Performance assessment normally includes the verification of compliance with applicable regulations, as well as site specific performance objectives. A performance assessment also provides a tool for the optimization of remediation measures and the identification of shortcomings in design, operating procedures and management systems. Since the design and operation of many tailings facilities relies on assumptions and models, performance assessment supplies information that is useful in the verification and calibration of these assumptions and models. Future research and development priorities can be identified on the basis of deficiencies within a design or system thus assessed.
8.2. Conceptual Approaches
Two basic approaches exist, risk-based assessment and design-based assessment. A risk-based performance is based on the calculated dose to a hypothetical receptor at a given point of compliance. A design-based assessment is based on compliance with engineering design
specifications. The risk-based assessment appears to be the more commonly accepted methodology, as it allows the development of optimised and cost-effective procedures. The design-based assessment is prescriptive, making it easily applied and verified. Design-based assessment was used in the UMTRA programme in the USA, while the WISMUT clean-up in Germany used a risk-based approach.
Performance parameters may be calculated using deterministic or probabilistic methods.
These are complimentary methods and are defined as follows:
The deterministic approach or ‘best estimate analysis’ is performed on separate scenarios and results in ‘best-estimate values’ for the input parameters, such as source term, recharge, tailings seepage, groundwater flow and transport, radon diffusion, and distribution coefficients. This approach is well suited for testing by parameter sensitivity analysis.
The probabilistic approach is based on a statistical distribution of multiple parameters using a Monte Carlo-simulator to calculate an average value, such as a range of a calculated dose at a given point of conpliance.8.3. Performance assessment approaches
The actual approach adopted for a performance assessment will depend on the conceptual starting point, site specific parameters, as well as possible requirements by the licensing authorities. The concept of performance assessment has been extensively developed in the context of radioactive waste disposal. For instance, the Sandia National Laboratory has adopted the following, six task process for the performance assessment of the Waste Isolation Pilot Plant near Carlsbad, New Mexico [201]:
(1) Disposal-system and regional characterization through collection of data on waste properties, facility design, and regional geology and hydrology.
(2) Scenario development that identifies combinations of events and processes whereby contaminants might be released outside a ‘controlled’ area, and subsequent determination of which scenarios to model.
(3) Probability modeling that provides estimates of the likelihood that the retained scenarios will occur.
(4) Consequence analysis, including uncertainty analysis, that predicts contaminant amounts released and associated uncertainties from the calculations.
(5) Regulatory-compliance assessment through construction of CCDFs (hypothetical CCDF illustrated below) for comparison of modeling results with long term performance criteria in [3].
(6) Sensitivity analysis that determines the parameters that most influence modeling results.
This assessment procedure was intended for use at high level waste repositories. Tailings disposal facilities present a lower level of risk, and, although similar principals may be applied, the rigor of design and level and intensity of monitoring will probably be far lower
8.4. Baseline data and regional characterization
It is necessary that the impoundment system in its regional context be understood well and relevant site investigations are needed. The data produced from these site investigations, both in a design or remediation context, will provide baseline conditions and allow the prediction of the behaviour of the facility in the short, medium and long term. Typically, these time frames would cover the operational, closure, and post-closure phases respectively.
Medium and long term predictions are based on data for the local conditions, models calibrated to local conditions, and possibly the study of engineered or natural analogues [113]
[123]. An important aspect of the use of predictive modelling is a procedure to verify the predictions of the model via monitoring as part of the performance assessment procedure.
Where observed conditions differ significantly from predictions, the regulatory regime should enforce corrective action if necessary.
8.5. Monitoring
Recently, the IAEA has published a comprehensive set of guidelines for the scope and performance of monitoring programmes for uranium mining and milling residues [121] [203].
The short term effectiveness of the impoundment structures is monitored during the disposal and environment surveillance stage, using the appropriate means (monitoring networks). In the longer term, effectiveness can no longer be demonstrated directly, due to the time-scale.
The isolating ability of the repository, therefore, has to be forecast using models that can be updated, if required, as new data are gathered.
Regardless of the time period in question, the approach followed to demonstrate that impact is acceptable typically consists of several iterative steps:
description and understanding of the behaviour of the various structures making up the impoundment,
selection of realistic exposure scenarios derived from events or processes that are representative of the potential risk of degradation of the impoundment,
analysis of the effects of these processes and events on the effectiveness and stability of the repository structures and how their environment changes, along with an assessment of the radiological consequences of the exposure scenarios selected.
The isolating ability of impoundments for uranium mill tailings depends on the effectiveness of the impoundment structures as a whole, be they man-made or natural.
The main items for assessing the behaviour of the repository are typically:
measurements and investigations carried out on the impoundment and its environment, from the start of operation to the present time:
as regards transfer through the atmosphere: radon concentrations, activity of dust transported in the form of aerosols, and external exposure;
as regards water pathways: physical, chemical and radiological characteristics of surface and groundwater;
description of the steps taken or planned to restrict use in order to prevent human activities (e.g. bans on house building or digging on the impoundment site, ban on use of water from the repository site etc.);
means implemented to maintain the repository structures and the means used to check them and ensure their integrity;
geotechnical stability in the short term with respect to settling, including differential settling, changes in gradients and materials properties (changes in porosity, migration of fines, changes in volume or loss of cohesion), fluctuations in piezometric level (saturation risk);
geotechnical stability in the medium and long term;
description of the radionuclide pathways in relation to their speciation, hydrology and the hydrogeology of the repository and its environment as well as the consequences resulting from geotechnical changes.
