1.1. BACKGROUND
Residues from the mining and milling of uranium ores as well as from a number of other resource extraction industries (for example, phosphogypsum from the phosphate industry and residues from mineral sands processing and from the oil, gas and coal industries) contain uranium or thorium series radionuclides or both [1] and are therefore naturally occurring radioactive materials (NORM). One important pathway for radiological impact arising from NORM residues is the release of radon isotopes into the atmosphere. As a result, the prediction of such releases, including the effect of various covers, is an important element in rehabilitation planning. In addition, measurement of releases from existing residue repositories is often necessary to provide an input parameter to models of radon dispersion in the atmosphere for final assessment of radiological impact and for the investigation of any required ameliorative actions.
In response to the need of its Member States, in 1992 the IAEA published Technical Reports Series No. 333 (TRS 333), Measurement and Calculation of Radon Releases from Uranium Mill Tailings [2]. TRS 333 reviewed the major aspects of radon release, control and monitoring as they relate to the management of uranium mill tailings.
Since the publication of TRS 333, a number of developments have taken place in techniques for the measurement and modelling of radon exhalation, and practical experience has been gained with various types of residue repository. Additionally, the IAEA has organized a series of projects from 1992 to the present aimed at improving environmental assessment and remediation. Through these projects, the environmental behaviour of radionuclides has been considered in many other IAEA publications, mainly in the context of contaminated site characterization and environmental remediation. These publications include: the characterization of contaminated sites [3], technical and non-technical factors relevant for the selection of the preferred remediation strategy and technology [4-6], an overview of applicable technologies for environmental remediation [7-9], options for the cleanup of contaminated groundwater [10] and planning and management issues [11]. Other IAEA publications on related aspects include reports on the remediation of uranium mill tailings [12] and of dispersed contamination [13, 14], on the decontamination of buildings and roads,on the characterization of decommissioned sites and on radiation protection and the management of radioactive waste in the oil and gas industry [15]. In view of these new publications as well as of new findings regarding radon, it was deemed necessary to produce a revision of TRS 333.
This report covers radon releases from repositories of NORM residues. For simplicity, in this report the term 'residue' includes tailings from processing,residues from heap leaching, waste rock, sludges, filter cakes and scales. Specific examples include uranium mill tailings, phosphogypsum, coal fly ash, and thorium refinery residues.
Such residues may be present in the environment in repositories in various physical forms, such as tailings ponds, dumps or stacks. Rehabilitated waste is typically covered with natural and engineered materials such as layers of clay,soil, water, rock, geosynthetics, vegetation, a combination of any of these or it may be exposed to the atmosphere.
The factors affecting radon releases from residue repositories will vary considerably with the residue type, the repository characteristics and the radon isotope of interest. This report describes approaches to the measurement,calculation and monitoring of radon releases that can be applied to many situations, together with specific examples of important representative scenarios.
There are three radon (Rn) isotopes naturally present in the environment: 222Rn, a member of the uranium decay series,220 Rn, a member of the thorium decay series, and 219Rn, a member of the actinium decay series. 220 Rn and 219 Rn are often referred to as thoron and actinon, respectively. In the case of actinon,its short half-life (3.98 s) and its very low activity concentration relative to 222 Rn in the environment mean that for all practical situations its radiological impact is negligible compared with that of 222Rn. The impact of 220Rn is minimal, as its half-life is 55. 6 s and it has a higher activity concentration than 219Rn but still much lower relative to 222 Rn. Consequently, the primary focus of this report is 222 Rn, and to some extent 220 Rn, wherever applicable.
1.2. OBJECTIVE AND SCOPE
This report covers the prediction, measurement and monitoring of radon releases from NORM residues, including uranium mining and milling residues. The intention is to support environmental investigations and assessments, as well as remediation in areas contaminated by NORM, by presenting state of the art concepts, models and parameters.
The primary focus of this report is the release of 222Rn, although 220Rn and 219 Rn are also discussed, and the material herein discussed can also be applied to those isotopes, with modifications. The report covers methods of prediction,measurement and monitoring of radon emanation from grains into interstitial spaces, transport within the residue and soil profile and exhalation into the atmosphere from the surface. The related topics of emanation and exhalation from non-residue materials, the behaviour of radon in the atmosphere following exhalation, implications for dose assessment and applications of radon as a tracer of environmental processes are all outside the scope of this report.
This report is intended to be used in conjunction with other IAEA publications related to the assessment of the radiological impact of radioactive waste, such as Safety Reports Series No. 19, Generic Models for Use in Assessing the Impact of Discharges of Radioactive Substances to the Environment [16], IAEA Safety Standards Series No. WS-G-2.3, Regulatory Control of Radioactive Discharges to the Environment [17], and other related publications.
1.3. STRUCTURE
This report is comprised of nine sections. Section 2 concerns factors controlling radon releases from residue materials. Sections 3 and 4 describe the measurement and estimation of variables affecting the radon exhalation rate. Section 5 considers measurement methods for radon concentrations in soil gas and for radon exhalation flux density from a surface in detail, including instrumentation and its calibration. Section 6 provides methods for predicting radon exhalation flux density. Section 7 addresses waste repository cover characteristics and meteorological conditions. Section 8 briefly discusses aspects of radon monitoring programmes including data collection, factors in design and implementation and quality assurance. Section 9 outlines a case study in which multiparametric measurements at a uranium waste repository are compared with model predictions.
The report also includes two appendices presenting the main mathematical development of the radon diffusion and radon flux equations.

