Xref: utzoo sci.space:4585 sci.space.shuttle:553
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From: karn@thumper.bellcore.com (Phil R. Karn)
Newsgroups: sci.space,sci.space.shuttle
Subject: Re: Payload of shuttle flight directly after last Challenger.
Summary: Pu 238 *is* fissionable
Message-ID: <947@thumper.bellcore.com>
Date: 15 Feb 88 09:07:43 GMT
References: <347@flatline.UUCP> <1988Feb3.133415.12432@utzoo.uucp>, <1988Feb8.192335.27728@utzoo.uucp>
Organization: Bell Communications Research, Inc
Lines: 25

> alpha and beta particles.  Plutonium 238 (note, 238, *not* 239) is one
> fairly good choice.  Such isotopes do not have to be fissionable and in
> fact usually aren't.  Small isotope generators can use thermoelectric

From everything I've read about plutonium, all of its long-lived
isotopes are fissionable.  Pu-239 is favored for bombs because it is
easy to make, and because its spontaneous fission rate is low enough to
make bomb design simpler. (It was the high neutron levels caused by
Pu-240 contamination that ruled out use of the gun-type bomb with
plutonium during the Manhattan Project; implosion brings the fissionable
material together faster than a gun can, avoiding a fizzle).

Pu-238 is used in RTGs because its much shorter half-life (86 years vs
24,400 years for Pu-239) gives much more power per unit weight.

It is the lack of a nonfissionable isotope of plutonium that would make
the controlled destruction of the superpowers' plutonium stocks so
difficult.  Enriched uranium, on the other hand, can be effectively
"denatured" by mixing it with depleted uranium (U-238).

Refs: The Making of the Atomic Bomb, by Richard Rhodes; Stopping the
Production of Fissile Materials for Weapons, by von Hippel et al,
Scientific American, September 1985.  Also see the CRC Handbook.

Phil