Not my field but air is used (drawn in for Bessamer process steel or processed air used for more modern steelmaking) and that will contain minute traces of radionucliedes.
This is a reason that sunken pre WW2 era ships are especially attractive, apart from the obvious scrap value. Known as 'low background steel'. Parts of the scuttled WW1 German fleet at Scapa Flow has been used for this.
It's useful for tools that need to sense minute traces of radioactive material and for MRI scanners.
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