Getting the material is not that difficult for a determined group. Truly dangerous isotopes like caesium-137 or strontium-90 are both used for medical and other

Getting the material is not that difficult for a determined group. Truly dangerous isotopes like caesium-137 or strontium-90 are both used for medical and other applications, and have both been accidentally released from non-military sources after having been forgotten in scrapyards, then causing fatalities. They also have considerable half-lives, about 30 years, so you can't just wait a few months for it to decay out of an area. Dispersing them is also not that hard, and you don't even necessarily have to use an explosive. You could do an indoors release of a powder, or a release of a water-soluble salt into a pond or a fountain, or what have you. Of course, other motivations, like the panic generated by a physical attack, may be more important to someone choosing methods.