Unenriched uranium. They might make a bomb that could cause cancer 10 years later, but it’s no “sum of all fears” threat.
Put one of these in NY City, and watch people suddenly decide that they just can’t live in the city....even if not a single building was toppled or destroyed. They’d lose ten percent of their population within eighteen months. Who’d want to live around such a city at age twenty, when they know they might be dead by age forty from radioactive poisoning.