Nah.. See post #46 for the HEU stuff. Scaaaary..
Scary indeed. Even if the HEU is unsuitable for a bomb* it can be used in a "delayed critical assembly" that will generate lots of radiation. This type of device would be easier to deploy than one using "conventional" radioactives like cesium-137 or cobalt-60, because if you had enough Cs-137 to make ten acres uninhabitable, all in one cannister, how would you approach the thing to deliver it? The shielding requirements would be immense and the probability of detection would be very very high (there are radiation detectors here and there).
An assembly of U235 designed to act like a truck-sized Chernobyl meltdown, however, would not be as detectible and could be "turned on" when put in place.
*Even if the U235 is 90% enriched the remaining 10% can contain something that prevents it's making a nuclear bomb. Lots of research reactor fuel is so arranged, just for safety's sake.