Reprocessing contaminates air and water to some extent. The most difficult product to control is Kr 85 which is an inert gas that usually gets vented to the air but could be stored cryogenically.
Waste products include Cs 137 and some other fission products that decay in 500 years or so. If these are concentrated they generate heat that must be managed. You also get some isotopes of uranium that are not good for fission like U 236, the plutonium goes back into reactors and so can some other transuranics, but others pose challenges. Np 237 is probably the isotope that would be most desirable to steal from a reprocessing plant to make a gun-type bomb, put it back into the reactor and it will fission but also breed Pu 238 which will make huge amounts of decay heat. This complicates the handling of the fuel but it might be a good thing because there is no way anybody is making a bomb out of Pu that is so contaminated…
Waste products include Cs 137 and some other fission products that decay in 500 years or so. If these are concentrated they generate heat that must be managed. You also get some isotopes of uranium that are not good for fission like U 236, the plutonium goes back into reactors and so can some other transuranics, but others pose challenges. Np 237 is probably the isotope that would be most desirable to steal from a reprocessing plant to make a gun-type bomb, put it back into the reactor and it will fission but also breed Pu 238 which will make huge amounts of decay heat. This complicates the handling of the fuel but it might be a good thing because there is no way anybody is making a bomb out of Pu that is so contaminated…