Japanese police say they've arrested a businessman has been arrested for stealing a small amount of radioactive material and throwing it in a river.

Police say Tomonori Iso stole a metal container holding a sealed, 2-millimetre section of iridium-192 from a firm he was working for as a subcontractor.

They say he broke the container in two parts, and then threw it in a river, from where the material's now been retrieved.

It's believed Mr Iso may have held a grudge against his company or the contractor.

The Science Ministry has told police the incident has had no apparent impact on human health.

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