Over time, radioactive objects lose their ability to emit radiation. This process is known as decay. The time it takes for half of the atoms of a given radioactive element to decay into an inert element is called its half-life. The half-life of uranium-235, which is used to fuel nuclear fission reactions, is more than 700 million years. Waste produced in nuclear power generation, then, must be stored for vast periods of time in a contained environment.

Once a year, about one-third of the fuel inside a nuclear reactor is removed and replaced with fresh fuel. Spent fuel is highly radioactive. Its potential radiation dosage amounts to millions of rems per hour. (A rem is a unit of dose that quantifies how efficient radiation is.) Considering that a lethal dose for humans is just 400 rems, the spent fuel must be treated with extreme care, as it is capable of doing great harm not only to people but also to the environment.

When spent fuel rods are removed from a reactor, they are temporarily placed to cool in a deep, purified pool of water near the reactor. Water helps absorb some of the harmful gamma radiation given off by the rods. Spent fuel rods are supposed to stay in the pool for only about six months, but lacking a permanent storage site, they often stay there for years. After cooling several years, spent fuel can be moved from the pools into sealed metal cylinders called dry casks. These steel- or concrete-encased containers are designed to provide safe, above-ground storage over a period of at least five years, even in such extreme conditions as flooding, tornadoes, missile strikes, and prolonged hot or cold.

As pools and casks at sites throughout the world fill up, it is becoming more and more critical to find safe, permanent storage for the spent fuel rods. Because reprocessing is not presently an option in the U.S. -- it is more expensive than making new fuel and creates very hazardous radioactive and chemical wastes besides -- many believe a long-term repository must be designed and built to protect public and environmental health and safety.

A permanent storage site in the U.S. has been selected at Yucca Mountain, on uninhabited federal land located 100 miles northwest of Las Vegas. The plans call for geologic disposal, or placing radioactive materials in tunnels deep underground, inside double-layered, corrosion-resistant containers. A series of natural and manmade barriers would further help prevent or slow any possible leaks that could contaminate the surrounding area. Tests show that a scenario in which surface water reached stored radioactive waste would take longer than the 10,000-year federally mandated period over which waste must be kept safely away from human populations. Even so, most nearby residents oppose the planned repository, because no one can guarantee that waste will not leak.