I'm not sure what you mean by your question.

It takes a few Kg of Pu to make a nuclear weapon. Probably a lot less to just make a crude dirty bomb. The amount dispersed in spent fuel is probably quite small, but I don't know, probably a few Kg. In Mox fuel, the amount is probably equivalent to the amount of U235 in regular fuel, around a few hundred Kg, but it is dispersed in inert uranium and hard to get out.

The amount that is toxic is in the milligram range (yeah, this stuff is bad news) but I don't know what kind of delivery system could achieve that concentration uniformly over a wide area. As far as a terrorist is concerned, a few grams of it spread in a subway or city reservoir could force the permanent evacuation of a city.

The health physics consequences of this plutonium spill are outside my area of expertise, but what I am really concerned about is that there appears to be a legitimate plutonium economy in the world right now, with countries making, storing, transporting and trading this stuff. All the plutonium the militaries of the world can ever need already exists in decommissioned weapons and can be recycled if needed for defense purposes. But tons of this stuff moving around for nuclear power generation changes everything.