[GSlob]

With half-life of 710 million years, U235 is an extremely poor choice of a material for a dirty bomb. Too long-lived [and thus too weakly radioactive]for such a use.

[7thOF7th]

Uranium and associated decay products thorium-230 and radium-226 will remain hazardous for thousands of years.

U235 enriched to a 20%-50% mix with U238 will render the surface area it settles on inhabitable for centuries