Civilian and military plutonium:
The plutonium used in atomic bombs contains more of the 239 isotope than the plutonium found in civilian reactors. Weapons-grade plutonium is enriched to 93% whereas the plutonium found in PWRs contains around 60%. The presence of 40% of other isotopes makes it inadequate for military uses - in particular, if the concentration of plutonium 240 is greater than 20% any resulting bomb will be highly unstable.