Comparative Analysis of Approaches to Regulation and Monitoring of Workers for Internal Radiation Exposure

Purpose: Harmonization and improvement of the system for regulating the internal radiation exposure of workers and the basic requirements for ensuring radiation safety with international requirements and recommendations. Material and methods: Issues related to the development of approaches to regulation and monitoring of workers for internal radiation exposure in the process of evolution of the ICRP recommendations and the national radiation safety standards, are considered. The subject of analysis is the standardized values: dose limits for workers and permissible levels as well as directly related methods of monitoring of workers for internal radiation exposure, whose purpose is to determine the degree of compliance with the principles of radiation safety and regulatory requirements, including non-exceeding the basic dose limits and permissible levels. The permissible levels of inhalation intake of insoluble compounds (dioxide) of plutonium-239 are considered as a numerical example. Results: Based on the analysis of approaches to the regulation and monitoring of workers for internal radiation exposure for the period from 1959 to 2019, it is shown that a qualitative change in the approach occurred in the 1990s. It was due to a decrease in the number of standardized values by introducing a single dose limit for all types of exposure: the effective dose E, which takes into account the different sensitivity of organs and tissues for stochastic radiation effects (WT), using the previously accepted concepts of the equivalent dose H and groups of critical organs. From the analysis it follows that the committed effective dose is a linear transformation of the intake, linking these two quantities by the dose coefficient, which does not depend on the time during which the intake occurred, and reflects certain exposure conditions of the radionuclide intake (intake routes, parameters of aerosols and type of radionuclide compounds). It was also shown that the reference value of the function z(t) linking the measured value of activity in an organ (tissue) or in excretion products with the committed effective dose for a reference person, which is introduced for the first time in the publications of the ICRP OIR 2015-2019, makes it possible to standardize the method of measuring the normalized value of the effective dose. Based on the comparison of the predicted values of the lung and daily urine excretion activities following constant chronic inhalation intake of insoluble plutonium compounds at a rate equal annual limit of intake (ALI) during the period of occupational activity 50 years it was shown that the modern biokinetic models give a slightly lower level (on average 2 times) of the lungs exposure compared to the models of the previous generation and a proportionally lower level (on average 1.4 times) of plutonium urine excretion for the standard type of insoluble plutonium compounds S. However, for the specially defined insoluble plutonium compound, PuO2, the level of plutonium urine excretion differs significantly downward (on average 11.5 times) compared to the models of the previous generation. Conclusion: With the practical implementation of new ICRP OIR models, in particular for PuO2 compounds, additional studies should be carried out on the behavior of insoluble industrial plutonium compounds in the human body. Besides, additional possibilities should be used to determine the intake of plutonium by measuring in the human body the radionuclide Am-241, which is the Pu-241 daughter. To determine the plutonium urine excretion, the most sensitive measurement techniques should be used, having a decision threshold about fractions of mBq in a daily urine for S-type compounds and an order of magnitude lower for PuO2 compounds. This may require the development and implementation in monitoring practice the plutonium-DTPA Biokinetic Model.