When inspectors found suspicious nuclear material at this site, the Iranians said it was just contamination.

BBC

Some of this material will be used to scrub the machinery's innards of any contamination that may have travelled with the rover from Earth.

BBC: Curiosity Mars rover takes historic drill sample

Relatively low contamination levels in western Japan could be explained by mountain ranges sheltering those regions from the dispersal of radioactive material, the authors said.

BBC: Fukushima fallout fears over Japan farms