At the back end of the nuclear fuel cycle, there is a need to ensure the safety of radioactive waste at all stages including its final disposal. A deep geological repository is evaluated in a safety case, which presents evidence and analyses to support decision making at each stage of the repository’s development.
The NEA assists its member countries in their needs for developing, reviewing and updating effective safety cases supported by a robust scientific-technical basis. The work of the Agency in this area is co-ordinated through the Radioactive Waste Management Committee (RWMC) and its Integration Group for the Safety Case (IGSC).
Since 2000, the IGSC has identified, documented and evaluated the structure and content of safety cases for deep geological repositories and established consensus positions on good practices in their development. In this context, the Agency hosted an online expert roundtable discussion on 24 June 2021 on the IGSC’s experience and how it could be used to develop safety cases for different types of radioactive waste disposal facilities.
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“The NEA developed the concept of the safety case years ago,” noted NEA Director-General William D. Magwood, IV during his opening remarks at the event. “And the safety case is a discipline by which the issues associated with the disposal of radioactive waste are assessed through a very careful safety process to support decision making, putting safety at the core of the proposed facility.”
The panelists agreed there are some methodological and strategic differences in compiling and presenting safety cases for near surface, intermediate depth and deep geological repositories, arising largely from the very different timescales and hence the uncertainties and associated risks involved.
For example, near surface disposal facilities cannot guarantee waste isolation and protection from human intrusion for long periods of time (i.e. hundreds of thousands of years). Therefore, safety cases for near surface facilities rely more on waste acceptance criteria, consider primarily low- and very low-level radioactive waste and put confidence in the institutional control period. As such, the safety case strategy for a deep geological repository would focus on reducing the likelihood of human intrusion, whereas the safety case strategy for a near surface facility would focus more on minimising the consequences of potential human intrusion.
Despite such differences, the overall philosophy of designing a robust safety case is common to all disposal types. “The expertise, approach and strategies required for the development of assessment scenarios, the management of uncertainties and the associated risk evaluations in compiling and presenting safety cases for different disposal facilities share some commonalities,” said Hiroyuki Umeki, RWMC Chair and Executive Director of the Nuclear Waste Management Organization of Japan (NUMO).
Regardless of the type of disposal facility, a robust safety case is built on multiple barriers and safety functions for defense-in-depth. These include a stable waste form to limit release rate, a waste package to provide containment, a buffer to protect the container and geological barriers to isolate the waste. A multi-factor safety case should also present multiple independent arguments for safety assurance, including a presentation and analysis of the scientific research, detailed total system modelling and natural analogue studies.
As many radioactive waste disposal programmes rely on volunteer or consent-based siting, safety case communication is another important aspect that is common to all types of disposal facilities. The scientific and technical basis for disposal facilities should be accessible to all stakeholders to enable them to participate in the decision making process. In this respect, it is key to build trust and confidence in the licensing process, tailor communication, explanation or concertation towards different audiences, and be concerned about stakeholder needs.
“Safety cases need to be developed throughout with engagement and communications in mind,” said Lucy Bailey, IGSC Chair and Head of Research Support Office at United Kingdom Radioactive Waste Management. “This is not just an add-on activity at the end.”
This expert roundtable is a precursor to an international workshop that the NEA will hold in collaboration with the Romanian Nuclear and Radioactive Waste Agency (ANDR) on 8-10 February 2022 in Bucharest, Romania. This workshop will facilitate exchanges on the experiences of NEA member countries to better understand how the experience accumulated by the IGSC can be shared in the development, use and communication of safety cases for deep geological and other types of repositories.