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Re: Article: Let a Thousand Reactors Bloom
Hi all,
I was starting to reply to this when Douglas's reply came through. I would
echo what he said, and add or at least reinforce one more thing. Since the
moderator is hot, thermal takes on a different meaning. The energy spectrum
is higher to begin with which means more neutrons are in the U238 resonance
capture energy range. So even if the area under the integral remained the
same the shift in energy up at higher temperatures favors capture over
fission.
Dale
daleboyce@charter.net
----- Original Message -----
From: "Minnema, Douglas" <Douglas.Minnema@nnsa.doe.gov>
To: "'Bill Prestwich'" <prestwic@mcmaster.ca>; "John Jacobus"
<crispy_bird@YAHOO.COM>
Cc: "radsafe" <radsafe@list.vanderbilt.edu>
Sent: Friday, September 10, 2004 3:17 PM
Subject: RE: Article: Let a Thousand Reactors Bloom
> Bill,
>
> First, the story has the Doppler effect wrong. The effect does not
describe
> the atoms moving apart when heated, but rather changes in the
cross-sections
> as a function of neutron energy. But you apparently recognize that
already.
> The catch is that the resonance peaks are non-fission captures, and
> therefore take neutrons away from the system before they can cause a
> fission.
>
> I have to rely on my memory here because my brains (oops, I mean books :-)
> are at home, but I think I can explain it. The reactivity of a critical
> system is based on the probability of a neutron to cause a fission event.
> Most typical reactors use thermal neutrons, which require slowing down the
> higher energy fission neutrons without losing them to non-fission
> absorption. The resonance peaks in the epithermal of some elements, U-238
> being a prime example, are absorption captures that remove the neutrons
> before they are slowed completely to the thermal energy range. The
> probability of resonance capture is based on the integral of the resonance
> peaks. Therefore, as the material heats up, the peak maximum
cross-section
> value lowers (the Doppler effect), but the area under the peak increases
> because of the broadening, which results in an increase of the integral
> non-fission capture probability in that energy region. Therefore the
> probability of fission capture goes down, and the system reactivity goes
> down.
>
> In normal water-moderated reactors, the concept is to have the neutron
> escape the fuel into the water before it slowed down, and to not have it
go
> back into fuel until it's energy is below the resonance region. In the
> particle bed reactors, the fuel and moderator are more closely mixed, so
the
> probability of resonance capture goes up, and you can take advantage of it
> more as a temperature feedback mechanism.
>
> It really does work, many TRIGA type pulse reactors rely on this as part
of
> the shutdown mechanism.
>
> Hope this helps,
>
> Doug Minnema
> NNSA
>
> -----Original Message-----
> From: Bill Prestwich [mailto:prestwic@mcmaster.ca]
> Sent: Friday, September 10, 2004 3:01 PM
> To: John Jacobus
> Cc: radsafe; know_nukes
> Subject: Re: Article: Let a Thousand Reactors Bloom
>
>
> Can someone who had time to read this interesting article explain to me
how
> the Doppler shift lowers reactivity-(I think that is what is implied)? I
> would have thought it would just broaden the resonances in the fission
cross
> section.
> Cheers,
> Bill Prestwich
>
> John Jacobus wrote:
>
> > >From another list serve I belong to. Sorry if this
> > was previously posted.
> >
> > -----Original Message-----
> > From: mailadmin@magportal.com
> > [mailto:mailadmin@magportal.com]
> > Sent: Thursday, September 09, 2004 1:25 AM
> > To: Jacobus, John (NIH/OD/ORS)
> > Subject: Hot Articles 2004-09-08
> >
> > . . .
> >
> > Wired looks at China's plan for nuclear power.
> > http://www.wired.com/wired/archive/12.09/china.html
> >
> > . . .
> > Copyright (c) 2004 Hot Neuron LLC.
> >
> > -------------------------------------------------------
> >
> > =====
> > +++++++++++++++++++
> > "Everyone is ignorant, only on different subjects."
> > Will Rogers
> >
> > -- John
> > John Jacobus, MS
> > Certified Health Physicist
> > e-mail: crispy_bird@yahoo.com
> >
> >
> >
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