Chapter 58: Radioactive Waste Repositories – Technical Challenges

This chapter was published on “Inuitech – Intuitech Technologies for Sustainability” on April 22, 2013.

A radioactive waste repository is a facility or designated site for storage or disposal of radioactive wastes.  The International Panel on Fissile Materials has stated:

  • “It is widely accepted that spent nuclear fuel and high-level reprocessing and plutonium wastes require well-designed storage for periods ranging from tens of thousands to a million years, to minimize releases of the contained radioactivity into the environment. Safeguards are also required to ensure that neither plutonium nor highly enriched uranium is diverted to weapon use. There is general agreement that placing spent nuclear fuel in repositories hundreds of meters below the surface would be safer than indefinite storage of spent fuel on the surface.”

1.      TECHNICAL CHALLENGES:

Implementing a deep repository involves designing or selecting engineered and natural safety barriers, undertaking major underground construction, building and operating equipment and facilities for transporting, encapsulating, and emplacing Spent Nuclear Fuel (SNF) or High Level Waste (HLW), running emplacement operations, and backfilling and sealing the facility after many decades. Most waste-management organizations agree that a staged or stepwise program is the best approach to this major long-lasting project.

Despite the scale and complexity of the engineering and science involved, however, the only really controversial technical issue is the credibility of predictions of repository system behavior for tens or hundreds of thousands of years into the future, and the debate on this issue has been intensifying. On one hand, opponents of nuclear power fear that accepting geologic disposal as a safe end-point might encourage the use of nuclear technology. On the other hand, proponents of geologic disposal often overstate the certainty of their arguments, failing to make it clear that absolute certainty and zero risk are unattainable.

The fact of the matter is that no strictly technical issues prevent implementation of geologic repositories, although the task is by no means trivial.  In fact, this opinion is held by the majority of the scientific and technical community.  Even many technical experts who are still concerned about uncertainties would be prepared to initiate the disposal process, if it could be reversible for the coming decades. The common impression that a great technical controversy exists is largely attributable to the media’s tendency to give equal coverage to both sides of every argument, regardless of where the weight of opinion lies.

Most of the controversial issues associated with geologic disposal are, therefore, questions of policy rather than questions of technology: Are there credible alternatives to geologic disposal? When should disposal projects be implemented? Where, if anywhere, can repositories be equitably sited? Can they satisfy technical safety criteria and also win sufficient support from the public? Can the disposal strategy be reversed if unforeseen problems arise? And these are the key questions repeatedly being posed about geologic disposal.

Although recent documentation by various organizations has confirmed the confidence of the scientific community in geological disposal, a significant fraction of the public does not share this confidence. The lack of public support is often related directly to the controversial issue of siting nuclear facilities, which has a troubled history that has led to continual changes in the selection processes.  Early on, some sites were chosen purely by experts and officials behind closed doors. The selection of the Gorleben site in Germany in the 1970s is a prime example.  In the 1980s, international bodies, primarily the International Atomic Energy Association (IAEA), mapped out a top-down, technical procedure intended to narrow a range of choices through objective criteria to a single site that would be recognized by all stakeholders as the most appropriate.  However, experience in various countries (e.g., France, U.K., and Switzerland) showed that this “decide, announce, and defend” (DAD) strategy could lead to controversy, delays, or failures.

Since then, more importance has been put on societal criteria, particularly the degree of acceptance in potential host communities, although the capability of a site to provide long-term isolation remains a condition sine qua non. This approach has been successful, particularly in the Scandinavian countries. In Finland, the implementer, the local community, the parliament, and the government have agreed on a geological repository site. In Sweden, local communities at two potential sites have agreed to investigations that could lead to implementation. Japan has requested that interested municipalities volunteer (more than 3,000 have been contacted).

Must every country have its own geological repository? In fact, the nuclear fuel cycle is already international, and there are no ethical, technical, or other reasons to compel countries to implement national solutions. Mining, enrichment, fuel fabrication, and reactor construction are all carried out by relatively few nations for the dozens of countries that use nuclear power. In a similar way, other toxic wastes are imported and exported when better environmental results can be achieved. International agreements (e.g., the IAEA Radioactive Waste and Spent Nuclear Fuel Convention) recognize the legitimacy of these transfers, as long as the waste-producing country is responsible for the safe and secure management of the waste. Nevertheless, some individual countries have legislated against importing waste. This is a national prerogative that must be respected, even though it is based more on considerations of public opinion and political feasibility than on ethical considerations.

