From: Robert J. Bradbury (bradbury@aeiveos.com)
Date: Tue Oct 22 2002 - 11:40:17 MDT
On Mon, 14 Oct 2002, Robert J. Bradbury (hey thats me!) wrote:
> Anyone enough of a nuclear engineer to know what the critical
> parameters are? Is it rate of alpha particle production?
> Or perhaps neutron capture rates (thermal neutron cross section???).
> Also -- nuclear reactions as I was taught them were based on
> an exponential rate in the increase of neutron production
> (alpha particles shouldn't be able to enter a nucleus) --
> how does the release of an alpha particle get translated into
> neutrons? (Does the alpha particle itself decay very quickly?).
Well it turns out scientists at Los Alaomos are have done an
experiment to work out the critical mass for Neptunium (the
element between Uranium and Plutonium).
Neptunium Goes Critical During Experiment At Los Alamos
http://www.spacedaily.com/news/energy-tech-02s.html
[Watch out for the pop-ups -- they may not work with Netscape]
So I guess part of the answer to my question is that you need
less critical mass if the element is more radioactive.
Half-lives:
U-238: 4.5x10^9 yrs
U-235: 7.0x10^8 yrs
Np-237: 2.1x10^6 yrs
Pu-239: 2.4x10^4 yrs
Am-242: 141 yrs (proposed for nuclear fission rocket engine)
Pu-238: 87.7 yrs (used in RTGs)
So my guess would be that the amount of Np-237 required for
a nuclear weapon would be several kg more than that required
for a Pu-239 based weapon but less than that for a U-235 based
weapon.
Robert
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