Repository Performance Assessment

What is Performance Assessment?

Performance assessment (my own professional activity) is the development of conceptual and mathematical representations of a repository and its environment, such that numerical estimates of detriment can be made. In this context, the term "detriment" refers to any general measure of harm, and in most assessments detriment is either dose or risk, because of the regulatory requirements to estimate these quantities.

Assessment Inputs and Outputs

The following diagram illustrates the various inputs and outputs from a programme of performance assessment modelling.

The inputs to the assessment reflect the understanding and knowledge gained in the course of an overall work programme to design and build a waste disposal facility. The reader will appreciate that these inputs are in fact very substantial studies that require significant expenditure. However, the following table gives a brief summary of what is involved.

Inventory:	Numerical measures of the amount of radionuclides in the wastes to be disposed, usually in Becquerels (Bq) for radioactive wastes and in mass units (kg) for chemically toxic wastes.

Design:	The arrangement of structures and barriers within the repository itself, designed to keep the wastes contained within the repository and to prevent the ingress of groundwater.

Site Characterisation:	Development of an understanding of the disposal system environment, including major features of the environment and groundwater flow characteristics. Note that these are often multi-million pound studies!

Detailed Research:	Research programmes designed to develop knowledge for use in assessments, for example experimental studies to define sorption coefficients, or theoretical studies to improve modelling techniques. Again, these are often multi-million pound studies.

Uncertainties:	Statements of, and approach to, the lack of knowledge about certain aspects of the disposal system, for example parameter values, system evolution, conceptual modelling uncertainty.

Regulatory Requirements:	The standards and “rules of the game” that performance assessments are expected to meet, including the range of outputs that are deemed necessary to form a safety case.

At the start of this article, we noted that performance assessments primarily are used to estimate health detriments and impacts. However, the lower half of the figure above shows that performance assessments also provide a number of other useful outputs, in addition to numerical estimates of detriment. Brief descriptions of these are given in the following table.

Expected Impacts:	Estimates of the doses and risks that will arise to future human populations and the environment, under a range of possible modes of disposal system evolution.

Comparison with targets:	The results of assessment studies can be used to determine whether a disposal system is compliant with regulatory targets.

Understanding:	Developing an understanding of why the disposal system gives rise to a particular level of detriment, and the roles played by the various barrier in the system in retarding the return of radionuclides to the accessible environment.

Key Controls:	Determining which system parameter variations control the magnitudes of dose and risk. That is, determining which are the “key parameters” for the system.

Communication:	Enabling stakeholders to understand the consequences of a particular waste disposal system, and hence to consider additional actions and requirements to ensure safe disposal of the wastes.

Impact of Uncertainties:	Developing an understanding of where additional research is needed, to try to reduce or remove those uncertainties that give rise to the greatest uncertainty in estimated doses and risks.

It is important to note that performance assessments do not attempt to make firm “predictions” about the doses and risks that will be received from disposed wastes at some time in the future. The problem is too complex, and with too many inherent uncertainties about system and future human behaviour, for this to be possible.

Instead, assessments consider a "stylised" representation of the disposal system, in which reasonable or cautious assumptions are made about disposal system behaviour, in order to investigate the range of possible detriments that could arise. This approach ensures that the assessments provide estimates of dose and risk that are towards the upper end of the range of those that might be expected.

Developing a Performance Assessment

Throughout the international radioactive waste disposal community, there is general consensus about the procedures that should be followed when developing a performance assessment for a radioactive waste repository. This is illustrated in the following figure, and was developed by the International Atomic Energy Agency (IAEA).

In this figure, the "scenarios" referred to in the third box from the top are alternative modes of future evolution of the repository system. The definition of an appropriate (but manageable) set of scenarios is a standard technique for investigating the range of possible ways in which the repository system could evolve over the very long timescales that the performance and safety assessment must cover.

In the left-most box, the term "assessment criteria" refers to the regulatory requirements for the performance and safety assessments, including the dose and risk targets that must be satisfied.

An interesting feature of this approach is that it allows the possibility of iteration between the results of the safety assessment, the design of the repository system, and the assessment approach. This is important, as it ensures that features and concerns about the performance of the disposal system can be highlighted in the performance assessment, fed back to other members of the project team, and improvements made. Such "improvements" could, for example, be changes to the repository design or location.

Outputs of a Performance Assessment

To conclude, the following figure shows the type of output that is obtained from performance assessments used to estimate radiological risk. The risks shown in this figure are to farm occupants, located on a farm near to the point of discharge of radioactivity. The farm occupants are assumed to consume plant and animal produce that is cultivated on the farm, and which therefore has the potential to be contaminated with radioactivity. The figure is obtained from reference [1].

A number of interesting features can be seen in this plot.

1. Risks do not become significant until several thousand years after repository closure (assumed to be at time = 0 on the x-axis). This is due to the time required for the disposed radionuclides to leave the repository in groundwater, and to travel through the geosphere to the biosphere.

2. The contributions to risk from the various radionuclides are different in shape and form. This is due to the different chemical behaviours of the various radionuclide elements, and also due to the different amounts of the radionuclides that are disposed.

3. The peak annual risk in this plot is about 10^-6. This is equal to the regulatory risk target, and hence would be considered acceptable. However, it would still be necessary to explore means by which this risk could be reduced, to make it as low as reasonably achievable. This could be done by making changes to the repository design. These changes could then be investigated in further performance assessments, to determine if a reasonable risk reduction had been achieved.

References

[1] United Kingdom Nirex Limited, Generic Post‑closure Performance Assessment, Nirex Report N/080, 2003.

This report is freely available from the document library at http://www.nda.gov.uk.

THE END