The release of the highly volatile radioxenon over several months last year was so concentrated that the plumes were detected in Melbourne up to two days later.

Other plumes were dragged out to sea by winds before drifting back over Sydney.

The Sunday Telegraph understands the Australian Nuclear Science and Technology Organisation (ANSTO) decided against releasing a public statement at the time to avoid causing alarm.

Scientists at a nuclear testing station in Melbourne traced the source of the radioactive gases to Sydney after they picked up 10 specific events between November, 2008 and February last year.

The Comprehensive Test Ban Treaty Organisation International Monitoring System site in Melbourne contacted Lucas Heights after detecting the radioxenon isotope Xe-133.

 

They were told that 36 hours earlier the first "hot commissioning trials" at ANSTO's Lucas Heights radioisotope facility for Molybdenum-99 had taken place.

Molybdenum-99 is produced by the fission technique - the intense neutron-bombardment of a highly purified uranium-235 - and is used in nuclear medicine.

 

While the nuclear reactor - and the government body that oversees it - insists the release of the radioxenon by-product were no threat to public safety, no one, including neighbours of the suburban Sydney plant, were informed.

"Xenon gases are highly volatile and, being inert, they are not susceptible to wet or dry atmospheric removal mechanisms," a scientific report obtained by The Sunday Telegraph says.

"Consequently, once released to the atmosphere they are simply transported down-wind while radioactively decaying away."

Significant amounts of the main gas detected - Xenon-133 - can be released during a nuclear reaction or a nuclear explosion.

While it is used in medical procedures, specialists are urged not to administer it to pregnant women and children.

Side effects of its use in medical procedures can include allergic reactions such as itching or hives, swelling of the face or hands, swelling or tingling in the mouth or throat, chest tightness, and trouble breathing.

The report into the release from Lucas Heights says the doses were "well below the annual limit for public exposure".

Officials from the Australian Radiation Protection and Nuclear Safety Agency said it was notified at the time and that the emissions were within public safety guidelines.

In 2006, ANSTO was forced to allay public fears after a leaked memo revealed xenon and krypton were released into the atmosphere following the rupture of a pipe.