Orphan source: you wouldn’t want WA’s missing radioactive capsule on your desk

They call them “orphan sources,” like when a tiny radioactive capsule smaller than a 10-cent piece goes missing in remote Western Australia.

It’s so called because it’s abandoned, missing or stolen and no longer under regulatory control.

The small silver capsule measuring 6mm by 8mm went missing somewhere on a 1400km truck route between a Pilbara mine site and Perth between 10-16 January; details the WA health department made public on Friday, 27 January.

WA Chief Health Officer Andrew Robertson says the missing capsule is a 19-gigabequerel caesium 137 source, described as emitting a large radiation dose, the equivalent of receiving 10 x-rays in an hour, AAP reports.

Curtin University’s Associate Professor Nigel Marks has extensive experience in nuclear materials. He tells Cosmos, Caesium 137 is commonly used in university, industrial and medical applications. He says the missing capsule would normally form part of an instrument, which in this case was used for measuring the density of iron ore.

19-gigabequerel is a measure of the amount of radioactivity, the gamma rays emitted as the source decays over time. Caesium 137 has a half-life of 30 years, which means its radiation output will halve in three decades.

Marks, a physicist with experience in radiation and radioactive waste, says the incident seems like “a weird failure of regulation, containment and engineering”.

He wonders why the radioactive capsule was packed together with its instrument on the back of the truck, instead of contained within a locked, lead box inside the car.

“You’re much better off having a small thing, in a small box, where you can see it.” A Geiger counter on board the vehicle could have helped to identify the problem sooner, he says.

“Something really has gone wrong with their QA [Quality Assurance],” he says.

He says the capsule would mainly pose a problem if someone picked it up, or it got trapped in someone’s car.

“You definitely wouldn’t want it on your desk, but if it was 10 metres away it wouldn’t hurt you at all”

“It looks pretty harmless. But if you put it in your pocket, you’d have radiation burns to your skin in a fairly short period of time.”

Members of the public who see the capsule or something similar are advised to stay at least five metres away and call 13 33 37.

Dr Pradip Deb, senior lecturer in medical radiation at RMIT, tells Cosmos it’s a very unusual event, as this kind of radiation source should normally be stored and transported in a shielded container.

On Monday Rio Tinto, the operator of Pilbara iron ore mine, apologised for the incident. The company opened the mine in question, named Gudai-Darri, in 2022, calling it its “most technologically advanced mine”.

Tony Irwin, an engineer specialising in nuclear and an honorary associate professor at Australian National University expects the capsule will be difficult to find given there are 1400 kilometres of remote road to search.

There’s also the risk that if the source fell on the road that it could have become lodged in the tyre of another truck. In which case it could be anywhere, he says.

The Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) is working with the WA Government to help locate the radioactive source.

The agency has sent a deployment team with specialised car-mounted and portable detection equipment to support the search of the transport route between the Pilbara region and Perth. They will be operational from 31 January.

In a statement, ARPANSA says the equipment and expertise forms part of the organisation’s national radiation protection and emergency response capability. The Australian Nuclear Science and Technology Organisation (ANSTO) have also sent deployment teams of radiation services specialists, and detection and imaging equipment to assist in the search.  

Radiation sources are regulated by state and territory authorities, in line with codes such as the Code for the Safe Transport of Radioactive Material, ARPANSA says.

Irwin says, usually radiation sources are listed, monitored, kept securely, and require a licence to handle them.

“It’s quite uncommon to lose a source in this way”, he says, so it will be important to review the incident to find out what went wrong.

“You’ve got to learn from these things and make sure this doesn’t happen again.”