"Dear Arjun"

Dear Flummoxed,

In 15th century Scotland, DUF₆ was not dangerous and was known as "duff," another way of saying dough, the paste made from flour. Back then women were making the dough. Unfortunately men, who always tended to meddle in women's affairs, took over the making of the bread dough and turned it into green dough. As they attempted to make greater amounts of green dough, the men added more and more flour, changing the spelling of the name from "duff" to "dufff" and eventually to "duffffff" to reflect the immense flour content. This was eventually shortened to "DUF₆."

The nuclear establishment has given an entirely new meaning to DUF₆. Today it stands for Depleted Uranium Hexafluoride, the by-product of uranium enrichment, and the chemical form of most depleted uranium. Depleted uranium (DU) is also stored in other chemical forms, such as metal and oxide. (See table.)

Natural uranium is composed of three isotopes: uranium-238 (99.284 percent); uranium-235 (0.711 percent); and, uranium-234 (0.005 percent), all of which are radioactive. The purpose of uranium enrichment is to concentrate uranium-235, the fissile isotope, in one stream. The other stream which is low in uranium-235, is called depleted uranium (DU), which typically contains only 0.2 to 0.3 percent uranium-235.

The enriched uranium is then further processed to varying degrees of enrichment. Uranium with between 3 and 5 percent uranium-235 (Low Enriched Uranium or LEU) is used as nuclear fuel for commercial nuclear power plants. An enrichment over 93.5 percent uranium-235 (Highly Enriched Uranium or HEU) can be used as material for nuclear weapons. In the U.S. is it also used in naval reactors. About 180 kilograms (kg) of depleted uranium result from the production of 1 kg of HEU with 93.5 percent uranium-235. Five to 10 kilograms of depleted uranium result from the production of 1 kg of LEU, depending on the degree of enrichment. Enrichment plants generally require uranium to be converted into the hexafluoride chemical form for processing reasons.

Storage of DUF₆ and Environmental, Health and Safety Hazards

Currently there are about 560,000 metric tons of DUF₆ stored primarily in 14-ton cylinders located near Portsmouth, Ohio; Oak Ridge, Tennessee; and Paducah, Kentucky. The long-term storage of DUF₆ presents environmental, health and safety hazards due to the chemical instability of UF₆. When UF₆ is exposed to moist air, it reacts with the water in the air to produce UO2F2 (uranyl fluoride) and HF (hydrogen fluoride) both of which are toxic. Storage cylinders must be regularly inspected for evidence of corrosion and leakage. Continuing to store depleted uranium in cylinders would require constant maintenance and monitoring of the stockpile because the estimated life-time of the cylinders is measured in decades, while the half-life of the main constituent of DU, uranium-238 is about 4.5 billion years.

Classification of Depleted Uranium

Currently, depleted uranium is still classified as a source material although its possible uses are few and the quantities involved are small. The major uses of depleted uranium ( to produce armor-piercing shells and armor plating for tanks -- are likely to be phased out due to concerns about its radioactivity and heavy metal toxicity. Hence, DU is essentially a radioactive waste, though it has not been declared as such. The Department of Energy (DOE) has begun a process for considering how DU ought to be managed and how it should be disposed of if it is declared a waste.

In its consideration of a license application for a new uranium enrichment plant in Louisiana, the Nuclear Regulatory Commission (NRC), declared that DU from the plant would be considered "Class A" "low-level" radioactive waste. "Class A" is the category for the least dangerous "low-level" radioactive waste. The NRC made this declaration under the default provision for unclassified wastes in the Code of Federal Regulations 10 CFR 61.55. This classification is fundamentally flawed and potentially dangerous.

The NRC's own research demonstrates why this default classification is wrong. In a 1994 report, it determined that shallow-land burial, the usual means for disposing of Class A low-level radioactive waste, would be inappropriate for DU because it could result in unacceptably high doses in the future.¹

A sound disposal program for managing DU as waste needs to be based on the properties of depleted uranium, not a flawed and arbitrary classification system.

Properties of Depleted Uranium

Health and environmental effects of radioactive materials are affected by several factors: the specific activity of the radioactive material (the radioactivity per unit weight); the nature of the radiation being emitted during the radioactive decay (alpha or beta, and whether the decay is accompanied by gamma radiation); the energy per radioactive decay, the half-life; and the behavior of the specific radionuclide and its various chemical forms in the body. As illustrated in Tables 1 and 2, depleted uranium is the same as transuranic waste (TRU waste) in all essential respects that matter to health and the environment.² The difference is terminologically not substantive.

Table 1 illustrates that the specific activity (here, radioactivity per gram) of depleted uranium in any form is 2.7 to 4 times more than the minimum specific activity of transuranic waste.