Russia has shut down the second of its three plutonium-producing reactors as part of a nuclear nonproliferation program with the United States, the Energy Department said. The remaining reactor at the once-closed city of Seversk in south-central Russia was shut down Thursday, ending 43 years of plutonium production there for Russia's nuclear weapons. The other Seversk reactor stopped operating last April. During the Cold War years, Seversk, known then only by its code name Tomsk-7, was a so-called secret city because of its weapons production activities. It did not appear on any official map until 1992. Russia continues to produce plutonium from a third reactor in the central Siberian town of Zheleznogorsk. It is scheduled to be shuttered by 2010.