Russia ships 8 grams of polonium-210 abroad annually

"Allegations that someone stole it during production are absolutely unfounded," said Sergei Kiriyenko, director of the nuclear agency Rosatom. "The controls are very tough."

Kiriyenko said Russia exports 8 grams of polonium-210 monthly, all of it to the United States. He said there had been no exports to Britain in five years.

Polonium-210 is highly toxic substance with 138 days half-life.