The [9 megaton] B53 was designed in the 1950s and replaced 500 B41s, which had a yield of 25 megatons. The largest nuclear device ever built and tested was the 27 ton Russian AN602, which had a yield of 50 megatons. It was eight meters (26 feet) long and 2.1 meters (6.9 feet) in diameter. Only one of these was built. The Russians followed the Americans in phasing out large yield bombs in favor of smaller, and more numerous ones. This was largely the result of ICBM warheads designed to carry multiple nuclear weapons.