Cesium-137 for example has a 30-year half-life. So after e.g. 60 years you still have 1/4 of the radioactive cesium-137 remaining. For context, that's less than the length of time since the Three Mile Island accident.

Edit: according to Wikipedia, cesium-137 decays to barium-137m, which decays to barium-137 by gamma ray emission (https://en.m.wikipedia.org/wiki/Caesium-137). So even though cesium-137 decays through beta radiation it needs shielding against gamma radiation.