On 30/04/2018 11:35, Jethro_uk wrote:
On Mon, 30 Apr 2018 07:57:37 +0100, Chris Hogg wrote:

Maybe the tree ferns and other vegetation that eventually became the
carbon in coal were able to concentrate those heavy metals from the
groundwater.

Brazil nuts are faintly radioactive (as are bananas).

The extensive root systems work to concentrate *already existing*
radioactive compounds.

Seem to remember that Brazil nuts are one of the most radioactive
foodstuffs around ( radioactivity by weight).