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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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