Re: What is Weapons Grade

[Bernard L Cohen <blc+@pitt.edu>]

	"Weapons grade" plutonium is normally defined as having a low
ratio of Pu240 to Pu239. Since Pu240 has a high rate of spontaneous
fission, it releases neutrons which causes pre-detonation of a bomb. Bomb
grade Pu is made from U238 by leaving it in a reactor for a short time,
typically one month. The ratio of Pu240 to Pu239 increases linearly with
time in the reactor, so for LWRs where fuel is typically left in the
reactor for 3 years, it is very high, making it unsuitable for use in a
bomb. It is called "Reactor grade" Pu. U.S. government bomb designers have
been able to use Reactor grade Pu for a bomb, but it was not easy and the
bomb is not reliable and has low explosive yield. This bomb was ordered 
by the Carter Administration to support its anti-proliferation political
positions -- if a bomb cannot be made from Reactor grade Pu there is no
proliferation problem from LWR power plants. The Carter Administration
anti-proliferation politics was roundly denounced all over the world and 
did not succeed in its objective of stopping reprocessing in foreign
countries. But it did shut down the completed Barnwell plant, thereby
destroying the reprocessing plans for commercial spent fuel in U.S.,
which was probably the real goal of the Carter Administration anti-nukes 
	No military bombs in any nation's arsenal would use Reactor grade
Pu, according to my understanding.

Bernard L. Cohen
Physics Dept.
University of Pittsburgh
Pittsburgh, PA 15260
Tel: (412)624-9245
Fax: (412)624-9163
e-mail: blc+@pitt.edu


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