Drawbacks of current dose limits on intake for plutonium

We have examined current national regulatory dose limits for occupational internal exposure and explained their weakness with reference to limits for inhalation of plutonium oxide. The purpose of the study is to analyze adequacy of standardized dose values underlying current national radiation safety standards, in order to improve radiation safety of workers contacting unsealed plutonium sources, and to set the dose limits, which allow keeping radiation risk at the acceptable social level. Our calculations show that committed effective dose from plutonium does not reflect the actual level of individual radiation exposure and potential radiation risk, because it reduces dose limit set in NRB-99/2009 (Radiation Safety Standards-99/2009) by three and more times depending on a plutonium compound and route of entry. That is why the committed effective dose cannot be used for standardization of internal dose from plutonium. Results of the study show that radiation risk coefficient depends on the age; within a working period it varies six-fold. The use of the constant tissue factor results in significant uncertainties of lifetime excess risk. Since annual excess risk associated with inhalation of plutonium oxide is formed due to exposure to lungs, we propose to consider the annual equivalent lung dose as the basic standardized dose limit, at which excess annual radiation risk is 1×10-3. Radiation internal dose from exposure due to different routes of plutonium enter can be limited using annual increment of plutonium content in a body or in a critical organ. For standardization of monofactorial radiation exposure due to contacts with plutonium special regulations for monitoring and limiting radiation doses from plutonium should be developed.