Nuclear power plants produce two types of radioactive waste that must be disposed of (see Chapter 9). Low-level nuclear waste includes any item that has become contaminated with small amounts of radioactive particles, including instruments, protective suits, or clothing from nuclear facilities. High-level nuclear waste is radioactive waste consisting primarily of nuclear fuel rods that have been depleted to the point that they can no longer contribute to efficient production of electricity. These fuel rods still contain uranium in addition to several other waste products of nuclear fission.

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Over very long periods of time, radioactive waste will completely decay and become harmless. However, most radioactive isotopes take so long to decompose that the timeframe is essentially meaningless in terms of human timescales. Meanwhile, tiny particles and rays of energy are released during active decay, which can damage living tissue, including the DNA blueprint found within the nucleus of all organisms. Safety requires that nuclear wastes be disposed of properly so that they do not contaminate the environment and endanger humans and other organisms.

Developing a permanent repository for high-level nuclear waste requires addressing complex technical issues. To prevent radiation from escaping, nuclear waste must be securely stored within several feet of steel and concrete. In the short term, this is a relatively simple task. What makes storing nuclear waste so difficult is the great length of time needed before some unstable elements decay into harmless ones.

Radioactive isotopes all decay, but at different rates. The time needed for half of a given amount of a radioactive isotope to decay is known as its half-life. While some nuclear fission waste products decay quickly, in a matter of hours or days, others require thousands of years. The half-life of plutonium-239, for example, is approximately 24,000 years. The durability of even the oldest known human structures, such as the 4,500-year-old pyramids at Giza, pales in comparison. Finding a suitable storage structure and location to accommodate these slow rates of radioactive decay presents a tremendous technical challenge. For the time being, most of the high-level nuclear waste in the United States is temporarily stored in concrete and steel towers called dry casks on the site of the reactors themselves, as the country searches for a long-term solution. These casks, while stable, cannot realistically be expected to last for the full length of time needed for the radioactive isotopes to decay to safe levels (Figure 12.16).

One of the biggest challenges to creating a long-term solution for nuclear waste is that no community wants it in their backyard, due to fears of radioactive contamination. The United States has been successful in building and operating a repository for defense-related radioactive wastes at the Waste Isolation Pilot Plant in southern New Mexico. The proposed facility for storing high-level waste from civilian nuclear reactors is located at Yucca Mountain, Nevada, where nuclear wastes would be stored approximately 300 meters below ground and a similar distance above the level of groundwater (Figure 12.17). However, plans for the facility were halted in 2009, a year before it was to begin storing waste (see Chapter 9). The main reason for the cancellation was political opposition by local communities, who didn’t want nuclear waste transported through their towns.

There were also questions concerning the suitability of the Yucca Mountain geology. Fractures in the geologic formation could provide an avenue for radioactive materials to contaminate groundwater below the storage facility. This would be particularly likely if the regional climate became wetter, which is possible during the million-year time frame that the facility is supposed to remain secure. Despite these concerns, in 2015, the U.S. Congress renewed efforts to open the Yucca Mountain repository, but whether the proposal will be approved remains uncertain. Meanwhile, the country remains without a site for disposing of high-level nuclear waste.

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