How a Radioactive Capsule Was Lost and Improbably Found in the Australian Outback

“We’re a little bit more careful about what we pick up,” Ms. Schmidt, who manages the BP-branded roadhouse in the small town of Wubin, said as the search continued. “You give it a good look before you go close to it.”

Officials were preparing for the possibility of a long search, but there was a breakthrough after just seven days. On Wednesday morning, government teams using vehicle-mounted detectors got a hit while they were traveling on a stretch of road about 50 miles south of Newman, the closest large town to Rio Tinto’s mine. They then used portable detectors to locate the capsule, which was found lying in red desert dirt and surrounded by a few pebbles about 7 feet from the side of the road.

Authorities say they are now investigating how the capsule of radioactive material, transport for which is heavily regulated, went missing, and whether there were any issues in the packaging or trucking of the object. Professionals who work with these devices are puzzled how the capsule—just five-sixteenths of an inch long—could come loose and bounce out of a crate from the back of a truck.

A leading theory among authorities is that vibrations experienced by the truck on its journey damaged the box and shook loose the capsule, which then fell through a bolt hole onto the road.

“We’re scratching our heads,” said Lauren Steen, general manager at consulting firm Radiation Services WA, noting that such devices are commonly used in a number of industries and usually transported without incident.

Although it still isn’t clear what went wrong, the incident is the latest public affairs challenge for the mining industry, which is fighting to change investor perceptions that its activities are problematic. The world is gearing up for a new round of investment in resources that will power the global economy and drive the energy transition, and miners are facing greater scrutiny of their environmental records and struggle to maintain community support.

High-profile incidents such as this stick in people’s minds and can be recalled when companies propose new projects, said Saleem Ali, chair of the department of geography and spatial sciences at the University of Delaware. “These things can come back to haunt them,” said Prof. Ali, who also serves on the United Nations International Resource Panel.

“If you lose your reputational capital, you can end up with that situation where you have a good project and you can’t get it through,” he said.

Rio Tinto has apologized for the uproar caused by the capsule’s disappearance and has said it would be willing to reimburse authorities for the search, which officials said involved 100 people from various agencies. The effort drew in police, firefighters, the military, Australia’s nuclear-safety body and its national nuclear-science organization. 

“We are incredibly grateful for the hard work of everyone involved in finding the missing capsule,” said

Simon Trott,

head of Rio Tinto’s iron ore business. “We will be assessing whether our processes and protocols, including the use of specialist contractors to package and transport radioactive materials, are appropriate.”

The contractor that packaged the material and the trucking company that transported it have both said they followed appropriate guidelines.

The episode began on Jan. 10, when the capsule was packaged, and it was eventually placed in a wooden box that was fastened to a pallet on a truck. A Geiger counter was used to determine the presence of the capsule inside the package, Rio Tinto said. The truck departed the mine on Jan. 12 for the nearly 900-mile journey to Perth. But the capsule wasn’t discovered to be missing until Jan. 25, when the box was opened at a laboratory in the Perth suburbs.

The start and finish of the journey, near the mine site and the transport depot in the northeastern Perth suburbs, were searched early on, officials said. By Saturday, teams included firefighters who were looking at strategic sites along the GPS-tracked highway route that the truck had taken. At one point, authorities zeroed in on a part of the highway where a member of the public found some unusual activity on a Geiger counter used for prospecting, though the capsule wasn’t found.

Initially, authorities were relying on portable radiation survey meters to look for the capsule, which had a radius of about 65 feet. By Monday, however, local authorities said they had received specialized equipment from the national government, including car-mounted detectors that could be used to retrace the truck’s journey across the Outback. Crews soon began moving in both directions along the highway at roughly 40 miles an hour and officials said it could take five days to survey the entire route.

One challenge for authorities was the road—the Great Northern Highway—where the capsule was lost. Hundreds of trucks travel along the highway every day to replenish supplies at the region’s mines, raising the possibility that the capsule could have become lodged in another vehicle’s tire. Rain could have washed the capsule away, or it could have been taken away by an animal—an emu, eagle or kangaroo, for example.

The highway also runs through several towns, which sparked concerns among some residents that the capsule could have bounced off in the middle of their community.

In Mount Magnet, a town of about 600 people, local officials had no communication from the state emergency authorities who were looking for the capsule, said Shire President Jorgen Jensen, the equivalent of the mayor. Mr. Jensen, speaking the day before the capsule was found, suggested that authorities could have tried to give priority to searching for the capsule in towns first, which would have put community members at ease.

Many of Mount Magnet’s major amenities are on the main highway, including the post office, local government offices, a swimming pool, pubs and a supermarket, meaning that most people in town could have been exposed to radiation if the capsule fell off there. Some people also have houses in the main street.

“You’d have to question why something so small isn’t contained somewhere other than on the back of a truck,” Mr. Jensen said. “I would hazard a guess to say someone didn’t follow the rules.”

The capsule contained Cesium-137. Health officials had warned that it packed enough radiation to deliver the equivalent of 10 X-rays per hour. The radioactive material, or source, was used in a gauge that was measuring iron-ore feed in the crushing circuit of a plant at Rio Tinto’s Gudai-Darri mine site. The devices determine levels and density of material using the radiation emitted from the capsule.

The Cesium-137 was in a ceramic form and was encapsulated in stainless steel, officials said. Typically, that is highly protected within a housing, and the devices are designed to be installed and left in place for 15 years without much attention, aside from regular auditing, according to Radiation Services WA. The housings are supposed to be able to withstand corrosion, vibration, a roughly 30-foot fall and 30 minutes at temperatures nearing 1,500 degrees Fahrenheit, according to the company.

Still, when the box was opened, the gauge was broken apart with one of the four mounting bolts missing. The source itself and all screws on the gauge were also not there, officials said.

Although the loss of such radioactive material is rare, it does happen. In 2021, roughly 300 U.S. sources were reported lost and only about two-thirds were recovered, a spokesman for the U.S. Nuclear Regulatory Commission said. 

Most contained small or very small amounts of radioactive materials that would present minimal risk to the public, and are typically in density gauges used in construction or well-logging devices used in drilling operations, he said. The gauges are often reported missing after falling off vehicles in transit or being inadvertently left at a worksite.

In the U.S., the Nuclear Regulatory Commission and individual states with which it has signed specific agreements oversee roughly 2 million radioactive sources and devices.

Back at the roadhouse in Wubin, Ms. Schmidt initially thought it would take search teams a long time to find the capsule, if they ever found it at all. On Wednesday, she expressed relief that the capsule wasn’t near her roadhouse, given the health risks it could have posed.

“I didn’t think it was,” Ms. Schmidt said. “But then you never know.”

Write to Mike Cherney at mike.cherney@wsj.com and Rhiannon Hoyle at rhiannon.hoyle@wsj.com