Lower Bound of Optimization for the Public Considering Dose Distribution of Radiation due to Natural Background Radiation

The International Commission on Radiological Protection (ICRP) released drafts of new recommendations in June 2004, June 2006 and January 2007 and finally approved a new set of fundamental recommendations on the protection of humans and the environment from ionizing radiation at its meeting in Essen, Germany, 19-21 March 2007. In the process of the completion of the draft recommendations, the numerical value for the minimum dose constraint ensuring public protection was deleted or redescribed using other expressions. This issue remains undetermined even in the recently released Publication 103. On the other hand, ICRP also recently published a new concept of a representative person in Publication 101. This representative person is a hypothetical person exposed to a dose that is representative of the most highly exposed persons in the population. On the basis of this new concept, it is theoretically reasonable that the 95th percentile of the dose received by such representative persons is always lower than the dose constraint, which indicates that the main part of the dose distribution is considerably lower than the dose constraint. In this study, by using the relationship between the dose constraint and the dose distribution of the representative persons and a probabilistic approach using Monte Carlo calculation techniques, the effects of the dose distribution of radiation due to manmade radioactive nuclides when added to those of natural background radiation have been carefully investigated. The results show that additional exposure to manmade radiation of up to 0.5 mSv/y (as a dose constraint) would not significantly change the distribution of the public dose. Taking into consideration such probabilistic analysis and the rationale behind the derivations of exemption and clearance levels, it can be concluded that the minimum dose constraint that requires optimization in radiation protection should be set to 0.1 mSv/y, which is one order of magnitude higher than 0.01 mSv/y, the current dose criterion for exemption and clearance.