The term irty bomb is primarily used to refer to a radiological dispersal device (RDD), a radiological weapon which combines radioactive material with conventional explosives. Though an RDD is designed to disperse radioactive material over a large area, a bomb that uses conventional explosives would likely have more immediate lethal effect than the radioactive material. At levels created from most probable sources, not enough radiation would be present to cause severe illness or death. A test explosion and subsequent calculations done by the United States Department of Energy found that assuming nothing is done to clean up the affected area and everyone stays in the affected area for one year, the radiation exposure would be "fairly high", but not fatal. Recent analysis of the Chernobyl accident fallout confirms this, showing that the effect on many people in the surrounding area, although not those in close proximity, was almost negligible.