The safety of a geological disposal system can be assessed using defined indicators such as risk and dose level, which directly measure radiological effects on human beings. Since the safety assessment of a geological disposal system covers a very long period of time, there are inherent uncertainties involved in assumptions about future human activities and environmental conditions, which increase the longer the time frame becomes.
To allow for some of these uncertainties, it has been proposed to supplement assessment results based on dose levels (or equivalent risks) by other independent indicators, without relying on assumptions made in the biosphere model. Safety indicators such as nuclide concentration, flux and radiotoxicity can usefully supplement dose calculations in the evaluation of overall system performance. In particular, they may be more indicative of the isolation capability of a disposal system and the potential risks of radioactive waste disposal over the very long timescales of safety assessment.
In the H12 study, which is the project aimed at establishing a technical basis for HLW disposal in Japan, supplementary safety indicators have been applied to increase confidence in the safety assessment. The nuclide concentrations evaluated in the geosphere and biosphere in the H12 repository system were compared with measurements of naturally occurring nuclides. The comparison indicated that the concentration of radionuclides released from the repository would be several orders of magnitude lower than that of natural radionuclides. This suggests that application of supplementary safety indicators could increase the reliability of long-term safety by more rigorous measurements of concentrations of naturally occurring radionuclides in the characterization phase at a specific site, which will be compared with concentrations of relevant nuclides predicted by site-specific performance assessment models.