Reversibility as a design consideration for geological repository
the idea of reversibility | intergenerational fairness | achieving confidence | the limits.
This reading is part of a series: Nuclear Waste Disposal
For a more detailed treatment of the ideas discussed here, please look at the excellent report of the Nuclear Energy Agency - "Reversibility and Retrievability (R&R) for the Deep Disposal of High-Level Radioactive Waste and Spent Fuel - Final Report of the NEA R&R Project (2007-2011)". (link)
Building a geological repository is increasingly viewed as a long-term undertaking involving multiple generations. It should not be expected that all generations involved in the construction will hold similar views about waste disposal. The reversibility of construction decisions over the short to medium term and the retrievability of the waste in the long term are considered essential design criteria to preserve choice for future generations. These considerations hope to increase confidence in the project and make it safer and more socially acceptable [1].
Given the nature of large-scale technology projects, where reliance on perfect knowledge is reckless, methods or designs that offer the greatest room to maneuver in the future should be favored, all else being equal. Therefore, reversibility of action should be counted as a significant benefit, and irreversibility as a major cost [2].
What do these design concepts mean, and why are they taken so seriously by nuclear waste storage planners and designers?
Reversibility aims to give builders the ability to reverse decisions and return to a previous state. For example, you had planned to build a 50-story building. After building the first 10 stories, it was decided the building needed 80 stories. So you deconstruct, return to the start, build a deeper foundation to accommodate 30 more floors and start again. Reversibility applies only to a certain point and only up to the repository closure. Once the waste is placed in the repository, retrievability is the design idea to consider. It is about making it easier for future society to retrieve the waste if it wants to.
In this note, the focus will be on reversibility.
A discussion on retrievability, which deals with the aspect of retrieving the waste package once it has been placed, can be found here: (link)
What is it?
Reversibility is a design concept that aims to facilitate decision-making. It offers the ability to reverse a single or multiple steps in the building process. This is achieved by developing the repository in stages and making each stage reversible.
This step-by-step approach involves careful consideration and deliberate decision-making at each step on whether to move to the next step or to reverse course.
Such an approach allows experience, new knowledge, and testing to inform the best course of action. Decisions that produce unwanted effects can be reversed.
The additional benefit of this approach is that it makes a repository socially easier to implement. The smaller each step is, the more socially acceptable it will be since significant changes and big decisions are more likely to cause disagreements and conflict [3].
It is about intergenerational fairness:
Reversibility ensures participatory decision-making continues during the lifetime of the project. It ensures that a decision can be corrected if it is later found faulty or questionable.
With increasing knowledge and changing value judgments, choices in the future may not match our own. Reversibility is about giving future generations the freedom of choice. It allows adaptation to societal preferences.
Does achieving intergenerational fairness have a cost? There may be situations where safety and fairness become conflicting ideas. This is an argument often put in the context of reversibility and retrievability. For fairness, designs that allow reversibility and retrievability may, in some instances, be considered less ideal than designs where "safety first" is an overriding requirement. However, safety should be a carefully considered judgment [3]. Especially where ethical values may need to be considered.
Achieving confidence in the design:
If reversibility is not predefined and the authorities are forced to reverse a decision, it could damage the plan's credibility and trust in the institution in charge. Thus, incorporating it into the design provides credibility.
If, at each step, all relevant stakeholders deliberately decide to move forward over the decision to change course or reverse steps, then at the time of final closure, a higher level of confidence can be achieved [1]. A decision to move forward is also a reaffirmation of previous decisions.
With a stepwise approach, there is an opportunity to incorporate the knowledge and experience gained from the previous steps. The decisions are better informed, which provides more confidence in the design's capabilities.
There are limits to reversibility:
Although the idea of reversibility is conceptually straightforward, implementing it is anything but. It is reasonable to expect that a future generation could be provided with the ability to reverse a few steps. But one has a hard time believing that they will have the option to reverse several steps and revert to a phase of several decades before.
It would also not be possible to make all steps fully reversible. For example, it would not be possible to change the tunnels' geometry and placement once they have been dug. Some steps would be difficult and costly to reverse, e.g., changing the canister design after the waste has already been encased in canisters and placed in the tunnels.
It would become increasingly more difficult and complex to revert to early phases when the repository is nearing completion. Therefore, reversibility is only achievable to a certain extent [3].
A point of view:
The idea of reversibility is about flexibility. It is about going from point A to point B, where the nature of point B is only vaguely known in the beginning. The more room to maneuver there is, the more knowledge one can have of the exact nature of point B and how to reach it. In this case, being on a predefined course leaves room for missteps, and the lack of complete knowledge means the possibility of ever reaching point B is based on guesswork.
References:
[1] NEA, “Reversibility of Decisions and Retrievability of Radioactive Waste: An Overview of Regulatory Positions and Issues,” Nuclear Energy Agency of the OECD (NEA), NEA-RWM-R--2015-1, 2015.
[2] National Research Council, Technology: Processes of Assessment and Choice. Washington, DC: The National Academies Press, 1969. doi: 10.17226/21060.
[3] NEA, “Reversibility and Retrievability (R&R) for the Deep Disposal of High-Level Radioactive Waste and Spent Fuel - Final Report of the NEA R&R Project (2007-2011),” OECD Publishing, Paris, NEA/RWM/R(2011)4, 2011.