WASHINGTON (Reuters) - U.S. nuclear reactor builders will likely have to weigh the potential for a commercial aircraft strike when they design new plants, the Nuclear Regulatory Commission said on Tuesday. The NRC's proposed rules are meant to protect new reactors against a deliberate hit by a jet like those that rammed into the World Trade Center and Pentagon on September 11, 2001, the commission said. "This is the most recent step in a broad, proactive effort to improve the security of reactors initiated by the NRC after September 11," NRC Chairman Dale Klein said. "We need more technical analysis...