Russian defector Alexander Litvinenko, an outspoken critic of President Vladimir Putin's administration and a close associate of exiled oligarch Boris Berezovsky, died in a London hospital with symptoms of radioactive poisoning November 23.
British health officials said a lethal dose of polonium 210, a toxic uranium by-product, was found in his body.
"The reactor was shut down two years ago," the source at the Federal Agency for Nuclear Power said, without revealing the reactor's location.
But he said Russia has produced eight grams of polonium monthly from reserves that remain in stock following the reactor's shut-down.
"We have supplied it [polonium-210] to U.S. companies, and there were deliveries to British firms. The eight grams we have produced cannot have disappeared in Russia, but we do not keep track of the material after selling it," the source said.
Polonium supplies to the United Kingdom ended in 2001.
According to the latest report published by the International Atomic Energy Agency (IAEA), no cases of disappearance of radioactive materials have been registered in Russia in recent years.
The global nuclear watchdog has been keeping a database on illegal trafficking of radioactive materials since 1993.
Following Litvinenko's death, Western media circulated a message purporting to be his deathbed note, in which he accused President Putin of orchestrating his death. The Kremlin has denied any involvement.
An Italian contact of Litvinenko, Mario Scaramella, has also been diagnosed with polonium 210 poisoning. He said that he and Litvinenko were poisoned because of secret information they shared, but did not specify details.
In ongoing investigations run by Scotland Yard, trace amounts of radiation have been discovered at 12 sites in Britain, and on two British Airways planes that flew the Moscow-London route.
Russia's Transportation Ministry announced Saturday that radiation was discovered on a Finnair plane, which arrived from Berlin via Helsinki, at a Moscow airport.
The results of Litvinenko's post-mortem examination Friday have been passed on to toxicologists for analysis, and have yet to be announced.