F. Marc de Piolenc wrote:
Consider that nuclear weapons could not be built if the fissiles had high rates of spontaneous decay - the stuff would detonate prematurely, resulting in a fizzle. That, incidentally, is why plutonium cannot be used in a gun-type device - two isotopes are inevitably present, one of which (forgot the mass number, but you can look that up, too) has a spontaneous decay rate that is too high for the (relatively) slow assembly rate of a gun.
Weapons grade plutonium contains less than 7% of the non-fissile isotope plutonium-240. This is created if you leave the plutonium-239 in the reactor after it is formed, and it manages to absorb an additional neutron. Small amounts of higher numbered plutonium isotopes are also created by the same process. For this reason, spent fuel from power reactors is not a suitable source of weapons grade plutonium, although it is theoretically possible to build a bomb from reactor grade plutonium if you use enough of it. Bear in mind you will have big thermal problems from short-lived plutonium isotopes if that you employ plutonium from spent fuel, as well as problems from neutrons getting soaked up by non-productive species. -- Eric Michael Cordian 0+ O:.T:.O:. Mathematical Munitions Division "Do What Thou Wilt Shall Be The Whole Of The Law"