1.1. BACKGROUND
The International Basic Safety Standards for Protection against Ionizing Radiation and for the Safety of Radiation Sources (BSS) [1] establish basic and detailed requirements for protection against the risks associated with exposure to radiation and for the safety of radiation sources that may deliver such exposure. The standards are based primarily on the 1990 Recommendations of the International Commission on Radiological Protection (ICRP) [2] and other IAEA Safety Series publications. The BSS [1] place requirements on both the Regulatory Authority and on the legal person responsible for a source. These requirements and the procedures required to fulfill them are outlined in more detail in Ref. [3]. This Safety Report supports that publication and, in particular, provides the information necessary to allow the legal person responsible to “make an assessment of the nature, magnitude and likelihood of the exposures attributed to the source” [1]. It provides a practical generic methodology for assessing the impact of radionuclide discharges in terms of the resulting individual and collective radiation doses.
Previous guidance on models for predicting environmental transfer for assessing doses to the most exposed individuals (critical groups) was given in Safety Series No. 57 [4]. Since the publication of that report, the IAEA has produced Safety Series No. 100 on methods for evaluating the reliability of environmental transfer model predictions [5]. A handbook of transfer data for the terrestrial and freshwater environment [6] has also been produced which brings together relevant information from the major data collections in the world. Many of the parameter values used in this report are derived from the data in that handbook [6]. While Safety Series No. 57 contained much useful information and has become, to some extent, a standard text, in practice it was incomplete since it did not include all the models that were needed for assessment purposes. Moreover, considerable skill, expertise and resources were needed to derive and use appropriate data in the models.
This Safety Report expands on and supersedes the previous report [4]. It includes a new section on radiation dosimetry for intakes of radionuclides by members of the public and revised sections on atmospheric and aquatic dispersion. A section on calculating collective doses for screening purposes is also included to help determine whether further optimization procedures would be warranted. This Safety Report is intended to be a complete and self-contained manual describing a simple but robust assessment methodology that may be implemented without the need for special computing facilities.
1.2. OBJECTIVES
The main purpose of this Safety Report is to provide simple methods for calculating doses1arising from radioactive discharges into the environment, for the purpose of evaluating suitable discharge limits and to allow comparison with the relevant dose limiting criteria specified by the relevant Regulatory Authority.
1.3. SCOPE
The models in this Safety Report have been developed for the purpose of screening proposed radioactive discharges (either from a new or existing practice);
that is for determining through a simplified but conservative assessment the likely magnitude of the impact, and whether it can be neglected from further consideration or whether more detailed analysis is necessary. The use of simple screening models for dose assessment is one of the first steps in registering or licensing a practice, as explained in more detail in Ref. [3]. A dose assessment will normally be required either to demonstrate that the source may be exempted from the requirements of the BSS, or as part of the authorization or licence application. A step-wise procedure for setting discharge limits is outlined in Ref. [3]. The function of the dose assessment within this process, and the value of an iterative procedure in which the complexity of the dose assessment method increases as the magnitude of the predicted doses increase, is outlined in Ref. [3] and discussed in Sections 2 and 8 of this report.
This Safety Report provides the information required to assess rapidly doses using a minimum of site specific information. Two alternative methods are presented
— a ‘no dilution’ approach that assumes members of the public are exposed at the point of discharge, and a generic environmental screening methodology that takes account of dilution and dispersion of discharges into the environment.
The screening models contained in this report are expected to be particularly useful for assessing the radiological impact of discharges from small scale facilities, for example hospitals or research laboratories. In these situations the development of special local arrangements for dose assessment is likely to be unwarranted because the environmental discharges will usually be of a low level, and the methodology described in this report will usually be adequate. However, for many larger scale
1 Unless otherwise stated, the term ‘dose’ refers to the sum of the effective dose from external exposure in a given period and the committed effective dose from radionuclides taken into the body in the same period.
nuclear facilities the assessed doses from the screening models presented in this report are more likely to approach the dose limiting criteria set by the Regulatory Authority (e.g. dose constraint), and users are more likely to need to follow a screening calculation with a more realistic, site specific and detailed assessment.
Such a re-evaluation may necessitate consultation with professionals in radiological assessment and the application of more advanced models. The description of these advanced models is outside the scope of this Safety Report.
Doses calculated using the screening models presented in this report do not represent actual doses received by particular individuals. Furthermore, it would not be reasonable to use these models to reconstruct discharges from environmental monitoring measurements, because the pessimistic nature of the models might lead to a significant underestimation of the magnitude of the release.
The modelling approaches described in this report are applicable to continuous or prolonged releases into the environment when it is reasonable to assume that an equilibrium or quasi-equilibrium has been established with respect to the released radionuclides and the relevant components of the environment. The approaches described here are not intended for application to instantaneous or short period releases such as might occur in uncontrolled or accident situations.
1.4. STRUCTURE
Section 2 provides an overview of the assessment methodology and discusses the basic procedures for screening radionuclide discharges. The parameters and models for assessing the transfer between various environmental compartments for releases of radionuclides to the atmosphere, into surface waters and to sewerage systems are described in Sections 3 to 5 of this report. Section 6 provides the necessary dosimetric data and the equations by which individual doses may be evaluated. Section 7 considers collective doses, and Section 8 discusses the procedures to be followed when calculated doses approach the relevant dose limiting criteria.
In each section a simplified modelling procedure is described. Limitations in the models and their use are discussed. Default values are provided for each of the parameters needed for the assessment — these are chosen from observed values in such a manner as to produce only a small probability of underestimation of doses.
Annex I provides two types of dose calculation factors. The first, known as no dilution factors, allow rapid estimates to be made of the critical group doses arising from a concentration in air or water (resulting from a discharge to the atmosphere or a river). These factors are intended to be used with the predicted maximum radionuclide concentrations at the point of discharge. This approach is likely to overestimate significantly the doses received by members of the critical group in
reality. It is expected that these data will provide a useful screening method to determine whether the discharge source may be automatically exempted from the requirements of the BSS (see Refs [1] and [3] for further discussion). Annex I also provides generic dose calculation factors based on the generic environmental methods presented in this report, and standardized assumptions regarding the discharge conditions and the location of the critical group. These factors give the dose for a unit discharge to the atmosphere or to a river or sewer. It is recommended that site conditions should be taken into account in generic assessments if predicted doses exceed a reference level, as explained in more detail in Section 2.
Radionuclide half-lives and decay constants are provided in Annex II, and special methods for calculating doses from 3H and 14C are described in Annex III.
Annex IV provides a number of example calculations that illustrate the main features of the model.
Annexes V–VII provide more detailed information on some of the models included in this report. Annex V is a description of the Gaussian plume model, Annex VI covers the model for radionuclide transport in surface waters and Annex VII gives an explanation of the methods used to assess collective doses.
A full listing of the parameter symbols used in the equations that describe the model is provided at the end of the report. These symbols are listed, for ease of reference, by the section in which they are used. A glossary of the terms used in this Safety Report is also provided.