Long-term isolation of nuclear waste - technical discussion
a technical discussion
This reading is part of a series: Nuclear Waste Disposal
What follows is a discussion of the technical obstacles to long-term nuclear waste isolation. Even if no technical obstacles existed to constructing a functional geological disposal facility, choosing a geological repository over surface storage is about intergenerational fairness and not imposing the cost of surveillance and management on future generations.
Further discussion on the social aspects can be found here: (link)
For a more detailed treatment of the ideas discussed here, please look at the excellent report of the National Academies — “Disposition of High-Level Waste and Spent Nuclear Fuel: The Continuing Societal and Technical Challenges.” (link)
Why does nuclear waste require isolation?
Nuclear waste can be categorized into many types. The most hazardous is called high-level waste (HLW) — it has the highest radioactive content. Two concerns require that we isolate HLW. First, it can stay hazardous to humans for over 100,000 years. And second, it is a proliferation concern.
Its hazardous nature has been discussed before here: (link)
Its proliferation concern originates from the fact that it can be used to make nuclear weapons. HLW can be in the form of spent nuclear fuel (from a power station) or highly enriched uranium (used in specialized reactors, e.g. naval submarines) and plutonium (from dismantled weapons built during the Cold War). Spent nuclear fuel itself contains uranium and plutonium that can be used to make nuclear weapons by a sufficiently sophisticated entity.
Typically, weapons-grade plutonium is made by running the reactor in a very short cycle, extracting the fuel and processing it to extract the plutonium. In a normal energy-producing cycle (in modern reactors today), plutonium has a lower concentration of Pu-239 and a higher concentration of other isotopes, which makes it less attractive for bomb manufacturing. However, plutonium of any isotopic composition can be used to make nuclear weapons.
During the initial 50 to 100 years after it exits the reactor, spent nuclear fuel contains highly radioactive fission products, which make it challenging to handle and extract the plutonium without a high level of sophistication. But as the short-lived fission products decay and the vital barrier of intense radioactivity diminishes, the proliferation risk increases.
These concerns require that we ensure the safety and security of the facility that stores the waste for a very long time.
An alternate solution is to separate the uranium and plutonium from HLW and reuse it for energy production. What remains in HLW is fission products that are not a proliferation concern and that decay to harmless levels relatively quickly. However attractive this option may be, it has substantial challenges, and it is not something we do today.
Short-term isolation and long-term isolation:
Isolation of nuclear waste can be thought of as having two phases: the short-term and the long-term.
In the short term, safety and security are ensured through active management, i.e. high vigilance and continuous monitoring. This could be achieved in surface storage or an unsealed geological repository. I have written previously about short-term storage solutions (here and here).
In the long term, the best way to ensure safety is through passive means, i.e. a hands-off approach that relies only on engineered structures and natural processes — this entails entombing the waste in a geological repository. This phase starts when the geological repository is sealed closed.
The length of the short-term phase depends on how long the society feels comfortable with continued management and has confidence in the capabilities of future generations. It is believed that most societies will eventually choose to transition from active to passive management of the waste.
During the active management phase, there is still time to rethink things, reverse decisions, and easily retrieve the waste if needed. But once passive management starts, i.e. when the repository is sealed and closed, the degree of retrievability decreases with time. This later phase has the benefit of freeing society from future resource commitments.
Is surface storage feasible as long-term storage?
Long-term storage solutions talked about today usually point to a geological solution. However, can surface storage provide a long-term storage solution? We have a long history of using surface storage of over 70 years and know how to do it well. Numerous dry cask storage facilities are operating throughout the US and other parts of the world. Although surface storage today is intended for 50 to 100 years, there is no technical barrier to extending this indefinitely. Surface storage can provide secure containment for a few centuries if properly managed [1]
Safety of surface storage is achieved by encasing waste in robust canisters. There is really no technical limit to how tough these canisters can be built. Security of the facility is achieved by restricting access to individuals that may divert fissionable material. Given the small volume of HLW and the safety and security that surface storage provides, there is no urgency, technically speaking, for a geological repository.
Limitations:
There are several limitations to relying on surface storage for too long. Surface storage facilities will always have a limited lifetime and will have to be rebuilt again and again to provide continued capability. Leaks in such storage facilities usually result from using them for longer than their intended life. That means constant funding must be ensured. The facility will require continuous monitoring for the entire life of the nuclear waste (over 100,000 years). And that is where the problem is.
The uncertainty then is not technical in nature but rather of the society and the future state of the world and whether continuous vigilance will be maintained. It is impossible to be certain that a future society will be stable enough and capable of continuous monitoring to ensure the safety and security of the facility. Social structures of our society have proved to be delicate, and relying on them for continued assurance is not prudent. The geological repository is an answer to the uncertainty in our society’s ability to forestall chaos and ruin forever. Apart from trusting the strength of the social structures, there is also the question of ethics. Is it fair that we transfer the responsibility of nuclear waste to future generations (link)?
Reasons to use surface storage:
Using surface storage for at least several decades offers time for the waste to cool down and the radioactivity to go down, which serves as an advantage in designing geological repositories. Given the glacial pace of geological repository development so far, extended use of surface storage is unavoidable.
