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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

> > >

> > >

> > >

> > > __________________________________

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