Nuclear waste disposal

Because of the dangers nuclear waste poses to human and environmental health, disposal of this hazard is an important consideration. It is also problematic, as the threat of radioactivity can persist for extremely long periods of time. Nuclear waste disposal has taken several forms, including above-ground storage, shallow burial, and ocean dumping. Alternative methods have been proposed, including unconventional plans such as ejection into outer space or dropping into subduction zones. However, storage in a geologic repository is considered the safest and surest way to achieve isolation of the wastes from the surface environment. There is no assurance a repository will last forever, but its combined geologic characteristics can minimize damage in the event of failure.

High- Versus Low-Level Wastes

Nuclear waste disposal is a necessary evil in a world where radioactive isotopes and the energy produced by their decay are used, among other things, for generating electricity, diagnosing, and treating diseases, and making nuclear weapons. Nuclear wastes are not all the same. Because of their composition, some of these materials are more dangerous than others and for longer periods of time. When evaluating a geologic site for disposal of nuclear wastes, the characteristics of the site must be identified so that the waste will be isolated from the earth surface environment for a minimally acceptable period of time.

Ensuring the minimal length of isolation is most important for those wastes that are dangerous for a long time and those that contain large concentrations of dangerous isotopes. Such wastes, usually called high-level wastes, consist of isotopes of uranium and plutonium produced in the use and processing of nuclear fuel. The length of isolation is not as important for those wastes that are dangerous for shorter times and less concentrated. These wastes are termed low-level wastes and may include some long-lasting wastes but in lower concentrations. An example of a low-level waste is slightly contaminated garbage, such as disposable laboratory equipment, disposable medical equipment, and disposable gloves and overalls (used in the handling of radioactive materials). The length of time that a particular waste is dangerous depends on which radioactive isotopes are in the waste and on the half-life of each isotope. The half-life is the time required for half of the radioactive isotopes in a sample to decay.

During the natural process of radioactive decay, the nucleus of a radioactive isotope (the “parent”) is changed into that of another isotope (the “daughter”), and energy is released. This radioactive decay from parent to daughter is usually one step in a series of many, as the original parent isotope changes into a nonradioactive, stable isotope. At each step, energy is released in the form of energetic particles or energetic rays. Thus, in a sample of uranium, the parent uranium isotopes are constantly decaying in a series of steps through various daughter isotopes until the sample consists of pure lead, the ultimate daughter isotope in the uranium decay series. It takes a long time for all the uranium to decay completely to lead. The half-life of one uranium isotope, uranium-238, is 4.5 billion years. The long half-life of uranium suggests that the half-lives for some of the daughter isotopes in the decay series are also long.

Isolation in Geologic Repository

Exposure to radiation is never without risk. The problem with a long half-life is that the isotope is giving off energetic particles or rays that are dangerous to plants and animals for an extended period. Thus, high-level wastes, where the isotopes are in concentrated form, must be isolated from the surface environment for a long time. Low-level wastes are less dangerous only because the concentrations of radioactive isotopes are lower. While they need to be isolated from the surface environment, they are not as destructive to life as high-level wastes.

Plants and animals are exposed to radiation through air, water, and food. Naturally occurring radioactive isotopes may be in the air (in the form of radon, for example) and taken into the lungs. Some radioactive isotopes are dissolved in the water we drink (potassium-40) and in the food we eat (carbon-14). To minimize excessive exposure to radiation, a geologic repository must isolate radioactive waste from the surface environment. A geologic repository is any structure in either rock or soil that uses, in part, the natural abilities of these materials to isolate the wastes from the surface environment. A geologic repository may be a shallow trench dug in the soil, partially filled with low-level waste and covered with the excavated soil, or it may be a cavern constructed deep underground in hard rock. Regardless, the purpose of the repository is to isolate the waste in such a way that there is no excess exposure of organisms to radiation as a result of the presence of the repository. In this sense, isolation means that the waste must be kept from contaminating the surrounding air, water, and food.

