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Publications N288.1-14

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N288.1-14 - Guidelines for calculating derived release limits for radioactive material in airborne and liquid effluents for normal operation of nuclear facilities

Publication Year:

  • 2014

Total Pages:

460

Publisher:

  • CSA
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Preface

This is the third edition of CSA N288.1, Guidelines for calculating derived release limits for radioactive material in airborne and liquid effluents for normal operation of nuclear facilities. It supersedes the previous editions published in 2008 and 1987.

This edition has been updated to provide improved direction on the applicability of the Guideline.

Major changes to this edition include

(a) updates to the following:

    (i) energy expenditures and dietary intake rates for humans;

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

    (iii) values for many parameters based largely on a new International Atomic Energy Agency (IAEA) handbook of parameter values (IAEA, 2010);

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

    (v) specific activity model for tritium in animals;
(b) introduction of a model for wild waterfowl as an additional source of human exposure through ingestion;

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

(d) provision of equations for explicit accounting of decay and ingrowth in all physical media; and

(e) improved direction on when the Guideline can be used to calculate derived release limits (DRLs) for intermittent releases.

Users of this Guideline are reminded that the site selection, design, manufacture, construction, installation, commissioning, operation, and decommissioning of nuclear facilities in Canada are subject to the Nuclear Safety and Control Act and its Regulations.

CSA N286 provides overall direction to management to develop and implement sound management practices and controls, while the other CSA Group nuclear Standards provide technical requirements and guidance that support the management system. This Guideline works in harmony with CSA N286 and does not duplicate the generic requirements of CSA N286; however, it may provide more specific direction for those requirements.

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).

Scope

1.1 Facilities

This Guideline and the CDG are intended to apply primarily to CANDU nuclear power stations in Canada. However, the radionuclides and environmental pathways addressed make this Guideline applicable to releases from many other nuclear facilities, including research reactors, radioisotope processing facilities, waste processing facilities such as incinerators, and power reactors other than those of CANDU design, subject to the limitations detailed in Clauses 1.2 to 1.8. Application to other types of facilities such as fuel fabrication plants and refineries is limited by the radionuclides considered here (see Clause 4.3). This Guideline may be adapted to cover part of the needs of such facilities, but additional models or methodologies might be necessary for other parts. However, neither the radionuclides nor the models included in this Guideline are complete enough to cover releases from sources such as uranium mines and mills, permanent geologic disposal facilities, and other facilities where extensive modelling of groundwater pathways is required.

1.2 Release paths

This Guideline covers releases to the atmosphere and to surface water (both fresh and marine). It does not address releases to groundwater, although transfers from other media to groundwater wells and ponds are considered. Direct gamma irradiation from radioactivity inside the facility is not modelled because it does not involve a release.

1.3 Release duration

The methods specified in this Guideline are designed for routine, continuous, low-level emissions. They also apply to periodic, short-term releases (see Clause 8.2), provided that

(a) the releases are controlled and associated with normal operations;

(b) the release rate is roughly the same from event to event;

(c) for atmospheric releases, the total release duration exceeds approximately 1000 h in the year; for aquatic releases, at least one or two releases occur in each month of the year; and

(d) the releases occur randomly over time.

Where the requirement of Item (d) is not met but the releases are known to occur at a particular time of day or year, this Guideline applies only if the air (water) concentrations are calculated using the meteorological (hydrological) data in effect for that time.

Notes:

(1) Where non-random releases are calculated using time-appropriate data, it might be possible to relax the conditions on the release frequencies.

(2) Releases that do not meet these conditions can use another model, such as that specified in CAN/CSA-N288.2 for atmospheric releases.

(3) For some facilities, intermittent releases occur predictably as spikes on a continuous base release. Such releases can be considered part of routine emissions and included in the DRL without special treatment if the total activity released in intermittent form is less than approximately 30% of the total release from the facility. The 30% cut-off is considered a small fraction of the overall uncertainty of the DRL estimates.

1.4 Contaminants

This Guideline applies to the radiation effects of radionuclides. It does not apply to chemicals or to the chemical toxicity of radionuclides.

1.5 Receptors

DRLs are derived for a representative person having the average characteristics of a group of individuals who, by reason of their location and habits, are likely to receive the highest exposures to a given radionuclide released from a particular source (see Clause 4.2). By maintaining release rates well below the DRL, the annual dose to these individuals (and therefore to all members of the public) will be below regulatory limits. This Guideline does not apply to nuclear energy workers (NEWs), or to non-NEWs working at a nuclear facility, who are assumed to be covered by on-site radiation protection programs. The DRLs calculated using the models specified in this Guideline apply to human end-points only; non-human biota are not covered.

1.6 Downwind distance of validity

This Guideline is not applicable to receptors located close to a source affected by building-induced turbulence because the atmospheric dispersion model does not simulate the cavity that forms in the lee of the building. Because the cavity extends approximately three building heights downwind, this Guideline applies only beyond this distance. Moreover, the dispersion model should be used with caution beyond an approximate distance of 20 km from the facility because the assumption of steady-state meteorological conditions implicit in the model becomes less valid at greater distances. This is not an issue in practice because the representative person is usually found closer to the facility than 20 km.

1.7 Site specificity

Local parameter values should be used wherever possible in calculating DRLs for a specific site. Where local values are not available, the default values given in this Guideline for the region closest to the site of interest may be used. These regional values represent conditions at the main nuclear sites in Canada (i.e., Pickering/Darlington, Bruce, CRL, G-2, and Point Lepreau) but may be interpreted as default values for the regional areas of southern Ontario, western Ontario, eastern Ontario, Québec, and the Maritimes, respectively.

1.8 Level of complexity

1.8.1 Simpler approaches

The models specified in this Guideline are comprehensive and in some cases include considerable detail. This level of complexity might not be warranted for all assessments. Less complex approaches, involving fewer pathways and/or less detail, might be appropriate under some circumstances. Any pathway that can be shown to not contribute significantly to the total dose may be neglected. In these cases, simpler models such as those described by the IAEA Safety Report Series No. 19 may be used, provided that justification for using a simpler approach is provided.

Note: For example, the following need not be considered:

(a) radionuclides that are not released from the site of interest; and

(b) pathways related to wells that are not used as a source of water by members of the public near the site.

1.8.2 Default transfer parameters

A simplified approach is available for application of this Guideline without implementation of the models. Annex A lists default transfer parameters for each radionuclide for each pathway in the model, together with the assumptions made in calculating the values. If the assumptions hold for the application in question, these default values may be used to obtain estimates of the DRLs without implementing the model itself, as demonstrated in Annex B. This allows all of the models and parameter values in this Guideline to be accessed in a simple way. Because conservative assumptions were made in calculating the default parameter values, the DRLs calculated using this approach will be more conservative than those obtained by implementing the model.

1.9 Terminology

In this Guideline, "should" is used to express a recommendation or that which is advised but not required and "may" is used to express an option or that which is permissible within the limits of the Guideline.

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