The head of the IAEA had warned the Seibersdorf facility was outdated
There has been a plutonium leak at a site run by the International Atomic Energy Agency (IAEA) in Austria.
UN nuclear monitors said pressure had built up and plutonium had contaminated a storage-room at the Seibersdorf laboratory, south of Vienna.
Last year the head of the IAEA warned the facility was outdated and did not meet UN safety standards.
Austrian officials said the empty lab had been sealed off and no-one was at risk. An investigation is planned.
The lab is used to carry out tests on samples taken during IAEA inspection missions.
"Pressure build-up in a small sealed sample bottle in a storage safe resulted in plutonium contamination of a storage room... at the IAEA's Safeguards Analytical Laboratory in Seibersdorf," said the agency.
The leak, which occurred in the early hours of Sunday, automatically set off an alarm via an air-monitoring system, said Daniel Kapp, a spokesman for Austria's environment ministry.
He added that radioactivity in the air would have been completely contained by the lab's filters, and Austrian monitoring centres had detected no increase in radioactivity, meaning no-one was in danger.
Last November, IAEA director general Mohammed ElBaradei said the site, constructed in 1970, did not meet UN safety standards.
He warned there was an "ever-growing risk" key components of the lab might break down, although the IAEA said there was no connection between Sunday's leak and the modernisation requirements.