EXCLUSIVE- WASHINGTON - The manhunt for Osama Bin Laden has become more urgent because of growing fears among intelligence officials that he is obsessed with building a nuclear bomb to explode on U.S. soil. Federal intelligence and energy officials worry that Bin Laden's terrorists will build an improvised nuclear weapon and smuggle it into the U.S., possibly in a container ship, according to well-placed sources. His goal, they believe, is a nuclear hellstorm like the 1945 Hiroshima blast that killed 140,000 Japanese. Unlike smaller-scale attacks, which can be planned and authorized by lower-level leaders, the chain of command in Al...