[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: Article: Let a Thousand Reactors Bloom
Many thanks to you and Douglas for your very informative replies, proving once
again just how valuable radsafe is.
Sincerely,
Bill Prestwich.
daleboyce@charter.net wrote:
> 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
> > >
> > >
> > >
> > > __________________________________
> > > Do you Yahoo!?
> > > New and Improved Yahoo! Mail - 100MB free storage!
> > > http://promotions.yahoo.com/new_mail
> > > ************************************************************************
> > > You are currently subscribed to the Radsafe mailing list. To
> > > unsubscribe, send an e-mail to Majordomo@list.vanderbilt.edu Put the
> > > text "unsubscribe radsafe" (no quote marks) in the body of the e-mail,
> > > with no subject line. You can view the Radsafe archives at
> > > http://www.vanderbilt.edu/radsafe/
> >
> > ************************************************************************
> > You are currently subscribed to the Radsafe mailing list. To
> > unsubscribe, send an e-mail to Majordomo@list.vanderbilt.edu Put the
> > text "unsubscribe radsafe" (no quote marks) in the body of the e-mail,
> > with no subject line. You can view the Radsafe archives at
> > http://www.vanderbilt.edu/radsafe/
> >
> >
> > ************************************************************************
> > You are currently subscribed to the Radsafe mailing list. To
> > unsubscribe, send an e-mail to Majordomo@list.vanderbilt.edu Put the
> > text "unsubscribe radsafe" (no quote marks) in the body of the e-mail,
> > with no subject line. You can view the Radsafe archives at
> > http://www.vanderbilt.edu/radsafe/
> >
> >
************************************************************************
You are currently subscribed to the Radsafe mailing list. To
unsubscribe, send an e-mail to Majordomo@list.vanderbilt.edu Put the
text "unsubscribe radsafe" (no quote marks) in the body of the e-mail,
with no subject line. You can view the Radsafe archives at
http://www.vanderbilt.edu/radsafe/