There were fears that terrorists or rogue states might make off with the concentrated fuel to create dirty bombs or nuclear weapons. With modern developments, it sounds like such theft would be the hard way.
Which were nothing by Envirowackjob propaganda to justify Jimmy Carter killing the program. The French have had such a breeder reactor to recycle spent fuel for 50 years and never had any such problem.
It would be easier for terrorist or a rouge state to steal a working nuke then it would for them to "make off" with the concentrated fuel.