In any nuclear power plant, heavy metal atoms are consumed as the fuel “burns.” Even though the plants begin with fuel that has had its uranium 235 content enriched, most of that easily fissioned uranium is gone after about three years. When technicians remove the depleted fuel, only about one twentieth of the potentially fissionable atoms in it (uranium 235, plutonium and uranium 238) have been used up, so the so-called spent fuel still contains about 95 percent of its original energy. In addition, only about one tenth of the mined uranium ore is converted into fuel in the enrichment process (during which the concentration of uranium 235 is increased considerably), so less than a hundredth of the ore’s total energy content is used to generate power in today’s plants.
This fact means that the used fuel from current thermal reactors still has the potential to stoke many a nuclear fire. Because the world’s uranium supply is finite and the continued growth in the numbers of thermal reactors could exhaust the available low-cost uranium reserves in a few decades, it makes little sense to discard this spent fuel or the “tailings” left over from the enrichment process.
The spent fuel consists of three classes of materials. The fission products, which make up about 5 percent of the used fuel, are the true wastes—the ashes, if you will, of the fission fire. They comprise a mélange of lighter elements created when the heavy atoms split. The mix is highly radioactive for its first several years. After a decade or so, the activity is dominated by two isotopes, cesium 137 and strontium 90. Both are soluble in water, so they must be contained very securely. In around three centuries, those isotopes’ radioactivity declines by a factor of 1,000, by which point they have become virtually harmless.
Uranium makes up the bulk of the spent nuclear fuel (around 94 percent); this is unfissioned uranium that has lost most of its uranium 235 and resembles natural uranium (which is just 0.71 percent fissile uranium 235). This component is only mildly radioactive and, if separated from the fission products and the rest of the material in the spent fuel, could readily be stored safely for future use in lightly protected facilities.
The balance of the material—the truly troubling part—is the transuranic component, elements heavier than uranium. This part of the fuel is mainly a blend of plutonium isotopes, with a significant presence of americium. Although the transuranic elements make up only about 1 percent of the spent fuel, they constitute the main source of today’s nuclear waste problem. The half-lives (the period in which radioactivity halves) of these atoms range up to tens of thousands of years, a feature that led U.S. government regulators to require that the planned high-level nuclear waste repository at Yucca Mountain in Nevada isolate spent fuel for over 10,000 years.
An Outdated Strategy
Early nuclear engineers expected that the plutonium in the spent fuel of thermal reactors would be removed and then used in fast-neutron reactors, called fast breeders because they were designed to produce more plutonium than they consume. Nuclear power pioneers also envisioned an energy economy that would involve open commerce in plutonium. Plutonium can be used to make bombs, however. As nuclear technology spread beyond the major superpowers, this potential application led to worries over uncontrolled proliferation of atomic weapons to other states or even to terrorist groups.
The Nuclear Non-Proliferation Treaty partially addressed that problem in 1968. States that desired the benefits of nuclear power technology could sign the treaty and promise not to acquire nuclear weapons, whereupon the weapons- holding nations agreed to assist the others with peaceful applications. Although a cadre of international inspectors has since monitored member adherence to the treaty, the effectiveness of that international agreement has been spotty because it lacks effective authority and enforcement means.