Characteristics of Repository

The rocks or soil of a geologic repository must have certain characteristics to isolate wastes properly. Some of the more important characteristics are porosity, permeability, mineral solubility, and sorption capacity. Porosity and permeability are related but are not the same. Porosity is a measure of the ability of a rock or soil to hold a fluid, either liquid or gas. Expressed as a percentage of the volume of the sample, porosity is the volume of open, or pore, space in the rock or soil. If the soil or rocks of a geologic repository have high porosity, then large amounts of water or air may contact the waste; however, contact with the waste does not necessarily mean that the contaminated water or air will reach the surface environment. If the rock or soil is also permeable—that is, if the pores are interconnected enough so that the water or air can flow from the repository to the surface—the repository is not likely to provide adequate isolation. Porosity can exist without permeability when many small pores contain a large volume of fluid, but the connections between pores are too small to allow fluid flow. Permeability can exist with small porosity when a few penetrating cracks in an otherwise solid rock allow easy fluid flow.

Mineral solubility is an important characteristic in cases where water may contact the waste. Soluble minerals—those that tend to dissolve in water—may be removed from the surrounding rock or soil by moving water. When minerals dissolve, a space is left behind that adds to the porosity and may increase the permeability of the repository.

The sorption capacity of a mineral is the ability of that mineral to remove a dissolved molecule or ion from a fluid. Sorption may occur by either chemical or physical means. A type of chemical sorption is the ion exchange that takes place in a home water-softening unit. Water containing problem ions (in the case of a repository, radioactive ions) flows over the exchanging minerals. The problem ions attach to the mineral and, in the process, force the ion that was attached originally into the solution. In this way, the water is cleansed of problem ions. In physical sorption, the water and the contaminant are attached to the mineral by the force of friction, which results in a thin layer of water attached to the mineral surface. The attached, or physically sorbed, water and any material dissolved in it are not moving.

All these characteristics of rock and soil are related in some way to the possibility that the waste will escape from the repository and be transported by a fluid to the surface. It is unlikely that any repository will provide isolation perfectly when all the characteristics are taken into consideration. Some characteristics, however, may be more important than others, and a weakness in one may be offset by a strength in another.

Description of Repository Material

Shallow burial of low-level wastes and deep burial of high-level wastes are preceded by a careful description of the strengths and weaknesses of the geologic material of the repository. In the case of shallow burial, the repository material is usually soil or saprolite (near-surface rock that has been partially turned to soil by the actions of water). To determine the strengths and weaknesses of a particular soil containing low-level waste, scientists must study the structure, hydrology, mineralogy, and chemistry of that soil.

As water from rain and snow permeates the earth, it does not flow uniformly, but flows more easily through certain permeable paths, or preferred paths, and in isolated areas. The structure may be inherited from characteristics of the original rock that broke down into soil, such as fractures and other cracks. Structures inherited from the parent rock are especially evident in saprolite, which exists in a stage between rock and true soil.

Other structures are the products of soil processes. Such structures include the cracks developed when soil dries out, the tunnels formed by burrowing organisms such as worms and ants, and the openings left when the roots of dead plants decay. All these structures are important because they determine the manner and speed at which water may flow.

The means and speed of water flow through soil are important aspects of its hydrology. The flow of water into a shallow burial trench is usually restricted by engineered, low-permeability barriers such as compacted clay or plastic liners. If the liners fail and water flows through and out of the wastes, the surrounding soil should slow the flow of contaminants in two ways: physically and chemically. Physically impeding, or retarding, the flow of contaminated water from the waste trench means that the soil structure does not provide permeable flow paths; the water is forced to flow through many small, interconnected pores. As a result, the waste-transporting water contacts more of the mineral material in the earth. Chemical retardation of wastes occurs when the contaminants in the slowly moving fluid interact with the minerals of the soil. Some of those minerals have the capacity to sorb contaminants, further slowing their migration. Other reactions between the minerals and the wastewater result in the chemistry of the water changing in such a way that the contaminants may become insoluble and form new solids in the soil. The effect is the same: The flow of contaminants from the burial trench is slowed or stopped.

Stability of Repository

Disposal of high-level wastes in a geologic repository requires that many of the same characteristics of the host rock be determined. In addition to having the appropriate hydrological, mineralogical, and chemical characteristics, the host rock is required to be reasonably stable for ten thousand years. For example, the modern hydrological characteristics of the proposed Yucca Mountain repository site in Nevada are ideal. The proposed repository would be located below the desert surface, out of the reach of downward-percolating rainwater and well above the nearest fresh groundwater. In the desert environment, rainfall is so infrequent that it cannot reach the repository from above, nor can the infrequent rainfall raise the groundwater level to flood the repository. Construction of 8 kilometers of test tunnels in Yucca Mountain began in 1994, and evaluation of the site was to be completed by 2001, with construction to begin in 2005 and emplacement of high-level waste to start in 2010. However, controversy led to political delays, and in 2010 the administration of President Barack Obama planned to cancel the project, and in 2021, the Biden Administration noted that the Yucca Mountain plan was not being pursued.

