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Changes to dose coefficients for occupational exposures

Advisory note

Well-dressed man inspectin machinery in a long shot

Summary

The International Commission for Radiation Protection (ICRP) is an independent, international organisation that develops and maintains the International System of Radiological Protection. The work produced by the ICRP is published in their reports and is commonly used as a basis for radiation protection standards, legislation, guidelines, programmes and practice.

The ICRP are in the process of publishing a series of new reports on the Occupational Intake of Radionuclides (OIR) that update dose coefficients for occupational exposed workers. To date three parts of the five part series has been published. This series will impact the dose assessment methodology for determining occupational exposures to workers from the intake (inhalation and ingestion) of radioactive materials.

Notes to reader

ARPANSA has previously provided an advisory and additional information in regards to changes for the dose coefficient for radon and radon progeny as published by the ICRP in OIR Part 3.

This advisory is aimed at regulators with an understanding of the radiation protection framework and the use and implementation of dose coefficients. It is aimed to provide information regarding changes to the dose coefficients for all other radionuclides.

Changes in the international publications

The ICRP have recently published the third part of their updated OIR series, the first of which was released in 2015. These documents provide updated dose coefficients for all radionuclides and will replace ICRP Publications 54 and 78 (ICRP, 1988a & 1997b). The series will comprise of five parts:

  • Part 1 – Provides a description of biokinetic and dosimetric methodology, and the use of bioassay data (ICRP Publication 130, 2015)
  • Part 2 – hydrogen (H), carbon (C), phosphorus (P), sulphur (S), calcium (Ca), iron (Fe), cobalt (Co), zinc (Zn), strontium (Sr), yttrium (Y), zirconium (Zr), niobium (Nb), molybdenum (Mo), and technetium (Tc) (ICRP Publication 134, 2016)
  • Part 3 – ruthenium (Ru), antimony (Sb), tellurium (Te), iodine (I), caesium (Cs), barium (Ba), iridium  (Ir), lead (Pb), bismuth (Bi), polonium (Po), radon (Rn), radium (Ra), thorium (Th), and uranium (U) (ICRP Publication 137, 2018)
  • Part 4 – lanthanides and remaining actinides; and (Publication TBC)
  • Part 5 – remaining elements. (Publication TBC)

The ICRP’s Committee 2 is responsible for the development of dose coefficients for the assessment of internal and external radiation exposure; development of reference biokinetic and dosimetric models; and reference data for workers and members of the public. The task group working to update dose coefficients has also been assigned to update the anatomical models to those based on information obtained through medical imaging. This aligns with ensuring that the models used for dose assessment are based on the best available body of knowledge as part of the ongoing review process. The task group also updated the nuclear decay data and reviewed and updated, where necessary, all biokinetic models used for the determination of dose coefficients.

Part 1 contains the key information regarding the control of occupational exposures, biokinetic and dosimetric models, monitoring methods, monitoring programmes and retrospective dose assessment. This report provides a description of the changes made to the ICRP reference Human Respiratory Tract Model (HRTM) (ICRP, 1994a) and an overview of the ICRP reference Human Alimentary Tract Model (HATM) (ICRP, 2006). 

Parts 2 and 3 contain information for each of the radionuclides including the typical chemical forms found in workplaces, routes of entry, monitoring techniques, dosimetric data and key references. The documents published to date do not provide complete dosimetric data for all radionuclides or dose integration time and are limited to gases/vapour with typically data for particles with a 5 μm activity median aerodynamic diameter (AMAD) and 50 year integration time. Comprehensive information has been made available in electronic annexes which contain the information for all isotopes and varying particle sizes. The electronic annexes for OIR Part 2 and Part 3 are available from the ICRP publications website.

Advice to implement changes

Regulators are advised to review the above documents and associated annexes against their licence holders monitoring programs and dose assessment methodologies. They should decide on an implementation plan for changes from currently used dose coefficients to ones published in this series. Changes should be considered as soon as the new data for the relevant radionuclides is available.

Comprehensive data for uranium will soon be available upon publication of the electronic annex for OIR part 3 and regulators with active uranium mines should review the new dose coefficient for uranium product dusts once this is accessible.

Determination of dose coefficients for mixtures of radionuclides (i.e. uranium and thorium ores) will only be able to be completed upon publication of OIR Part 4 which includes the remaining actinide series.

Relevance to Radiation Protection Series No. 9 & 9.1

It has previously been recognised that the information provided for dose coefficients in Table 1 of the Code of Practice for Radiation Protection and Radioactive Waste Management in Mining and Mineral Processing (RPS 9) has not been consistently derived and the results have been calculated for different radon retention percentages. This has resulted in calculations demonstrated in the Safety Guide Monitoring, Assessment and Recording Occupational Radiation Doses in Mining and Mineral Processing (RPS 9.1) also being inconsistent. Both RPS 9 and RPS 9.1 are scheduled to be revised by ARPANSA and amendments to relevant data will be provided. This work will await the publication of OIR Part 4.

ARPANSA has previously provided advice to address the inconsistency with the dose coefficient values in RPS 9. Reference is made to the Western Australian document Managing Naturally Occurring Radioactive Material (NORM) in Mining and Mineral Processing – Guideline NORM 5, Dose Assessment, Department of Mines and petroleum, (2010) which provides appropriate consistent dose coefficients for 100% radon retention. A correction factor can be applied for varying radon retention requirements.

Medical sector

Dose assessment programs in the medical sector may require the consideration of doses arising from the intake of radionuclides primarily in radiopharmaceutical production and administration of gaseous or volatile radionuclides. As a result, the use of occupational dose coefficients in the medical sector is generally limited to doses occurring as a result of incidents. Regulators and radiation safety officer should be aware of the publication of these new dose coefficients and use appropriate factors when determining relevant occupational exposures.