A final policy-related issue in waste disposal concerns the security implications of moving wastes and spent fuel from distributed surface-storage facilities to centralized underground repositories. It seems clear that more safeguards against misuse and better physical protection against terrorists can be offered at repositories. Counterarguments are the risks involved in transporting wastes to repositories and the potential attraction of centralized sites for terrorist groups. This debate is, however, of little immediate relevance because there is no way of accelerating repository programs enough to have a major impact on security in the next 10 years or more.

2.     DISPOSAL CONCEPTS FOR REPOSITORIES:

According to a report prepared by the US Nuclear Waste Technical Review Board for Congress and the Secretary of Energy, in all countries, the implementer has the responsibility for developing a disposal concept that describes a repository system comprising natural and engineered barriers.  In most countries, limitations imposed by the geology constrain which disposal concepts can be considered.  The implementer typically ends up focusing on one particular geologic formation because of its prevalence or because other formations either are unsuitable technically or cannot be developed because of land-use conflicts.  Once a host rock has been chosen, the implementer considers the hydrogeological environment and determines what, if any, engineered barriers are appropriate as well as how the repository system as a whole will be designed.  The implementer then is expected to advance its safety case, a set of arguments and analyses demonstrating why the proposed deep-minded geologic repository will isolate and contain HLW and SNF for as long as society demands.  There is broad scientific agreement that deep-mined geologic repositories can be constructed in a wide variety of host-rock formations and hydrogeological environments, including in salt, crystalline rock such as granite, different clay formations, and unsaturated volcanic tuff.

2.1       The Concept of Deep-Mined Repositories – Salt:

Disposal of HLW and SNF in salt has been explored in detail in several countries for more than a one-half of a century.  The NAS had first proposed developing such a repository.  The great advantage is that no water can pass through salt and fractures are self-sealing.   Those two properties have been at the core of the salt disposal concept adopted, for example, by the German waste management program.

The salt disposal concept appears to be extremely elegant in its simplicity.  Put in the simplest terms, if the salt is there, water flow, the predominant mechanism for transporting radionuclides to the biosphere, is not occurring at rates of concern for waste disposal. It is then just a matter of carving out drifts in the formation.  Waste is lowered and emplaced into the drifts, most likely in boreholes.  The shafts leading to the repository, the drifts themselves, and the boreholes then are sealed with crushed and compacted salt.

Under lithostatic pressure exerted by the layers of rock above the repository, the salt flows slowly, closing around emplaced disposal packages and healing any fractures or voids that may have formed during the construction phase.  Waste packages are not considered long term barriers for isolating and containing HLW and SNF because localized brine inclusions could cause them to fail.  Even so, because the environment in the repository would evolve over several hundred years from being oxidizing to become reducing, the waste would remain in a relatively insoluble form.Slide1It is possible to analyze disposal of HLW and SNF in salt generically to determine whether it is an appropriate host rock for a deep-mined geologic repository.  Those generic studies would have to address questions about the effects of heat on brine migration and whether pressures become too great when hydrogen is generated as small amounts of water contact the waste. Moreover, extensive underground exploration of salt beds or domes would be required to determine whether a particular site might be suitable for a deep-mined geologic repository.  But at least some participants in the German program believe that an undisturbed deep-mined geologic repository in salt will have zero release of radionuclides to the biosphere for at least one million years.Slide2The safety case for disposing of HLW and SNF in salt involves a demonstration that the undisturbed geologic barrier would perform as anticipated and that engineered barriers (mainly shaft, drift, and borehole seals) would prevent brine inflow through the man-made penetrations of the salt barrier, thereby ensuring that any movement of dissolved radionuclides along those pathways will be minimal.

Sophisticated modeling work has been carried out to support the proposition that a disturbed salt repository holding non-heat-generating waste will isolate and contain radioactive waste for long periods of time.  Additional modeling appears to support the proposition for heat-generating waste as well, although that claim has not been subjected to a formal empirical or regulatory test.  Toward that end, experiments have been conducted to understand the compaction behavior of crushed salt. Investigations have been carried out to thermally simulate disposal in drifts of waste packages containing SNF.  Based on these studies, experts from the German program argue that the engineered elements of the repository it contemplates using (other than the waste packages) would function reliably.  So far however, no country has advanced a comprehensive safety case for disposing of HLW and SNF in salt.