The problem is that climates change. For example, the desert Southwest of the United States previously experienced much more precipitation. Thus, climates will undoubtedly change in the future. In addition, a repository may be disturbed, whether inadvertently or purposely, by human activities. The most likely inadvertent disturbance is of future generations, who, in a search for mineral deposits, might puncture the repository with drilling equipment. To avoid this scenario, a repository should be located in an area unlikely to yield mineral wealth. Deeply buried layers of rock salt, thought to be ideal repositories because of their stability and their resistance to water flow, are no longer being considered. Many of the rock salt deposits are already being mined for salt, and future generations may mine the remaining ones. In addition, some of these deposits are associated with oil and natural gas. Although these considerations make salt beds unsuitable for storing high-level waste, some are suitable for storing low-level waste.

The federal Low-Level Radioactive Waste Policy Act of 1980 delegates the responsibility for the disposal of low-level waste to the state in which that waste is produced. The Waste Isolation Pilot Plant (WIPP) was constructed 42 kilometers east of Carlsbad in the southeastern corner of New Mexico in order to develop and prove the required technology. By law, WIPP can only accept low-level waste from the nation’s nuclear weapons program; the first shipment arrived and was placed in storage during 1999. The waste is stored 650 meters below the surface in rooms excavated in an ancient, stable salt formation.

As part of its search for a means to dispose of nuclear waste, as prescribed by the Nuclear Waste Policy Act of 1982, the Department of Energy explored the possibility of storing waste in crystalline rocks—generally granite intrusions—in the eastern United States. Storage in such sites would have the advantage of shorter transport distance and geological stability. A provision in the act that allowed states and American Indian tribes to veto the construction of a site doomed the project from the outset, although a veto could be overridden by Congress. In the face of intense public opposition, the project was abandoned.

Political Considerations

Nuclear waste disposal, whether of low-level or high-level wastes, is a problem that will persist as more waste is created every day. A geologic repository and its host rock or soil must safely isolate the wastes from the surface environment for a specified period of time. Should the repository and host material fail, plants and animals will be exposed to radioactive elements. The risk of exposure to radiation can never be eliminated, but careful design of the repository and careful determination of the host material’s strengths and weaknesses can result in a disposal site that minimizes the risk.

Ultimately, the location of a nuclear waste disposal facility is a political decision that must be based on solid technical information and judgments. As nuclear wastes accumulate in temporary holding facilities (for example, spent fuel elements from nuclear reactor power generators are stored in large water-filled pools near the reactor), political pressures build to solve the disposal problem. An informed public, while pressing for a solution, will understand delays necessitated by the need to gather and interpret the data. It must also be understood that risk can be minimized but never eliminated.

As of the late 2010s, the long-term storage plan that had progressed most successfully was the Onkalo geological repository in Finland. The Finnish government had begun searching for a suitable site in 1983, and, in 2000, ultimately chose an area near the Olkiluoto nuclear power plant out of one hundred options due to its favorable geologic conditions. In 2019, Finland had about 263,000 tons of waste, and by 2022, the site had experienced few issues. However, experts noted that even repositories that are successful in terms of geologic stability face the challenge of communicating their purpose and the danger of the stored waste to humans who could potentially come across the site thousands or even tens of thousands of years into the future.

Principal Terms

half-life: the time required for half of the radioactive isotopes in a sample to decay

high-level wastes: wastes containing large amounts of dangerous radioactivity

isotopes: atoms of the same element that differ in the number of uncharged neutrons in their nuclei

leachate: water that has come into contact with waste and, as a result, is transporting some of the water-soluble parts of the waste

low-level wastes: wastes that are much less radioactive than high-level wastes and thus less likely to cause harm

permeability: a measure of the ease of flow of a fluid through a porous rock or sediment

porosity: a measure of the amount of open spaces capable of holding water or air in a rock or sediment

radioactivity: the spontaneous release of energy accompanying the decay of a nucleus

solubility: the tendency for a solid to dissolve

sorption: the process of removing a chemical from a fluid by either physical or chemical means

transuranic: an isotope of an element that is heavier than uranium and formed in the processing and use of nuclear fuel and plutonium

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