guidelines on the assessment of radiation hazards to members of the public from mining and minerals...

Guidelines on the Assessment of Guidelines on the Assessment of Radiation Hazards to Members Radiation Hazards to Members of the Public from Mining and of the Public from Mining and Minerals Processing FacilitiesMinerals Processing Facilities

SARPA WORKSHOPSARPA WORKSHOPMarch 2010March 2010

O Phillips: Senior Manager-NNRO Phillips: Senior Manager-NNR

Table of contentsTable of contents

Radiation Hazard Assessment Methodology• Overview of methodology (Site characterization, Source

term, Transport model development, Calculation of dose, Statistical considerations, Limitations of public exposure)

• Model validation and Quality assurance

• Submissions to NNR

Objectives and proposed changes for LG-1032

Radiation Hazard Assessment MethodologyRadiation Hazard Assessment Methodology

Purpose of assessment

• The purpose of the assessment is to quantify the dose to members of the public arising from radioactive materials released from mining and mineral processing facilities.

Scope

• This guideline is intended to be applied to all types of licensed mining, minerals processing and related facilities which carry out activities and operations involving radioactive ores and minerals containing uranium and thorium and their progeny.

Objectives

• The purpose of the guideline is to provide licensees with guidance regarding the requirements of the NNR relating to the assessment of radiation hazards to the public arising from activities on mining and minerals processing facilities.


STAGES OF THE ASSESSMENT PROCESS

• Identification of the source term which encompasses all aspects of the release of radioactive material relevant to the dose calculation, including the release rate of relevant radionuclides, physical and chemical form, airborne release velocity and temperature, time distribution of releases, elevation, area and location of source, release pathway into the environment, etc.

• The analysis of the transport (or passage) of radioactive material from the source through the environment (air, water, food-chain etc.) to humans, normally using a mathematical model of the environmental pathways and transport processes. The analysis of environmental transport may involve several exposure pathways and possible interactions between them.


• The estimation of doses to individuals arising from internal and external exposures due to radioactive materials in the environment. The models used require assumptions on age, gender and living habits and account for the transfer and metabolism of radioactive materials in human beings.


PROCESS OVERVIEW

• A typical assessment of the radiation hazards due to an existing mining or mineral processing facility, would comprise of the following stages:

• The source term must be determined. Relevant factors for the assessment in terms of potential sources of radioactive material, release pathways, intervening media in the environment (e.g. rivers, dams, vegetation etc.) must be identified.

• In terms of the overall hazard assessment, a screening assessment should be performed when it is believed that it can be demonstrated that the site in question has a minor impact on the public. This step may in any case be performed as a preliminary stage of the detailed site specific assessment, with the objective of refining the models and data in a tiered approach.


The SCREENING ASSESSMENT:

• Simple models

• Conservative data

• Site characterization

• The criterion applicable to a screening assessment is 25 µSv. The basis for this is that if an assessment is performed using conservative input data and recognized environmental transport models, with uncertainties accepted to be less than a factor of 10, then a model benchmarking exercise will not be required if the dose to the critical group from all pathways, determined on this basis, is less than one tenth of the 250 µSV criterion

Note: Since SSRP has come into force, exclusion and exemption criteria should be applied.


DETAILED SITE ASSESSMENT :

• For doses less than 250 µSv, detailed release monitoring, environmental surveillance and quality assurance programmes would be required.

• In the event that remedial action is proposed to reduce the radiation hazards, the hazard assessment must be updated to demonstrate that the proposed action will achieve the desired result.

• In the case of the detailed site specific assessment predicting doses less than 250 µSv, detailed release monitoring, environmental surveillance and quality assurance programmes would be required.


SITE CHARACTERIZATION

• Initial site characterization provides a first overview of all significant sources, possible contaminant migration, dispersion and public exposure pathways. This information will used for planning and implementation of a monitoring programme and the choice of suitable models.

• Information required:

– Historical information on radionuclide contamination levels e.g. past surveys on water.

– Input from operators and site inspections.

– Experiences from other sites.

– Information from EMPs and submissions to Department of Water Affairs and Forestry (DWAF) e.g. site water balance and sampling programmes of non radioactive materials.


– Studies identifying known points of chemical pollution e.g. identification of groundwater plumes and sampling points.

– Identification of water uses and users e.g. municipalities, gardening, irrigation, farmers, use of mine water for drinking purposes, use of mine water for on-site irrigation, export to other users.

– Other information required are general climate description; Seasonal variations; geological and hydrological factors (e.g. existence of aquifers); Other relevant factors: topography; biota; demography; Land use e.g. agricultural use, urban and industrial development.


