Paducah and Wilmington, page-13

I may be wrong about Pu239 but it and Pu240 would appear to sum up to much the same thing. Both U239 and Pu240 decay through emission of an alpha particle (a Helium nucleus) which results in a decay to U235 (still very fissile) from Pu239 or from Pu240 (half life 6561 years) to U236 (a nuclear poison). Whilst U236 is a nuisance it has a long half life (2.348 x10⁷ years) but decays by emission of alpha (low penetration). Thus in terms of HL waste is not a major problem (however, it is if you wish to use this decay product (U236) in fuel). From a proliferation perspective U236 is an advantage since it inhibits manufacture of weapons.

Where there is a concern from a proliferation viewpoint in that Pu239 is the perfect weapons material (like U235 HEU), its the presence of Pu 240 in reactor (or weapons grade) Pu which inhibits it from being used in simple gun like weapon. As such from a proliferation viewpoint there will be lot of sensitivity about a technology that enables proliferators to produce a simple weapon (an implosion weapon with Pu240 present is a lot more difficult to produce) and probably not within the capability of terrorists (or without testing). A weapon with Pu239 (or U235 HEU - as was used in Hiroshima) is just a gun (i.e a tube, some explosive, possibly a neutron source and 10 kg of Pu 239 - U235 critical mass is 52 kg).

From the perspective of Pu waste the isotope you do not want Pu241 (half life 14 yrs) since it decays to Americium 241 (half life 432 yrs) which is non fissile and a major contributor to radio active waste and difficult to separate as it does not fractionate with the other major fission products but associates with lanthanides, strontium, caesium, barium, yttrium, and is not recycled into nuclear fuel.