The second nuclear submarine, USS Seawolf was the only U.S. submarine to have a sodium-cooled, beryllium - moderated nuclear power plant. It was commissioned in 1957, but it had leaks in its superheaters, which were bypassed. In order to standardize the reactors in the fleet, the submarine's sodium-cooled, beryllium-moderated reactor was removed starting in 1958 and replaced with a pressurized water reactor.