Russia's customs service said on Friday it had seized radioactive sodium-22, an isotope that is used in medical equipment but has no weapons use, from the luggage of a passenger planning to fly from Moscow to Tehran.
The service said in a statement that the material could be obtained only "as a result of a nuclear reactor's operations" but did not say when it had been discovered at Moscow's Sheremetyevo international airport.
The material triggered an alarm in the airport's radiation control system and a luggage search led to the discovery of 18 pieces of the radioactive metal packed in individual steel casings, it said.
Sodium-22 can be used in medical equipment and nuclear detectors, nuclear experts said.
"There is no weapons aspect to this (material)," research director Lars-Erik De Geer of the Swedish Defence Research Institute.
Tension is rising between Western powers and Iran after a United Nations nuclear watchdog report last month that said Tehran appeared to have worked on designing a nuclear weapon, and that secret research to that end may be continuing.
Russia, which built Iran's first nuclear power station, has said it might help Tehran construct more atomic plants.