SOURCE TERM

• The establishment of a source term requires determination of the location and nature of releases of radioactive material to the environment, including both presently prevailing releases, future predicted releases, and potential releases due to accident conditions and infrequent events (e.g. flooding). The mode of release, chemical and physical characteristics, as well as the quantities of the radionuclides released, must be determined with the objective of computing the highest annual average dose to the critical group or groups during the operation of the facility or subsequent to closure thereof .


• Airborne pathways: Gaseous and particulate releases Elevation, area and location of release, release velocity, Mass flow rate, particle size distribution and activity (AMAD)

• Liquid borne pathways: Releases into surface water bodies, releases into groundwater, leaching and seepage, migration of groundwater off the site Extraction via wells or bore-holes, chemical characteristics impacting on radionuclide transport processes


Source term Monitoring Programme

• Objectives • To characterize the source term by sampling and

analysis of releases and environmental media, • To provide input for validation of transport models, • To identify unexpected environmental contamination,

transfer routes and pathways.

Note: Depending on the results obtained and the requirements of the transport models used, the monitoring programs should be amended from time to time.


Sampling for source term determination

General Principles • To obtain representative samples adequately

reflecting temporal and spatial variations in the medium sampled.

• Sampling at intervals short enough to cover all phases of facility operations; for example a change in operating conditions may require a change in the sampling programme. In the case of environmental sampling, seasonal variations must be taken into account.


• The sampling locations will be dictated by the objectives of the monitoring programme.

• The extent of the sampling programmes would be site specific and would vary according to many factors e.g. mine size, type of operations, water balance, population density.

• Sampling must be performed according to procedures acceptable to the NNR


Sampling for source term determination continued

Particulate Emissions

• Monitoring of particulate emissions should preferably be based upon long-term integrated measurements that adequately account for operational and seasonal variations. However a combination of modeling and measurements may also be employed.

Liquid Sampling

• Required to characterize the level of radionuclide contamination in liquid source terms on and around the site and for longer term monitoring of releases, surface and ground-water.

.


ANALYSIS

Introduction to analysis

• Any analytical method that is capable of performing the measurements at the required sensitivity (usually background level) and accuracy may be acceptable to the NNR. In order to determine the acceptability of a method, scientific validation of the methodology must be provided to the NNR.

Radionuclides analysis

• The relevant radionuclides - primarily those belonging to the 238U and 232Th decay chains.

• Public exposure to radioactive materials released from mining and minerals processing facilities may arise from the intake of a number of different radionuclides, in different chemical forms with widely varying dose coefficients.


Radionuclides analysis continued

• In general, the main radionuclides to be selected fall into the following categories:

– Long lived alpha emitters: 238U, 234U, 230Th, 226Ra, 210Pb; 232Th, 228Th, 224Ra.

– Beta emitters: 210Pb, 228Ra

– 222Rn and 220Rn (and their progeny).

• Radioactive decay must be taken into account in the hazard assessment, not only to avoid overly conservative results in the case of the slower transport processes, but also to account for the impact of the relevant decay products.

• The specific radionuclides to be included in the analysis programme or modeling assessment will vary according to site specific factors (e.g. the uranium and thorium contents of the ores, process materials and wastes) and the specific exposure pathway under examination (gaseous, particulate and liquid).

• The exclusion of radionuclides from analysis programmes and modeling assessments must therefore be indicated and justified.


Gross Alpha and Nuclide Specific Analysis

• Samples may be analyzed through an appropriate mixture of gross alpha counting and radionuclide specific analysis.

• Gross alpha activity analysis could be used as part of the assessment to identify the need for further nuclide specific analysis and also to identify critical source terms and critical sampling locations (e.g. those with significant and widely varying concentrations).


Gross Alpha and Nuclide Specific Analysis continued

• Initial sample characterization e.g. to identify those radionuclides which result in significant radiation hazards (e.g. source term characterization).

• Sample media that have been identified as critical samples.

• In those cases where bore-hole water was used for domestic purposes e.g. drinking, cooking and showers, the analysis approach should include gross alpha counting, radon determination and long lived radionuclides of the Thorium and Uranium decay chains.


TRANSPORT MODEL DEVELOPMENT

Methodology

• For the development of the hazard assessment model consideration must be given to a wide range of potentially significant factors including factors which may be relevant to the transport of nuclides in the environment.


Pathways that should be modelled

• Atmospheric pathway

• Water pathway

• Surface Water pathway


Transfer Factors

• In assessing the hazard from releases of radionuclides into the environment, mathematical models are used in which the pathways of radionuclides from the release point to humans are quantitatively described by transfers between "environmental compartments" (e.g. pasture, cattle). The radionuclide transfer between these compartments is usually described by transfer parameters. In simple models these transfer parameters represent the ratio of concentrations of a radionuclide in two compartments under equilibrium conditions (referred to as the Concentration Factor Method). In more complex dynamic models an attempt is made to represent the time dependent movement of radionuclides between the various environmental compartments . This model is a conceptual approximation of the kinetics of the real system.

