WASHINGTON (Reuters) -- IBM will build a next-generation supercomputer for the U.S. Energy Department with the potential to achieve a sustained speed of 1,000 trillion calculations per second, or one petaflop, the department said on Wednesday. The new computer, dubbed "Roadrunner", will be built at the Los Alamos National Laboratory in New Mexico. Congress provided $35 million in fiscal 2006, which ends on September 30, to launch the computer project. Roadrunner may eventually be used for an Energy Department program that ensures the U.S. stockpile of nuclear weapons is safe and reliable without the resumption of underground testing, the department...