For those in the disaster business, one nightmare involves radiation spewing from a ravaged nuclear power plant, a terrorist’s dirty bomb, the mishandling of nuclear waste, a transportation catastrophe or a scenario not yet imagined. So naturally a wave of anxiety spread across the Western states — especially Nevada and California — when the U.S. Environmental Protection Agency announced it was going to move — this weekend, in fact — one of its two Mobile Environmental Radiation Laboratories from Las Vegas to Montgomery, Ala., to join the other one there, 2,000 miles away.