How did a radioactive capsule go missing in Australia and how dangerous is it?

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Australian authorities are mounting an extensive search for a tiny radioactive capsule believed to have fallen out of a road train - a truck with multiple trailers - that travelled 1 400 km in Western Australia. Here's what you need to know:

What is it?

The silver capsule is just 6 mm in diameter and 8 mm long and was part of a gauge used to measure the density of iron ore feed at Rio Tinto's remote Gudai-Darri mine.

How did it go missing?

The gauge, packaged by a specialist contractor, was picked up from the mine on 12 January, by a separate logistics company. When it was unpacked on 25 January, it was found broken apart, with one of its four mounting bolts as well as screws and the capsule missing.

Authorities suspect vibrations from the road train caused the gauge to break apart and the capsule fell out.

How dangerous is the capsule?

Filled with Caesium-137, it emits radiation equal to 10 X-rays an hour. Authorities have issued a radiation alert for large parts of Western Australia and if it is spotted, recommend that people stay at least five meters away as exposure could cause radiation burns or sickness.

But just driving past it is believed to be of relatively low risk.

What does the search involve?

The road train travelled from the mine in the state's remote Kimberley region and arrived at a storage facility in the suburbs of Perth on 16 January.

Search crews are travelling north and south along the state's Great Northern Highway as well as other sections of the road train's journey with specialised radiation detection equipment. The Department of Fire and Emergency Services said on Monday it will take five days to cover the road train's route. It said on Tuesday that more than 660 km had been searched so far.

The search involves at least five other government agencies including the Department of Defence, the police, the Australian Nuclear and Science Technology Organisation and the Australian Radiation Protection and Nuclear Safety Agency.