Comments of the Institute for Energy and Environmental Research on the Department of Energy Notice of Intent addressing the Alternative Strategies for the Long-Term Management and Use of Depleted Uranium Hexafluoride Federal Register, Thursday, January 25, 1996
By: Annie Makhijani and Arjun Makhijani
22 March 1996

The Department of Energy has announced its intent to prepare a programmatic environmental impact statement addressing the use and/or disposal of the 560,000 tonnes of depleted uranium hexafluoride currently stored in cylinders at three sites located near Portsmouth, Ohio; Oak Ridge, Tennessee; and Paducah, Kentucky.

The DOE plans to consider a set of six preliminary alternatives:

  • The “no action” alternative which would be a continuation of the current management program, that is storage on site in cylinders of the depleted uranium hexafluoride.
  • Two storage alternatives based on retrievability
    • in the UF6 form
    • in the oxide form
  • Two use alternatives
    • radiation shielding after conversion to metal
    • radiation shielding after conversion to oxide
  • Disposal as low-level waste in the oxide form in drums placed in
    • engineered trenches
    • below-ground concrete vaults
    • mines

IEER welcomes the DOE’s effort to address the long-term management of the country’s depleted uranium hexafluoride. However the DOE list of alternatives does not include all reasonable alternatives. Specifically DOE should add another option to the above mentioned list. This option would be the disposal of depleted uranium according to the rules of 40 CFR 191 which govern the disposal of transuranic (TRU) wastes.

Currently, uranium is still classified as a source material. In cases where it might be disposed of as a waste the Nuclear Regulatory Commission has put it by default into the category of class A low-level radioactive waste according to 10 CFR 61.55 (6). 10 CFR 61.55 allows near surface disposal. The inappropriateness of this default classification is demonstrated by the NRC’s own assessment that shallow-land burial of depleted uranium could result in unacceptably high doses to future generations. However in its notice of intent, the DOE has left the low-level category of depleted uranium ambiguous. In the three scenarios of the disposal alternative, the DOE does not specify in which low-level waste class it puts depleted uranium. The first one – disposal in engineered trenches – corresponds to a classification of depleted uranium as class A low-level radioactive waste. The last two – disposal below-ground in concrete vaults and disposal in mines – imply that the current classification is inadequate. DOE should clearly include waste
classification as an issue as it relates to depleted uranium in its implementation plan and address it in detail in the PEIS.

We set forth below the scientific reasons why depleted uranium should be put in the same classification as transuranic wastes for the purpose of waste management and disposal. This would mean that depleted uranium would have to be placed in a deep geological repository.

The current definition of TRU waste according to 40 CFR 191.01 (i) is: “. . .waste containing more than 100 nanocuries of alpha-emitting transuranic isotopes, with half-lives greater than twenty years, per gram of waste . . . .”

What matters to health and environmental considerations is the specific activity of the radioactive wastes, the nature of the radiation being emitted during the radioactive decay (alpha or beta and whether the decay is accompanied by gamma radiation) and, the energy per radioactive decay. