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Depleted uranium, a byproduct of the uranium enrichment process, is initially less radioactive than naturally occurring uranium. However, the radioactivity of depleted uranium increases rather than decreases over time, posing significant challenges for safe and effective long-term disposal.
Natural uranium is composed of three primary isotopes: U-234, U-235, and U-238. Nuclear reactors use a higher concentration of fissionable uranium-235 (U-235) than occurs naturally, using the enrichment process to increase the proportion of U-235 available to create a nuclear reaction. The material left from the enrichment process is “depleted,” meaning it has proportionately less U-235 and proportionately more U-238.
The enrichment process produces small quantities of enriched uranium and large quantities of depleted uranium hexafluoride (DUF6), an unstable compound. To stabilize this depleted uranium, deconversion facilities chemically extract fluoride and replace it with oxygen. This produces uranium oxide, a chemically stable compound. Deconversion facilities place this uranium oxide into steel cylinders for long-term disposal.
Radioactivity Increases with Time
Unlike other low-level radioactive waste (LLRW), depleted uranium retains its radioactivity for a very long time. Uranium decays very slowly, with a half-life in the range of millions of years. The decay products of uranium become more radioactive over time due to ingrowth. This ingrowth occurs when “parent” uranium isotopes decay to produce “daughter” isotopes. If these daughter isotopes are unstable, they decay as well, producing even more daughters. As these daughter products grow, the total radioactivity from the uranium and its daughters increase. Radioactivity peaks after about one to two million years, at which point the daughter products decay as fast as they are generated, resulting in secular equilibrium. After billions of years, U238and its daughters decay to a stable form of lead.
Cylinders containing uranium oxide pose minimal risks to workers and the public because the radiation levels of DU are very low. Uranium is a low-toxicity alpha emitter—alpha rays can be stopped by a piece of paper—so radiation from DU cannot pass through these containers. Even the radiation from direct contact with DU is extremely low.
DU, however, poses a radiation hazard if it is released into the environment and inhaled, ingested, or pierces the skin. Inhalation can trap tiny particles in the lungs, and ingestion has been shown to cause kidney damage. Health impacts from exposure to DU are dependent upon the physical and chemical nature of the DU as well as the duration and level of exposure. Because DU creates radon gas and becomes more radioactive over time, scientists predict that these health impacts will grow along with the daughter ingrowth, although it will be tens of thousands of years before the health impacts, usually an increased risk of cancer, become considerably more severe.
Nuclear Regulatory Commission (NRC) rulemaking on low-level radioactive waste (LLRW) in the early 1980’s did not anticipate the dramatic increase in uranium enrichment activities by the private sector or the large quantities of depleted uranium that would be produced by these commercial activities. The NRC assumed DU, and other long-lived nuclides, would be found in only trace levels in LLRW. There were no commercial uranium enrichment facilities producing large quantities of depleted uranium at the time.
A 2008 NRC technical analysis concluded that the safe disposal of depleted uranium was dependent on the geological, hydrological, and climate characteristics of the proposed site. The NRC recommended site-specific performance assessments for depleted uranium to evaluate the waste stream, update and revise assumptions on the behavior of depleted uranium over time, and assess whether disposal met public health and safety requirements
In 2006 and 2007, the NRC issued licenses to commercial uranium enrichment facilities in Paducah, Kentucky and Portsmouth, Ohio. The Department of Energy (DOE) constructed two depleted uranium deconversion facilities next to these uranium enrichment plants to process the DU left from enrichment. Depleted uranium processing from these two facilities converts the DU to a more stable and safer form for storage and disposal.
Deep time is the time scale of geologic events on a scale beyond most human events or even human comprehension. Measured in tens to hundreds of thousands—even millions--of years, deep time takes into account the massive geologic changes that take place over these vast time spans. Because depleted uranium reaches its peak dose at approximately two million years, modeling scenarios for DU disposal need to take into account projected geologic changes over deep time, including 100,000 year global glacial cycles and the formation of pluvial lakes in proposed disposal sites.