The Bush administration announced yesterday the winner of a competition to design the nation’s first new nuclear weapon in nearly two decades and immediately set out to reassure Russia and China that the weapon, if built, would pose no new threat to either nation.

If President Bush decides to authorize production and Congress agrees, the research could lead to a long, expensive process to replace all American nuclear warheads in the next few decades with new designs.

The first to be replaced with the new Reliable Replacement Weapon would be the W-76, a warhead for missiles deployed on submarines.

Officials said the Lawrence Livermore National Laboratory in California would design the replacement warhead based on previously tested components, allowing the administration to argue that no new underground tests would be necessary before deploying the new weapon.