2.2       The Concept of Deep-Mined Repositories – Crystalline Rock:

The KBS-3 plan developed by the Swedish program, supplemented by work carried out by the Canadian and Finnish programs, has resulted in a well-articulated crystalline rock disposal concept. The viability of this disposal concept depends on finding a site where the acidity and the oxidation-reduction potential of the groundwater enveloping the crystalline rock formation fall into an appropriate range.  In that case, according to the laws of thermodynamics, elemental copper would not react with the groundwater and thus waste packages made of that material would contain the SNF virtually forever.  Sites that have few fractures add an extra layer of protection.Slide3KBS-3 envisions a repository system composed of multiple compatible natural and engineered barriers. Groundwater in Swedish crystalline basement rock possesses the requisite chemical and electrochemical properties. The crystalline rock, however, is not impenetrable.  Fractures permit groundwater to flow within the typical formation, although the flow usually is quite slow, thereby limiting release of radionuclides to the environment.  To reduce further the release of radionuclides, engineered barriers are critical elements of the KBS-3 plan.  Rings of bentonite clay are used to line the boreholes where the packages will be emplaced.  This material further limits exposure of the copper canisters that contain the SNF to the groundwater. The bentonite also protects the canisters in the event of small movements in the rock and delays the spread of radionuclides that might escape from the waste package. The repository is designed so that the drifts can be backfilled with bentonite. The waste package features a canister, which is constructed from five-centimeter thick copper. In addition to being corrosion-resistant in the repository’s environment, the canister can withstand some of the mechanical forces caused by the movement of the rock.  Inside the copper canister is a nodular cast-iron insert to increase the mechanical strength of the waste package.

The safety case for the crystalline rock disposal concept depends most importantly on groundwater having favorable chemical properties, including acidity, oxidation-reduction potential, and dissolved solutes. All alternative corrosion mechanisms to the one articulated in the disposal concept have to be investigated to ensure that copper will remain in its elemental state. Bentonite has to be shown to limit advective transport under the chemical, thermal, mechanical, and hydrologic conditions expected to be present in the deep-mined geologic repository. Techniques for flawlessly welding lids on to the waste package must be demonstrated. Slide4SKB has studied these issues.  It has constructed laboratories to test the properties of bentonite under a wide range of conditions. It also has built a laboratory to investigate methods for welding and inspecting canister lids at an industrial scale. It has undertaken preliminary work to investigate the claim that new mechanisms have been identified by which copper might corrode in the basement rock.  An extensive review of this particular issue, however, by the oversight body largely concluded that the claim was not technically supported (Swedish National Council for Nuclear Waste 2009). A report, however, to the Swedish Radiation Safety Authority (SSM) suggests that this controversy has not yet been put entirely to rest.

The KBS-3 plan has been subjected to rigorous national and international peer-review.   SSM and the National Council for Nuclear Waste have regularly evaluated SKB’s plans to address outstanding technical issues. Neither organization has found flaws with the disposal concept that would require its abandonment or radical revision.  Confidence in the concept is increased because intact copper nodules, millions of years old, have been found enclosed in the same type of formations that might someday host a deep-mined geologic repository.

All information obtained through its research program was analyzed by SKB as it prepared a license application, which was submitted to the authorities in March 2011. As part of that application, SKB carried out a quantitative post-closure safety analysis of the proposed facility, primarily by estimating the possible dispersion of radionuclides and how those releases would be distributed in time for a representative selection of future potential scenario sequences.

2.3       The Concept of Deep-Mined Repositories – Clay:

A repository mined out of a layer of clay or clay-like materials may be an effective approach to isolating and containing HLW and SNF because the rock has three important properties.  First, water typically moves very slowly through clay. Second, clay can have a high sorption capacity for radionuclides. Third, any fissures or fracture planes in the rock close by themselves over the course of time. Three countries are actively investigating the possibility of developing a deep-mined geologic repository in clay formations found within their borders: Belgium (boom clay); France (argillite); and Switzerland (opalinus clay).  Although some important differences exist among the three countries’ disposal concepts and the types of waste they will dispose of, the similarities in the disposal concepts are more substantial. For the purposes of the discussion below, Switzerland’s national program is described.Slide5The absence of significant advective groundwater flow in the clay means that radionuclides would move out of the engineered barriers and the undisturbed rock at a very slow rate.  Any such movement would thus be controlled by diffusion, which suggests that only the most mobile and longest-lived radionuclides can reach the edge of the clay formation.  Rock units surrounding the formation where a repository might be built, which also are rich in clay, would further slow the release of radionuclides to the biosphere.  Chemical conditions in clay would be reducing, thereby maintaining the constituents of SNF in a low-solubility state.

