The service said in a statement that tests showed the material was a radioactive isotope which could be obtained only "as a result of a nuclear reactor's operations".
The statement said the material had triggered an alarm in the airport's radiation control system. The luggage search led to a discovery of 18 pieces of radioactive metal packed in individual steel casings.
A Sheremetyevo airport customs spokeswoman said by telephone that the material had been identified as sodium-22 but gave no other details. Sodium-22 is a radioactive isotope of sodium that can be used in medical equipment.