Uranium-235 isn't useful for dirty bombs: it's too long-lived and not radioactive enough. Making dangerous levels of radiation requires short-lived isotopes. Some of these are the products of uranium fission, which is why spent uranium fuel is a radiation hazard. But isolating U-235 just to fission it to get radioactive isotopes like cesium-137 would not be the easiest way to get them.
alt Hacker News
Uranium-235 isn't useful for dirty bombs: it's too long-lived and not radioactive enough. Making dangerous levels of radiation requires short-lived isotopes. Some of these are the products of uranium fission, which is why spent uranium fuel is a radiation hazard. But isolating U-235 just to fission it to get radioactive isotopes like cesium-137 would not be the easiest way to get them.