Recommendations on Dose Coefficients for Assessing Doses From Accidental Radionuclide Releases to the Environment

Abstract

This report summarises the recommendations of a Joint Working Group comprising representatives from the Radiation Protection Bureau of Health Canada, the Atomic Energy Control Board (AECB), and Atomic Energy of Canada Limited (AECL) on standard dose assessment parameters for use in Canada.

In 1990, the International Commission on Radiological Protection issued new recommendations on basic radiation protection principles (ICRP 1991). Changes to the terminology and formulation for assessing the health detriment from radiation exposure have a direct impact on the derivation of dose coefficients used in radiological protection and dose assessment. The objectives of the Working Group were to review published human physiological parameters and dose coefficients consistent with the latest ICRP guidance and to recommend default values for use in dose projections and assessments. The goal is to make the information widely available for use by various federal and provincial government agencies responsible for radiation protection in Canada, and by Canadian nuclear utilities and facilities, thereby providing a degree of standardisation reflecting the latest scientific knowledge.

This document contains the recommendations of the Working Group on appropriate human physiological and dosimetric parameters for use in assessing doses in the short-term following an accidental release of radioactive contamination to the atmosphere during a nuclear emergency. It focusses on the exposure pathways expected to deliver the greatest radiation dose under this scenario, namely, external irradiation by photons and electrons from radionuclides in the plume or deposited on the ground, and internal irradiation following inhalation of airborne radionuclides. Specifically, this document provides:

• age groups and recommended age-specific breathing rates for assessment of Canadian populations;
• recommended age-specific effective dose coefficients for internal exposures from inhalation, and external exposures from contaminated air and ground surfaces for all radionuclides of interest; and
• guidance on the use of the data

The ICRP has recently published age-specific breathing rates and inhalation dose coefficients for six age groups covering infants to adults (ICRP 1994, 1995b, 1996). The Working Group recommends the use of these parameters as default values in the assessment of inhalation doses from airborne radionuclides.

Radionuclide-specific dose coefficients for external irradiation from radionuclides distributed in the environment have not yet been published by the ICRP. As a result, the Working Group concentrated its effort on the review of published compilations of dose coefficients for cloudshine and groundshine exposures that incorporated the latest ICRP risk formulations for the assessment of health detriment. The Working Group concluded that the dose coefficients of Eckerman and Legett (1996) represented the best values currently available for cloudshine and groundshine dose assessments. An extensive list of these external dose coefficients has been reproduced in this document.

The dose assessment parameters recommended in this report should be used as default values in the evaluation of the three critical pathways associated with accidental releases from nuclear facilities, or other radiological accident scenarios. These parameters are also applicable to non-emergency situations, notwithstanding that exposure scenarios and methodologies for calculating doses may be different. Modifying factors accounting for exposure scenarios that are different from those applicable to the default parameters were beyond the scope of this report, although these should be used where appropriate.

This report does not deal with the methodologies used to derive the environmental radionuclide concentrations from which doses are calculated. Other aspects of dose assessment, such as recommendations on modelling approaches or environmental parameters, were also beyond the scope of the Working Group.