Engineered barriers would be designed to work with the natural ones. The waste package would be constructed from steel and expected to prevent the inflow of water for several thousands of years. When the packages start to corrode, the resulting corrosion products might hydrolyze to create more acidic and aggressive near-field environments. The packages would be emplaced horizontally, and the drifts would be backfilled with bentonite. As with the crystalline rock disposal concept, the bentonite would retard the radionuclides and ensure that their transport is only by diffusion.

A separate but co-located, pilot facility would be constructed after a site for a deep-mined geologic repository is selected. Representative volumes of waste would be disposed of in this facility. Monitoring would take place to validate long-term predictions of how the host rock is evolving as well as to identify possible early indications of safety barrier failures.

Like the German waste-management program, the Swiss program is intended to demonstrate that there will be zero releases for at least a million years if the host rock is undisturbed.  Only if there is significant climate change, borehole penetration of the repository, or deep groundwater extraction at the site would there be any release to the biosphere. Work to enhance the technical basis for the clay safety case continues, especially for repository performance under disturbed conditions. The following are some of the key uncertainties being explored:

  • Solubility limits and sorption coefficients;
  • Rate at which the clay is re-saturated;
  • Impact of heat on the performance of the bentonite buffer; and
  • Gas generation by steel canister corrosion.

In creating its safety case, the waste-management program in Switzerland explicitly develops multiple lines of argument, including the use of alternative indicators that are complementary to those of dose and risk; natural analogues; and conservative performance assessments. Preliminary safety assessments using both deterministic and probabilistic methodologies have been carried out. Based on those assessments, the Swiss government has accepted the safety case advanced by NAGRA.  The safety case also has been peer reviewed by an international team assembled by the Nuclear Energy Agency (NEA).  The safety case also forms the basis for the Sectorial Plan, which currently guides efforts to identify suitable sites.

2.4       The Concept of Deep-Mined Repositories – Unsaturated Volcanic Tuff:

The United States is the only country that has developed a safety case for disposing of HLW and SNF in a deep-mined geologic repository located in unsaturated volcanic tuff.  That safety case was developed by Department of Energy (DOE) in parallel with the characterization of a specific site located at Yucca Mountain in Nevada.  The safety case rests on two main pillars:

  • Engineered barriers minimize the amount of water that can come in contact with the HLW and SNF; and
  • Transport of radionuclides to the biosphere is limited by the amount of water leaving the drifts.

Very little precipitation falls on Yucca Mountain.  A large fraction of what does returns to the atmosphere by evaporation, plant transpiration, and run-off; only a small amount of water infiltrates below the root zone, and even less seeps into the repository drifts.  The location of the proposed repository lies in the unsaturated zone, where the environment is oxidizing, so the constituents of SNF would react with oxygen and become more mobile. To limit the release of radionuclides, corrosion-resistant titanium drip shields would be installed to divert the water that enters the drifts, thereby protecting the waste packages that lie underneath. The packages themselves would be fabricated with an outer layer of a nickel-based material, Alloy 22, and an inner layer of stainless steel. The packages would degrade very slowly in repository environments because of this corrosion-resistant alloy.  Any radionuclides that escaped would move slowly because significant advective transport is unlikely.Slide6Relying on this disposal concept, DOE issued a final environmental impact statement.  That assessment was one of the reasons that Congress approved the selection of the Yucca Mountain site.  In June 2008, DOE submitted a license application to National Regulatory Commission (NRC) based on a Total System Performance Assessment, which is grounded on more than 30 years of site-specific scientific and technical investigations.

Nonetheless, questions remain about the safety case for unsaturated volcanic tuff.  Nearly 300 issues have been raised by supporters and opponents participating in the licensing hearing convened by the NRC. Moreover, the Nuclear Waste Technical Review Board (NWTRB), which is not a party to those hearings, has noted that there is only a poor understanding of how fast water moves in the unsaturated zone.  It also has suggested that deliquescence-induced localized corrosion could lead to more-rapid waste package degradation than DOE maintains.  These issues might eventually be resolved in the course of that hearing process. For the moment, at least, that hearing process has been suspended.

2.5               Foundations for Safety Case:

Here is a table which summarized the foundations of safety cases, highlighting the respective countries and the host rock for each foundation:Slide7

Resources:

  1. International Panel on Fissile Materials;
  2. International Perspective on the Reprocessing, Storage, and Disposal of Spent Nuclear Fuel; and
  3. Experienced Gained from Programs to Manage High-Level Radioactive Waste and Spent Nuclear Fuel in the United States and other Countries.

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