The reference to a "de minimus limit" engenders the obvious...

The reference to a "de minimus limit" engenders the obvious question: What should this limit be?

Below is a reference to a research study of Kvanefjeld radiation levels, conducted in 2015 by the Danish DTU.
It states: one, that radiation levels are measured in millisieverts (mSv); two, that levels below 20 mSv per annum over a 5 year period (no more than 50 mSv in any one year) are acceptable; and three, that a mine at Kvanefjeld will, conservatively, produce between 1 and 5 mSv per annum.

Question answered.

https://orbit.dtu.dk
Predicted Radiation Exposure from Mining at Kvanefjeld: Introduction to Radiation, Review of Baseline Information, and Predicted Radiation Exposures from Kvanefjeld Mining, Mineral Processing and Refining — Welcome to DTU Research Database

P.S. Any one interested in this report may need to dig a little to find it.