), but it compounds slightly the
   more important decay-heat removal class of accidents.  It solves the
   wrong problem.

   BTW, fuel rods don't fizzle - the fuel inside them absorbs neutrons and
   emits neutrons.

   So far as I know, no one has any plans to build this reactor, since it's
   basically, as I've shown above, a dumb idea.

   "Aren't the fuel rods still as radioactive as plutonium in any case?"

   Yep, you are quite perceptive for someone unschooled in the technology.
   And while we're here - plutonium-239 (the most common isotope in a
   nuclear reactor) has a half-life of roughly 24,000 years.  Strontium-90,
   a common fission product, has a half-life of 30 years.  If you have
   the same number of Pu-239 atoms and Sr-90 atoms, the Sr-90 will
   decay much faster.  In 300 years, the Sr-90 will be reduced to 0.1%
   of its original mass, while pretty much all of the Pu-239 will still
   be there.  For the first 300 years then, which was more radioactive?
   Obviously, the Sr-90 as a lot more of it decayed.  My point is that
   "as radioactive as plutonium" is kind of meaningless.

   "Can we transmute our waste up to atomic number 110 and let it
   decay with the half-life of seconds common there?"

   It's an idea which has been looked at for a long time (I studied
   it back in 1974 and 75, for instance).  First of all, as I noted
   above, most of the high-level radioactive waste is really from
   fission products, with atomic numbers between 35 and 55 or so.
   There is a good amount of transactinide waste (plutonium, americium,
   curium, etc) and it is an arguable proposition that it could be
   separated out and transmuted.

   First of all, you would transmute it using neutrons, not protons.
   That coloumbic repulsion is still there and is going preclude 
   transmuting anything more than a few target nuclei if protons are
   used.  Neutrons, having no charge, are of course not subject to the
   coloumbic repulsion.  

   The idea then would be to transmute the transactinides to a 
   fissionable isotope and then fission them to yield fission products
   with shorter half-lifes.  For instance, if Pu-240, which is not
   fissionable by thermal (slow moving) neutrons absorbs a neutron,
   it will become Pu-241, which is fissionable.  If the Pu-241
   absorbs a neutron, it will either fission (most likely) or 
   transmute to Pu-242.  Some of these isotopes decay to other
   isotopes as well, but they are still transactinides and will
   have, somewhere in their decay chains, some long-lived isotopes.
   
   A problem is getting the source of neutrons - a fission reactor
   needs its neutrons to sustain its chain reaction (and it is
   generating fission products and transactinides) so it really
   does have a whole lot to spare.
   
   It has been proposed to use a deuterium-tritium fusion reactor
   as a source of neutrons or to use an accelerator as a source
   of neutrons.  The cost, though, seems to be pretty high.
   
   Many of us argue that the plutonium is a valuable fuel resource,
   at least in the not-too-distant future, and we don't want to
   see it destroyed.  The difficulties of geologic disposal of
   the other transactinides and the fission products do not, to
   many of us, seem all that hard to solve, so that the
   emphasis on transmutation seems out-of-place.
   
   One last observation - suppose we did transmute the stuff
   to atomic numbers of 110 and it decayed - what would it decay
   to?  Right back to where we had transmuted it from - no 
   gain.
   
   Let me know if you have additional questions.
   
    


-- 
B. Alan Guthrie, III            |  When the going gets tough,
                                |  the tough hide under the table.
alan.guthrie@cnfd.pgh.wec.com   |
                                |                    E. Blackadder