To assess the overall safety of a geological repository by means of numerical radionuclide release calculations, specific magnitudes are needed which follow from the calculation and can be compared with reference values. Such magnitudes are often called safety indicators. The most common safety indicator used so far is the effective dose rate. The uncertainty of dose rate calculations, however, increases with the time under consideration, and therefore it seems desirable to have additional indicators to improve the reliability of performance assessments. As long as not the overall safety but the performance of individual barriers is assessed, or the functioning of the system is to be showed, another type of indicators can be used, these are sometimes called performance indicators. The SPIN project identified and tested seven safety and fourteen performance indicators. Safety indicators have been mainly identified by evaluating the open literature, performance indicators through systematic approaches. The indicators have been tested by re-calculating existing performance assessments of disposal systems for high level waste in crystalline formations in Spain, Germany, Finland and Switzerland. The results have been compared and assessed in view of the general applicability of the specific indicators. Although other geological formations than granite have not been tested, the conclusions of the project might be more generally applicable. The effective dose rate is taken as the basic safety indicator. Two other indicators were found to provide significant benefits and may therefore be used to complement the effective dose rate. The three proposed safety indicators and their preferred application to time frames are: 1) Effective dose rate: most relevant to early time frames, 2) Radiotoxicity concentration in biosphere water: preference for medium time frames, 3) Radiotoxicity flux from geosphere: preference for late time frames. For the effective dose rate the data from present regulations were used as a range of reference values. Reference values for radiotoxicity concentration and fluxes were taken from nature, based on the assumption that nature in general is radiologically safe. Widely reported concentrations and fluxes corresponding to crystalline sites were used as reference values. The project concluded that several performance indicators can be used to show different aspects of the functioning of the individual compartments of the multi-barrier system. These indicators and their preferred applications are: 1) Inventories in compartments: showing where the radionuclides are at different points in time, and the retention of radionuclides from the biosphere, 2) Fluxes from compartments: showing the decreasing release rates from successive compartments, including radioactive decay and ingrowth, and the delayed release, 3) Time-integrated fluxes from compartments: showing decay during delayed transport, 4) Concentrations in compartment water: showing the decrease of concentration by dilution, dispersion and decay in successive compartments, 5) Transport times through compartments: showing the potential importance of individual radionuclides to the release of radiotoxicity by comparing them to their half-lives For investigations relating to the total radionuclide spectrum, performance indicators based on radiotoxicities should be used. When investigating the behaviour of different types of radionuclides, indicators based on activity are considered appropriate.
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