A terrorism event may occur using radioactive material. Immediate radiation sickness and long-term illness will occur. The risk for developing cancer increases with exposure to radiation. Yet there are some basic ways to minimize that exposure.
Minimize the exposure time.
Radiation intensity decreases dramatically as the distance from the source increases.
Barriers of lead, concrete, or water give good protection from penetrating radiation. If at all possible, improvise some shielding. Even your home is better than nothing.
(Attribution: U.S. Nuclear Regulatory Commission)