MOSCOW  Russia’s state nuclear energy corporation is expected to switch off a nuclear reactor on Sunday in a closed city in Siberia. The reactor has been producing weapons-grade plutonium for four decades, a senior American nonproliferation official said Saturday.

The reactor, ADE-4, is one of two in the city of Seversk that have been extraneous remnants of the Soviet Union’s nuclear weapons program since the cold war. For 15 years, they produced plutonium that the Kremlin neither needed nor wanted.

Opened in secret in the 1960s to feed the arms race, the reactors have continued to operate because of their peculiar construction as defense-industry suppliers.

The Defense Ministry stopped purchasing plutonium in 1993, rendering the reactors’ primary purpose obsolete. But the reactors could not be closed, and plutonium was still produced, because the reactors were also a primary source of heat and power to the bitterly cold regions along the Tomsk River, where no equivalent utility sources had been built.