The worldwide average activities in the earth crust for the main natural radionuclides are [40K] = 400 Bq kg−1; [238U] = 35 Bq kg−1; [226Ra] = 35 Bq kg−1; [232Th] = 30 Bq kg−1. Using these activities, a simplified ratio of 238U:232Th:235U:40K equal to 1:1:0.05:10 was derived and used in the assessments carried out in this paper. As usual a ratio for 235U/238U of 4.6% was assumed