This, from SmartMoney.com(my emphasis added):
AMERICA HAS THE most nuclear reactors. Russia had the first (a five-megawatt small-fry). Britain has the oldest in operation. France has the biggest (1,445 megawatts). Constellation Energy Group (CEG: 68.61, -0.06, -0.1%), a regulated distributor of electricity to the city of Baltimore and to central Maryland and a nonregulated wholesaler to the rest of the country, has the distinction of owning the U.S.'s oldest operating nuclear reactor. (It's a two-way tie, really.) The smaller of its two General Electric-built reactors (621 megawatts) at Nine Mile Point in Oswego, N.Y., went online in 1974.