Since the migration of contaminants from a tailings disposal facility may occur over long periods of time and, hence, the long term performance of the facility must be assessed, it is often useful to look at analogues. These may be existing engineered structures or natural analogue sites.
8.6. Radiological impact 8.6.1. Concept of critical groups
The radiological performance of a tailings impoundment would be evaluated with respect to certain scenarios and critical groups. A realistic critical group is representative of individuals likely to receive the highest doses in the immediate environment of the tailings facility for a given scenario. Aspects to be considered in identifying a critical group include the degree of self-sufficiency, current local lifestyles and, in particular, the origin of water supply.
The assessment of the future radiological impact caused by potential releases of radioactivity to the environment typically is based on current technological levels and lifestyles. However, any extrapolations of living conditions into a distant future would be largely speculative.
8.6.2. Scenarios
8.6.2.1. Approaches for generating scenarios
Scenarios are collections of actual or predicted features and events that will lead to certain exposure patterns. The process for developing scenarios, however, typically is rather informal and relying on expert opinion. This holds in particular for ‘future’ scenarios. The diversity of events, and their sequences and consequences may lead to a complex system to be analysed.
In order to facilitate the development of scenarios, to formalise the reasoning, and to avoid any suspicion of ‘arbitrary’ selection of scenarios, a systematic approach in generating scenarios may be adopted:
1. identifying and establishing an inventory of all possible processes, their origin (natural or anthropogenic), the probability of their occurrence, and the effects that they will have on the integrity of the impoundment;
2. identifying all the different possible states of the impoundment, their combinations, and their possible changes;
3. the elimination of certain combinations of their states due to respective incompatibility or lack of relevance, considereing the events or processes which could produce them;
4. categorization of combinations of states with respect to the type of resulting exposure and critical group;
5. generation of formalised exposure scenarios that reflect these categories of combinations of states.
Such approach would be expected to result in a limited and tractable number of formalised exposure scenarios representative of different categories of events or sequences of events.
8.6.2.2. Resulting exposure scenarios
Typically two types of scenarios are developed using such approach:
A reference scenario that corresponds to a ‘normal’ development of the impoundment, taking into account:
normal changes in the natural environment around the repository,
all natural factors that can lead, for various reasons, to gradual modifications over time of the performance, the structural integrity, and the impounded residues.
‘Event’ scenarios: these scenarios would reflect increased deterioration of impoundment and its environment, or human actions that would endanger the integrity of the impoundment.
Superimposed on the development of scenarios may be probability analyses, which help to further eliminate from consideration improbable scenarios.
8.6.2.3. Radiological consequences of these scenarios
The possible radiological consequences of these scenarios would be assessed using the additional individual effective dose to members of the critical group. These assessments are based, as much as possible, and in particular for evaluation of short term scenarios, on the results of measurements made at the site of the environment and its immediate environment.
Assessment of the additional effective dose includes:
external exposure,
internal exposure due to intake by inhalation and ingestion of these radionuclides.
These assessments typically are supported by:
geometrical simplifications and/or simplifications of phenomena adopted, showing that they are conservative,
sensitivity studies used to identify important phenomena and parameters and providing a justification for simplifications made,
estimations of the uncertainty ranges.
The radiological consequences resulting from the presence of a tailings impoundment are then compared to the doses that would be received on the site without tailings. Reference would be made to baseline data, if available. Otherwise, the effective natural background dose would need to be estimated from measurements at nearby locations, but in an environment with characteristics comparable to those of the site. Variations in the natural background dose, estimated or measured, would also be taken into account.
8.6.2.4. Normal conditions- reference scenario
The radiological impact from the tailings impoundment would be assessed for reference scenarios using the additional effective dose calculated for a period at least equal to the length of time during which the disposal structures is estimated to remain effective. For this period explicit uncertainties can be taken into account. Beyond this period, the results will have an
additional uncertainty attributable to the uncertainty in the impoundment development. Dose estimates, therefore, would be supplemented by qualitative assessments of the factors that cause the uncertainties.
8.6.2.5. Hypothetical conditions — ‘event’ scenarios
Hypothetical conditions result from certain random events, whether naturally-occurring or related to human activity, are likely to involve a risk of higher addtional effective doses.
In order to capture the random nature of such events, a probability of their occurrence would be estimated whenever possible. The acceptability of the additional effective doses would be assessed as a function of the probability of occurrence, taking into account the characteristics of the random events, the duration and types of radionuclide transfer through the biosphere, the characteristics of the exposure pathways, and the type critical group(s) considered.
8.7. Non-radiological impacts
While the above discussion focused on radiation dose as an overall measure of performance, it should be noted that there are other environmental stressors that need to be taken into account.
Relevant stressors typically include inter alia toxic heavy metals, acid drainage, and dust.
The methods to derive scenarios and assess risk would be very similar to those discussed above. The evaluation, however, would not be carried out on the basis of a dose to a critical group, but with respect to resulting concentrations in certain environmental compartments as stipulated in relevant national legislation.
Similarly, methods exist to evaluate conventional risks from geotechnical failure [76] [80].
9. SUMMARY