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

Introduction

In February 1997, a Joint Working Group comprising representatives from the Radiation Protection Bureau of Health Canada (RPB), the Atomic Energy Control Board (AECB), and Atomic Energy of Canada Limited (AECL) was established to review and recommend a standard set of human physiological parameters and dose coefficients for radiation dose assessments in Canada. The impetus for this work arose out of separate initiatives on the part of RPB and the AECB to incorporate this information into models for assessing public doses arising from routine and accidental releases of radioactivity to the environment. It was realised that there were significant advantages to ensuring that common data, where applicable, were used in these models, and that these reflected the latest recommendations of the International Commission on Radiological Protection (ICRP 1991). After initial discussions, the decision was made to limit the scope of work to parameters directly applicable to assessments in the short-term period following a nuclear emergency, on account of methodological differences between short-term accidental and long-term chronic exposure assessments (although some relevance will be found for the latter situation). The Joint Working Group membership is given in Appendix A.

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 a release of radioactivity to the atmosphere during a nuclear emergency. 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

Due to the absence of an internationally recognised compilation of external dose coefficients for radionuclides distributed in the environment, the Working Group focussed its attention on the review of published collections of dose coefficients for cloudshine and groundshine exposures that incorporated the latest ICRP risk formulations for the assessment of health detriment.

In presenting its recommendations, the Working Group recognises the widespread use of guidance issued by the Canadian Standards Association in its N288 series of documents on Environmental Radiation Protection (CSA 1987, 1991). While not intended as a replacement to these documents, it is the aim of the Working Group that the recommendations in this report serve as a common source of updated parameters for the relevant aspects of dose assessment contained in the CSA guidelines.

Values recommended in this report are applicable to dose calculations for accidental releases from nuclear facilities, or other radiological accident scenarios. While calculational aspects of dose assessments for routine and emergency situations may differ, the breathing rates and dose coefficients recommended in this report are applicable to both types of assessments.

Calculation of Dose from Atmospheric Releases of Radionuclides

Following a release of radioactive material to the environment under either routine or accidental conditions, irradiation of members of the public can occur by a number of exposure pathways. These include immersion in radioactive airborne plumes or contaminated water, inhalation of airborne radioactivity, irradiation from deposited radionuclides and inhalation following their resuspension into the air, and ingestion of contaminated food and water. This report focusses on the assessment of those pathways expected to deliver the greatest radiation dose in the short-term following an accidental atmospheric release of radioactive contamination to the environment, 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. These routes of exposure are typically referred to as the cloudshine, groundshine, and inhalation pathways.

Exposure to external irradiation from airborne radionuclides occurs only during the passage of the plume. The exposure period depends on the duration of the release, the meteorological conditions, and the residence times of the exposed individuals. Material deposited on the ground can give rise to exposures both during and after the passing of the plume. Radioactive material inhaled by individuals immersed in the plume can be retained in the body, and give rise to internal irradiation over a protracted period of time.

Under idealised exposure conditions, the effective dose rate to an exposed individual from external sources, E_ext(t) (Sv s^-1), can generally be expressed as

where χ(t) is the radionuclide activity concentration at time t in the air or deposited on the ground surface (Bq m^-3 or Bq m^-2, respectively), and DC_ext is the instantaneous external dose rate factor per activity concentration, or the external dose coefficient for the given radionuclide (Sv s^-1 Bq^-1 m³ for cloudshine; Sv s^-1 Bq^-1m² for groundshine).

For inhalation doses, the committed effective dose rate (Sv s^-1) is given by

where χ(t) is the concentration in air, B is the ventilation, or breathing rate of the individual (m³ s^-1), and DC_int is the dose coefficient per unit intake of inhaled radioactivity, or the inhalation dose coefficient (Sv Bq^-1). The radionuclide concentration is determined from environmental measurements, or predicted from environmental models.

This report focusses on the human physiological data and dose coefficients used in the above equations, namely the age-dependent effective dose coefficients for internal and external exposures, and age-dependent breathing rates. Methodologies for calculating activity concentrations in the various environmental media were beyond the scope of the Working Group.

This report does not address the ingestion pathway, and ingestion dose coefficients are not included, as the control of exposures due to the consumption of contaminated food and water, while potentially significant in some instances, is generally not considered urgent immediately following a radiological accident. If doses from the ingestion of contaminated food and water are assessed, ingestion dose coefficients from ICRP Publication 72 (1996) are recommended.