You're also ignoring the larger threat to that plutonium; the issue that someone may not want to separate the contaminants. If they're making a dirty bomb, they don't care if it has contaminants or not.
So which is worse for that, having a fuck-ton of seriously nasty stuff sitting around forever, or concentrating the nasty stuff to a much more manageable volume and then fairly quickly shipping it off to be destroyed (in a way that just happens to provide useful energy out of the deal).
So which is worse for that, having a fuck-ton of seriously nasty stuff sitting around forever, or concentrating the nasty stuff to a much more manageable volume and then fairly quickly shipping it off to be destroyed (in a way that just happens to provide useful energy out of the deal).