Note: To be revised to take into consideration latest developments

• IAEA Technical Report Series 472.


CALCULATION OF DOSE

• ICRP has defined models, parameters and dose coefficients which are appropriate for the calculation of age specific doses to the critical group of the public arising from intakes of radionuclides (ICRP 1989, 1993, 1994, 1995a, 1995b, 1996). (To be revised)

• When assessing the resulting doses to a defined critical group arising from the internal and external exposure to radionuclides in the environment,


• The following factors must be considered as appropriate in the assessment:

– The age composition of the critical group and age related metabolic parameters

– e.g. breathing rates.

– Particle size and activity distribution i.e. AMAD.

– Radionuclide intakes and radionuclide specific dose coefficients,

– Gut uptake factors and lung clearance class (defined by radionuclide chemistry).

– Individual habits affecting exposure e.g. sleeping, activity, periods spent in and out of doors.

– Culturally appropriate dietary and age specific food and water consumption rates,

• Gamma shielding factors.


STATISTICAL CONSIDERATIONS

• For a screening assessment, models and data used must be sufficiently conservative to encompass estimated uncertainties. For the detailed site specific assessment, more accurate models and data may be required. Once a stage has been reached at which a conservative point-estimate value for a data item (or a conservative model) can no longer be tolerated, it may be necessary to use best estimate values combined with an uncertainty analysis as described below.

• Uncertainties generally arise from one or more of the following: Natural variations in the characteristics of the environment, Model uncertainties. Each of these uncertainties must be quantified and incorporated into the

hazard assessment.


LIMITATION OF PUBLIC EXPOSURE

• Since a critical group may be exposed to sources and exposure pathways arising from a number of sites, a site specific incremental dose limit is applied. This is set at 250 µSv per annum. The mean dose per site (accounting for all uncertainties) to the critical group (or groups) may not exceed this value.

• The control of releases must be optimized i.e. the resulting doses must also be kept ALARA (As Low As Reasonably Achievable), social and economic factors being taken into account.

• In addition to the dose to the critical group(s) from each site the cumulative impact of all sites needs to be addressed. In this regard the licensees will be consulted on the proposal of a suitable methodology whereby this can be achieved.


MODEL VALIDATION AND QUALITY ASSURANCE

• In terms of quality assurance on software and constituent models, the following must be addressed:

– Where the results of a code application are used to demonstrate compliance with the regulatory limits, the code must be bench-marked against actual measurements sufficiently representative of the situations for which the code is to be applied. This must demonstrate that the code is conservative for the particular application, or result in uncertainties which can be used in the overall analysis.

– For a screening assessment, code bench-marking may not be necessary. In this regard it may be sufficient to provide a rationale for the use of a particular code, for a given application, by referring to published information or expert opinion. The motivation should provide sufficient evidence to the effect that the uncertainties in the code do not result in underestimation of the dose to the critical group by more than a factor of 10 (which is the basis for the use of the 25 μSv screening criterion).


SUBMISSIONS TO THE NNR

• PROJECT PLAN

• SOURCE TERM DETERMINATION

• SCREENING ASSESSMENT

• DETAILED SITE SPECIFIC ASSESSMENT

Objectives and proposed changes for LG-1032Objectives and proposed changes for LG-1032

The primary objectives are as follows:

• The present LG-1032 emphasises the “Source term modelling approach”. The LG can be improved by essentially taking in account the “receptor assessment approach”

• To incorporate national and international developments regarding environmental monitoring into the NNR’s assessment and review process

• Ensure that exposure pathways and the relevant scenarios are completely and explicitly defined. This will improve dose modelling and estimation.

Objectives and proposed changes for LG-1032Objectives and proposed changes for LG-1032

• Transfer factors and consumption rates for performing dose calculations should agree with latest international guidelines.

• Pathways that lead to contamination in the environment must be defined so that build-up of contaminants in the environment is prevented.

• Ensure that the revised LG-1032 puts more emphasis on the sampling of sediments where the potential for build up of contamination is suspected.

Objectives and proposed changes for LG- Objectives and proposed changes for LG- 10321032

• Ensure that while the “receptor assessment approach” is introduced in order to achieve better controls of releases of contaminants to the environment, the fundamentals of ensuring that the source of activity is minimized before release to the environment is still enforced.

• To ensure a consistent approach/manner of the safety assessment, perhaps the status/type of regulatory document should be considered i.e. requirements vs. guide. For a regulatory guide deviations are allowed if it can be justified.

EndEnd

THANK-YOU FOR YOUR ATTENTION