Russia closed down a plutonium producing reactor Sunday, marking a major milestone in U.S. nuclear nonproliferation efforts, a Russian news agency reported.

The U.S. and Russia have been working for years on arrangements to close Russia's three remaining weapons-grade plutonium producing reactors.

A second reactor, also in the Siberian town of Seversk, is expected to be shut down in June, the ITAR-TASS news agency quoted a Siberian Chemicals Plant official as saying.

The reactor closed Sunday was temporarily deactivated in an unscheduled incident earlier this month.

Plant officials, who were not immediately available Sunday to confirm the report, have said the stoppage was caused by a power failure and that no leaks had been detected. Russian Federal Atomic Energy Agency officials also could not be reached for comment.

The third reactor, in the central Siberian town of Zheleznogorsk, is expected to be shuttered at the end of next year.

Closing the reactors has been a major U.S. nonproliferation goal. But the reactors provide electricity and heat to the nearby towns, and the Russians have been reluctant to shut them down before two fossil fuel plants are built.

An agreement was reached in March 2003 between the U.S. and Russia on Western access to Seversk and Zheleznogorsk. Washington committed US$926 million (euro586.4 million) to help build the fossil plants, with the one at Seversk almost completed.

The project to shut down ADE-4 was implemented by U.S. firm Washington Group International together with Russian contracting firm Rosatomstroi.

Seversk, formerly known as Tomsk-7, is among 10 cities that once were at the heart of the Soviet Union's nuclear weapons production complex.

The reactors were built in secret in the 1960s in the Soviet Union's race to challenge with the U.S. monopoly on nuclear weaponry. The government stopped buying plutonium from the site in 1993, however.

The design of the Seversk reactor was similar to the Chernobyl reactor involved in the 1986 nuclear disaster in Ukraine. But unlike U.S. reactors, it did not have concrete containment domes to hold in radiation in case of an accident or major leak.

The U.S. has already closed all 14 of its plutonium reactors.