Evaluation of uncertainties related to isolation and containment of a containment-providing rock zone

In [1] indicators have been developed to assess confinement and isolation of radionuclides within the essential barriers of a deep geological disposal repository (containment-providing rock zone and geotechnical barriers respectively according to § 23 (4) of the German site selection act). In total, nine indicators have been proposed which are related to different physical reference quantities. Functionality and impact of the indicators were illustrated by means of transport and dispersion calculations for a generic disposal system in clay, based on simplifying assumptions. As a basis for the calculations (using the computer programs MARNIE and TOUGH2-GRS), the inventory of high-level radioactive waste in Germany derived within the VSG (Vorläufige Sicherheitsanalyse Gorleben) [2] and parameter bandwidths of the geoscientific weighting criteria of the site selection act were assumed. The afore mentioned research work [1] was important for the justification of § 4 (5) of the safety requirement ordinance [3].

In the work presented here the prognosed inventory from the VSG in 2011 [2] was updated with the adjusted amount of high-level waste due to the shutdown of all nuclear power plants in Germany and other smaller changes in the nuclear waste management chain. In addition, transparent selection criteria for the radionuclides taken into account within the calculation were proposed. Subsequently, the assumptions of [1] were analysed according to their inherent uncertainties. E.g. the assumption of a linear sorption term for the nuclides overestimates the sorption capacity of the containment-providing rock zone and the assumption of an instant release of the full inventory overestimates the amount of radionuclides being transported. It is one goal of the work to deepen the understanding of the consequences of the inherent uncertainties of such assumptions. With the open-source reactive transport code PFLOTRAN, further developed at BASE, selected calculations from [1] were carried out again to ensure the reproducibility of the results with a different modelling tool. From the analyses of the afore mentioned uncertainties new assumptions had been derived and updated calculations were carried out to demonstrate the applicability of the indicators. Based on the parameter bandwidth of the geoscientific weighting criteria of the site selection act, a systematic sensitivity and uncertainty analyses was carried out. Similar to the findings in [1], the presented results may contribute to a possible review and enhancement of the rules and standards for nuclear waste disposal.

References

[1]	Navarro, M., Weyand, T., Eckel, J. & Fischer, H. (2019): Indikatoren zur Bewertung des Einschlusses und der Isolation mit exemplarischer Anwendung auf ein generisches Endlagersystem mit dem Wirtsgestein Tongestein, GRS-A-3985.
[2]	Peiffer et al. (2011): Abfallspezifikation und Mengengerüst, GRS-274, GRS-278.
[3]	Deutscher Bundestag (2020): Drucksache des deutschen Bundestages, 19/19291.