The mining and milling of uranium ore produce large quantities of residues containing natural decay series radionuclides. Although such residues are relatively small in magnitude compared with those from metal mining and extraction processes, their present worldwide production exceeds several million tonnes annually. In addition, a number of other resource extraction industries (such as phosphate, mineral sands, oil and gas, and coal) produce residues with similar radionuclide profiles. In recent years, these materials have become increasingly interesting from the point of view of radiological impact assessment. There is thus a need to reduce the environmental and health risks from these materials to an acceptable level.
In response to the needs of its Member States, the IAEA has for many years supported efforts to publish information on the environmental behaviour of radionuclides. In 1992, the IAEA published Technical Reports Series No. 333 (TRS 333), Measurement and Calculation of Radon Releases from Uranium Mill Tailings, which reviewed the major aspects of radon release, control and monitoring as they relate to the management of uranium mill tailings.
Since the publication of TRS 333, a number of developments have taken place in techniques for the measurement, modelling and prediction of radon exhalation, and practical experience has been gained with various residue repositories. It was therefore considered timely to produce a revision of TRS 333 and to expand its scope to include other NORM residues. The present report provides a comprehensive overview of the prediction, measurement and monitoring of radon releases from NORM residues, including uranium mining and milling residues.
The IAEA wishes to express its gratitude to P. Martin (Australia) for his assistance in chairing the meetings, and to K. Lange (Canada) for her assistance in editing this report, as well as to those experts who contributed to the development and completion of this report.
The IAEA officer responsible for this report was M. Phaneuf of the IAEA Environment Laboratories.

1. INTRODUCTION 1

1.1. Background 1

1.2. Objective and scope 2

1.3. Structure 3

2. FACTORS CONTROLLING RADON RELEASES 3

2.1. General 3

2.2. Emanation coefficient 7

      2.2.1. Factors affecting radon emanation 9

      2.2.2. Radon emanation coefficients for natural materials 11

2.3. Diffusion coefficient 12

2.4. Radon exhalation 16

3. MEASUREMENT OF VARIABLES AFFECTING RADON EXHALATION FLUX 16

3.1. Measurement of the emanation coefficient 16

3.2. Measurement of the diffusion coefficient 20

4. ESTIMATION OF VARIABLES AFFECTING RADON EXHALATION FLUX 24

4.1. Radon emanation coefficient (E) 24

4.2. Diffusion coefficient (Dr) 24

4.3. Radium activity concentration (R) 25

5. MEASUREMENT OF RADON CONCENTRATION AND EXHALATION FLUX DENSITY 25

5.1. General 25

5.2. Measurement of radon concentration 26

      5.2.1. Mode of measurement 26

      5.2.2. Nuclear track detector 28

      5.2.3. Solid surface barrier detectors 28

      5.2.4. Scintillation cell 29

      5.2.5. Electret 29

      5.2.6. Activated charcoal 30

      5.2.7. Ionization chamber 30

5.3. Measurement of exhalation flux density 30

      5.3.1. Accumulation 30

      5.3.2. Flow through method 33

      5.3.3. Adsorption 35

      5.3.4. Measurement of soil gas concentration 36

      5.3.5. Measurement of mass exhalation rate 37

5.4. Calibration and uncertainties 38

6. SIMPLIFIED EXPRESSIONS FOR ESTIMATING RADON EXHALATION FLUX DENSITY 39

6.1. General 39

6.2. Radon exhalation flux density from bare residue 40

      6.2.1. Residue thickness much greater than radon diffusion length 41

      6.2.2. Residue thickness comparable to or less than the diffusion length 41

6.3. Radon flux density from residue repositories with a single cover 43

6.4. Radon flux density from residue repositories with multiple covers 44

7. EFFECTS OF METEOROLOGICAL CONDITIONS AND THE REPOSITORY COVER 45

7.1. Meteorological effects 45

7.2. Aspects of repository cover design 47

      7.2.1. Surface cover effects 47

      7.2.2. Surface cover materials 48

      7.2.3. Effects of vegetation on the cover 51

8. MONITORING RADON RELEASES FROM NORM FACILITIES 51

8.1. General 51

8.2. Data collection at various stages 53

8.3. Factors in design and implementation 53

8.4. Quality assurance 54

9. CASE STUDY OF RADON EXHALATION FROM A URANIUM RESIDUE REPOSITORY 54

9.1. Introduction 54

9.2. Measuring radon flux directly at the residue surface 55

9.3. Estimating flux by measurement of Lr and E 56

      9.3.1. In situ radon diffusion length (Lr) 57

      9.3.2. In situ radon emanation coefficient (E) 59

      9.3.3. Estimation 59

9.4. Estimating flux by calculating Lr and E via empirical formulas 59

9.5. Illustrations of estimation and measurement results 60

      9.5.1. Determination of exhalation flux 60

      9.5.2. Determination of radon diffusion length 60

      9.5.3. Relating radium content to radon flux 62

APPENDIX I: MATHEMATICAL DEVELOPMENT OF RADON DIFFUSION EQUATIONS 65

APPENDIX II: REFINED METHODS OF RADON FLUX CALCULATION 71

REFERENCES 75

NOTATION 83

CONTRIBUTORS TO DRAFTING AND REVIEW 85

中国标准化研究院国家标准馆