[ RadSafe ] Fast neutron reactors

George Stanford gstanford at aya.yale.edu
Tue Dec 2 20:07:32 CST 2008


         The goal, which I am told is achievable, is to have less 
than 1% of the transuranics reach the waste stream.

         Per GWe-yr, there is roughly one tonne of waste that leaves 
a fast-reactor plant (e.g., IFR or S-PRISM) .  Annual output of 
transuranics would therefore be 10 kg or less per GWe-yr (there's an 
economic tradeoff).

         In the spent fuel from current (thermal) reactors, there's 
about 240 kg of transuranics per gWe-yr, currently destined for Yucca 
Mountain..  If instead it were to be used for input to fast-reactors, 
a 1% loss would mean that 2.4 kg of transuranics per GWe-yr of 
thermal-reactor power would be in the waste stream, mixed with a 
tonne of fission products.

         I'm sorry the Scientific American article did not make this clearer.

         Regarding the long-term hazard of nuclear waste, essential 
perspective is in Bernie Cohen's article, to be found at
<http://www.phyast.pitt.edu/~blc/pra-ppr.pdf>. Bottom line:  Yucca 
Mountain would be just fine, especially with a major reduction in 
transuranic content.

         Molten salt for the secondary loop sounds good to me (but 
that's not my field).


         --  George Stanford
         Reactor physicist, retired.


At 03:10 PM 11/30/2008, Jaro wrote:
The Sci. Am. article includes these statements :

Because the fast-reactor waste would contain no significant quantity of
long-lived transuranics, its radiation would decay to the level of the ore
from which it came in several hundred years, rather than tens of thousands.
Tailored waste forms that would only have to remain intact for 500 years,
after which material would no longer be hazardous.
<end quote>

Yet it is difficult to get any numbers on the actual amount of TRUs
(including Pu) that would end up in the waste stream.
The only clue we get is this :

....the pyroprocess collects virtually all the transuranic elements
(including the plutonium), with considerable carryover of uranium and
fission products.
Only a very small portion of the transuranic component ends up in the final
waste stream...
<end quote>

The problem is that since this is a U-Pu cycle, large amounts of Pu (many
tons) would pass through the pyroprocess, so that even a small level of
inefficiency could result in significant amounts of Pu (+ other TRUs) in the
waste -- possibly not sufficiently low to support the claim that the
"material would no longer be hazardous" after 500 years.

Has anyone seen some figures on pyroprocess Pu-TRU extraction efficacy ?

(By contrast, a fuel cycle based on U-Th has about two orders of magnitude
less Pu to begin with, so the amount that ends up in the waste is
correspondingly less also, assuming similar extraction efficacy....)

PS. as for the statement that "Engineers do not consider sodium's
flammability to be a major problem," it is interesting to note that recent
fast reactor design studies include the possibility of changing the
secondary heat transfer circuit sodium -- which passes through the water
boiler -- with molten salt :  The latter carries no risk of violent chemical
reaction with water or steam....


-----Original Message-----
From: radsafe-bounces at radlab.nl [mailto:radsafe-bounces at radlab.nl]On
Behalf Of Rainer.Facius at dlr.de
Sent: Saturday, November 29, 2008 1:00 PM
To: sjd at swcp.com; radsafe at radlab.nl
Subject: AW: [ RadSafe ] Fast neutron reactors


I forward a paper and a commentary by George Stanford (and colleagues) which
can answer your question authoritatively, much more so than I can. >From the
paper the following quote gives the gist of the argument.

"Fast reactors can extract more energy from nuclear fuel than thermal
reactors do because their rapidly moving (higher energy) neutrons cause
atomic fissions more efficiently than the slow thermal neutrons do. This
effectiveness stems from two phenomena. At slower speeds, many more neutrons
are absorbed in nonfission reactions and are lost. Second, the higher energy
of a fast neutron makes it much more likely that a fertile heavy metal atom
like uranium 238 will fission when struck. Because of this fact, not only
are uranium 235 and plutonium 239 likely to fission in a fast reactor, but
an appreciable fraction of the heavier transuranic atoms will do so as well.

 >From the commentary the following quote summarizes the consequence:

"Yes, reprocessing spent nuclear fuel in fast reactors can indeed solve the
'waste problem'. It will allow more than 99% of the energy in uranium to be
used -- unlike today's reactors, which utilize less than 1%."

Kind regards, Rainer

Dr. Rainer Facius
German Aerospace Center
Institute of Aerospace Medicine
Linder Hoehe
51147 Koeln
Voice: +49 2203 601 3147 or 3150
FAX:   +49 2203 61970


Von: radsafe-bounces at radlab.nl im Auftrag von Steven Dapra
Gesendet: Sa 29.11.2008 17:28
An: radsafe at radlab.nl
Betreff: [ RadSafe ] Fast neutron reactors

Nov. 29

         According to a book review in "Health Physics News" (Nov. 2008; p.
9, col. 3), fast neutron reactors will minimize waste and make better use of
uranium resources.  How will such reactors reduce rad waste and use uranium
more efficiently?

Steven Dapra
sjd at swcp.com


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