In natural U mill-tailings, the concentration of radioactive Ra isotopes generated in the decay chain reactions can be as high as ∼10 3 Bq/kg of 226 Ra in soils (Carvalho et al., 2014), ∼10 4 Bq/kg of 226 Ra and ∼10 2 Bq/kg of 228 Ra in mining waste heaps (Carvalho et al., 2007), and ∼10 –1 –10 2 Bq/L in water near U mill tailings ...