The US government selected a design Friday for a new generation of atomic warheads, taking a major step toward building the first new nuclear weapon since the end of the Cold War two decades ago. The military and the Energy Department selected a design developed by the Lawrence Livermore National Laboratory in California over a competing design by the Los Alamos National Laboratory in New Mexico, according to government sources who spoke on condition of anonymity in advance of a formal announcement. The decision to move ahead with the warhead, which eventually would replace the existing arsenal of weapons dating back 40 years, has been criticized as sending the wrong signal to the world at a time the United States is trying to contain nuclear weapons development in North Korea and Iran. But military and Energy Department officials have argued the new warhead will not add to the nuclear arsenal. They maintain the new design will make the weapons stockpile more secure and reliable without the need for actual underground testing.