A former Russian nuclear scientist has handed over to police eight containers of plutonium-238 he had stored at home for eight years.
Experts fear Soviet-era plutonium could be acquired by militants
The 400g (14oz) of plutonium-238 - a highly radioactive compound - came from a disused laboratory in Siberia.
Former employee Leonid Grigorov said he removed the containers for safekeeping after the lab was looted and stored them in a lead case, Russian media say.
He may face criminal proceedings, Russia's Itar-Tass news agency says.
A spokesman for Russia's Atomic Energy Agency (Rosatom), Nikolay Shingaryov, insisted that "this is not weapons-grade plutonium, but an isotope widely used in various instruments".
Counter-terrorism experts have repeatedly warned that radioactive material from decrepit Soviet-era installations could fall into the hands of militants.
Mr Grigorov is quoted as saying he had written letters to his former bosses warning of the risk posed by radioactive material left in the laboratory in Zmeinogorsk, which was abandoned and looted after the fall of the Soviet Union in 1991.
When his letters failed to elicit a response, Mr Grigorov says he was obliged to remove the material himself "to prevent anything bad from happening".
He says he took the plutonium from his garage to the local police, in response to a newspaper advertisement announcing a cash reward for surrendering weapons.
Zmeinogorsk police are quoted as saying Mr Grigorov was morally right to have hidden the hazardous material but he may nonetheless face criminal charges.
Itar-Tass said a legal case had been brought against the physicist for "illegal storage of radioactive substances".
Plutonium-238 can be used with ordinary explosives to make a "dirty bomb", potentially contaminating a large area with radiation.