VIENNA, Austria -- More than two tons of radioactive material stored in a rundown research facility in Serbia offer an easy target for terrorists seeking to build a "dirty bomb," according to the United Nations' nuclear watchdog. Nuclear inspectors have branded the lightly guarded storage facility at a communist-era reactor, which closed 22 years ago, the world's most dangerous disused nuclear site -- because of the potency of the material and the risk of leakage. The outdated facility is on a 48-acre site at the Institute of Nuclear Sciences in Vinca, 10 miles outside the capital, Belgrade, surrounded by a...