scope:

0 Introduction

0.1 Derived release limit (DRL)

The DRL for a given radionuclide is the release rate that would cause an individual of the most highly exposed group to receive and be committed to a dose equal to the regulatory annual dose limit due to release of the radionuclide to air or surface water during normal operation of a nuclear facility over the period of a calendar year. The DRL is derived using mathematical equations that describe the transfer of radioactive materials through the environment to humans. It takes into account all exposure pathways, including external exposure from immersion in contaminated air and water, external exposure to contaminated soil and beaches, and internal exposure from inhalation and ingestion of radioactivity. DRLs are normally used in a prospective sense to set release limits for individual radionuclides, but the pathways model described in this Guideline may be adapted for other purposes such as the estimation of annual public doses.

0.2 History of the Guideline

In 1987, CSA Group published CSA N288.1 to provide guidelines and a methodology for calculating DRLs for routine releases of radionuclides to air and surface water from nuclear facilities. Since 1987, significant scientific advances were made in dosimetry and in the understanding of radionuclide behaviour in the environment; thus, there was a general consensus in the Canadian nuclear industry that the models and data in CSA N288.1 needed to be updated. Revision began in 2000 with the development of a derived release limits guidance document prepared for Ontario Power Generation (OPG). This document was specifically designed to calculate DRLs at CANDU® nuclear power generation facilities in southern Ontario (i.e., the Bruce, Pickering, and Darlington stations). In 2006, the document was extended by the CANDU Owners Group (COG) to cover all CANDU facilities in Canada, including the power generation stations at Gentilly (G-2) and Point Lepreau and the research laboratories at Chalk River (CRL). This extended document (Hart, 2008) formed the basis for the second edition of this Guideline and included background material deemed too detailed for inclusion in the Guideline itself. Most of the models and parameter values in the first edition of the Guideline were updated and new exposure pathways were added in producing the second edition, which was issued in 2008. Following publication, users of the Guideline identified a number of minor errors, which were corrected in an amendment issued in July 2011.

0.3 Need for a new edition

Due to lack of time, resources, and information, it was not possible during the development of the second edition to implement all of the changes that were deemed desirable. These changes were set aside so that the second edition could be published in a timely fashion. Work on these changes has continued since through a number of research projects funded by COG. The findings of these projects form the basis for most of the improvements to the third edition. The COG background document (Hart, 2008) was revised in concert with the third edition; the updated version is referred to hereafter as the COG Derived Release Limits Guidance or CDG (Hart, 2013).

0.4 Changes in this edition

The major differences between this and the previous edition of this Guideline include

(a) updated energy expenditures and dietary intake rates for humans;

(b) updated half-lives, gamma energies, and photon yields for all radionuclides;

(c) updated values for many parameters based largely on a new International Atomic Energy Agency handbook of parameter values for environmental transfers of radionuclides (IAEA, 2010);

(d) improved direction on when the Guideline can be used to calculate DRLs for intermittent releases;

(e) updated wind direction and precipitation data for use in the wet deposition model;

(f) introduction of a model for wild waterfowl as an additional source of human exposure through ingestion;

(g) extension of the carbon-14 (C-14) specific activity model to cover plant to animal transfer;

(h) an improved specific activity model for tritium in animals, including an update and extension of the water intake source fractions for fresh and dry feed; and

(i) provision of equations for explicit accounting of decay and progeny ingrowth in all physical media, as an alternative to the use of progeny-inclusive dose coefficients.

Note: CANDU (CANada Deuterium Uranium) is a registered trademark of Atomic Energy of Canada Limited (AECL).