Radiological Consequences of Future Human Actions
Nagra (2024): Radiological Consequences of Future Human Actions. Nagra Arbeitsbericht NAB 24-09.
pdf NAB 24-09 Radiological Consequences of Future Human Actions(4.45 MB)
The Nuclear Energy Act (KEG) requires deep geological disposal of all of Switzerland’s radioactive waste. Given the extensive timescales involved, safety assessments for radioactive waste disposal inherently involve uncertainties related to the activities of future generations around the repository site, such as those involving exploratory investigations. A key characteristic of the Swiss repository site is the lack of any significant underground resources specific to the region that could attract future human actions, including inadvertent intrusion into the repository. While the likelihood of disturbance of the deep geological repository by humans in the future is considered extremely low, even after knowledge of the site is lost, a consequence analysis of potential future human actions, confirming the post-closure safety of the repository, is a regulatory requirement. The radiological consequence analysis of future human actions is performed by assessing future events that could have the potential to affect the safety of the repository.
The employed methodology is a bottom-up approach involving an assessment of features, events and processes and the development of stylised future human actions safety scenarios for comprehensive radiological consequence analyses. In line with national and international guidelines, it outlines the potential consequences of future human actions. The future human actions safety scenarios conservatively account for the potential direct interaction with radioactive waste. This includes safety scenarios such as borehole penetration of a spent fuel disposal canister within a high-level waste emplacement drift, as well as the penetration of waste packages in a low- and intermediate-level waste emplacement cavern. Additionally, the safety scenarios consider the consequences of repository abandonment during the monitoring phase. The times considered for borehole penetration of the repository range between 500 and 50,000 years post-closure.
Analyses of dose consequences, based on conservative conceptual models and pessimistic parameter assumptions, indicate the following: The highest dose rate due to dissolved radionuclides for borehole penetration of a spent fuel canister is 3.7 × 10-3 mSv/a, while gas release of volatile C-14 through boreholes results in a much smaller radiological impact with maximum dose rates of 2 × 10‑6 mSv/a. For borehole penetration of a low- and intermediate-level waste emplacement cavern, the highest short-term dose rate of 0.43 mSv/a occurs from the direct penetration of a single waste package after 500 years. The long-term release through the borehole with upward fluid flow produces a maximum dose rate of 6 × 10-2 mSv/a. Gas and volatile C-14 release calculations result in a maximum dose rate of 6 × 10-3 mSv/a. In the event of repository abandonment, the highest dose rate of 7 × 10-4 mSv/a is associated with volatile C-14 released from the low- and intermediate-level waste repository section. The results show that all the considered future human actions safety scenarios result in dose rates well below the Swiss and international regulatory guidelines for future human actions of 1 mSv/a.
The findings further provide evidence that, although surface markers and robust record-keeping practices may help to reduce the potential of early future human actions and thus can support post-closure safety, the Swiss deep geological repository is located and designed to securely contain the radioactive waste purely by its system of passive barriers until the waste reaches radiotoxicity levels equivalent to natural rock. The stylised approach for the assessment of future human actions demonstrates that the system is robust even under the most conservative cases of inadvertent human intrusion. Since the highest dose rates are below the regulatory guideline for future human actions, no further efforts to reduce future human actions are warranted. Host rock properties, repository depth and repository design will therefore ensure long-term passive radiological safety for both humans and the environment throughout the full post-closure period of the repository, taking future human actions into account. These findings highlight that post-closure safety requirements are maintained even if memory of the repository is lost.