Advantages of geological disposal:
The main advantage of a geological repository is that it provides safety and security passively, i.e. it would not require ongoing monitoring by future generations. Moving waste deep underground also limits human access and reduces proliferation risk. Scientific work has been going on about its efficacy for the past 50 years, and it is generally accepted to be a safe approach for the disposal of HLW.
Although thought of as a permanent solution, deep disposal in a geological repository need not be irreversible right away. A geological disposal facility can be kept open and offer the option of retrievability for many years. Any decision for final closure will be only after decades of close observation and deep reflection within the society, which will give time for the society to decide if other methods offer better solutions [2].
Dealing with uncertainty:
Practically, geological disposal is not a major technical undertaking. All techniques that would be required to construct a repository are already known. We regularly design and build facilities that are more complex and have higher hazard potential [2].
The reason the question of safety and security is so profound with geological disposal and requires a thorough and detailed answer is because the discussion involves an extraordinarily long time scale and must deal with uncertainties of a geological nature. It involves building things that would last longer than anything we have designed so far. Whatever we decide to build must also give confidence to the public in its ability and overcome the fear of “nuclear.”
Redundancy:
Some uncertainty can be reduced by using built-in redundancy at each stage and for crucial components to deal with possible future failure scenarios. Additional redundancy can be achieved using a multi-barrier approach with engineered and geological barriers working together to provide safety [2].
In many locations, geology has remained stable for hundreds of thousands of years and provides high confidence. However, some locations being considered have many geological unknowns. In these locations, designers have opted to reduce reliance on geological barriers in favour of barriers engineered to withstand harsh conditions and remain stable. For example, waste encapsulated in glass or ceramic material can remain stable in many geological environments for thousands of years [2].
Step-wise approach:
Uncertainty in design can be reduced, and knowledge gaps can be filled by incorporating learnings from observations and experience. This requires that design and construction be done in steps. At the end of each step, a careful study is conducted to evaluate the efficacy of previous steps and decide the next best step. This allows continued research, and the new knowledge acquired can be incorporated into the design. Going in steps allows for mid-course correction if the research points to a new direction.
Why such an approach is beneficial. The case of Yucca Mountain:
At Yucca Mountain, the flow of water from the surface through the repository at 250m to the groundwater at around 350m below the repository was initially assumed to occur over thousands of years. However, newer scientific measurements suggested that water may have been transported from the surface to the underground repository level in the previous 40 years [3].
Initially, it was also assumed that radionuclides released from the repository would stick to the volcanic rock and not travel far. However, newer measurements suggested a small number of radionuclides may be capable of being transported through the groundwater on microscopic particles called colloids [4].
These findings prompted a redesign of the engineered barriers. An example such as this raises the question of the extent of our understanding of the geologic medium. The uncertainties are precisely why it is essential to maintain flexibility in design and approach the development of a repository step-wise with the ability and the willingness to adjust design based on new findings [2].
Keeping the repository open for longer will also improve confidence in the design as more findings get tested.
Modelling can help reduce uncertainty:
Humans have only recently started constructing complex structures relative to the time scale we are discussing here. We have no observational data from a functional repository over its million-year life. This is one of the causes of uncertainty.
However, modelling can also help reduce uncertainty. We are not entirely blind. We know that natural processes like corrosion and diffusion don’t change with time. Natural analogies can also provide a mountain of data on geological processes over a million years' time frame.
Rather than looking for an exact prediction, achieving confidence with a range of potential future outcomes makes the task more achievable. We are not seeking to make an impenetrable and indestructible structure that lasts an eternity. It would be sufficient to build a structure that can contain the waste until it is rendered safe. Radionuclide release into the environment doesn’t have to be absolutely prevented as long as it is low enough not to pose a hazard to the life living on the surface [2].
Some uncertainty will always remain:
No matter how careful we are or how much we invest, some degree of uncertainty will always remain, and safety can never be fully assured. All we can hope for in a design is high confidence in its ability to function as intended for 100,000 years. A step-wise approach, multiple redundant barriers and extensive modelling cannot eliminate the uncertainty, but they can reduce it and build confidence in the design [2].
References:
[1] IAEA, “Safety Requirements. Near Surface Disposal of Radioactive Waste. WS-R-1,” IAEA, Vienna, 1999.
[2] National Research Council, Disposition of High-Level Waste and Spent Nuclear Fuel: The Continuing Societal and Technical Challenges. Washington, D.C.: National Academies Press, 2001. doi: 10.17226/10119.
[3] J. T. Fabryka-Martin et al., “Distribution of fast hydrologic paths in the unsaturated zone at Yucca Mountain,” in 8th Annual International High-Level Radioactive Waste Management Conference, American Nuclear Society, La Grange Park, 1998, pp. 93–96.
[4] A. B. Kersting, D. W. Efurd, D. L. Finnegan, D. J. Rokop, D. K. Smith, and J. L. Thompson, “Migration of plutonium in ground water at the Nevada Test Site,” Nature, vol. 397, p. 5659, 1999.