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3. RTDC3
In RTD component 3 methodologies and tools for integrated PA for various geological disposal
concepts will be further developed. The work is separated into four work packages and it includes
the development of scenarios by applying safety functions and stylised scenarios,
the PA approach to gas migration processes with respect to the determination and quantification
of the impact of gas on the engineered and natural barriers, and
the PA approach to radionuclide source term modelling where a more detailed modelling of the
chemical environment and the up-scaling from one canister or disposal cell to a large scale repository
is considered.
In some of the areas, PA representations are known to be less advanced. The last work package in
this RTD component concerns the use of safety and performance indicators with the objective of
testing the applicability of various safety indicators for host rock formations other than granite. First
results are described here.
The work in this work package is based on the review work in RTD component 1 concerning safety
and performance indicators. One of the main findings was that there is international consensus that
a repository safety case can be strengthened by the presentation of a range of safety indicators, to
complement the dose or risk calculations. There are different concepts of assessing repository safety
and performance by means of other indicators. The review also revealed that there is still a large
variety of different views on the terminology used for safety indicators and performance and function
indicators, respectively.
In the work in RTD component 3 an important goal is to develop a common understanding as follows.
Every safety indicator that is applied for the safety case must be defined by a safety statement,
based on a previously defined safety aspect. These safety aspects are based on a specific
safety concern, such as “protection of human life” and “protection of environment”. A safety statement
could, for example, be that all biological effects to a human individual, i.e. the incorporation
of radionuclides released from a repository by humans via different exposition paths, remain so
small that they have no impact on human health. The corresponding safety aspect is the human
health. For a complete safety statement a numerical measure is required, by which the biological
effects due to incorporation of radionuclides can be calculated, as well as a reference value in order
to define a safe level. A common measure for this type of safety statement is the individual effective
dose rate. In the case of the effective dose rate, the national legal limit could be used as reference
value. In general, reference values are either global or can be derived on a local scale. The use
of the latter must be justified thoroughly.
In order to employ a safety indicator in a safety case, it is necessary to calculate the outcome of the
considered numerical measure (e.g., the effective dose rate) by means of a PA model for the corresponding
repository system. The PA model allows the calculation of the radionuclide migration in
the repository system based on a reference design and a description of the geological site. By comparing
the results of the PA model with the reference value of the safety indicator, the determination
of the safety indicator is complete and the results can be added to the safety case.
In contrast to safety indicators there can be a close interaction between the PA model and the definition
and calculation of performance indicators, especially between the compartment structure and
the repository design. For every performance indicator a compartment structure has to be defined.
The chosen compartment structure depends on several conditions, e.g. the host rock or the type of
quantity (concentration, flux) that is calculated. If several performance indicators are used within
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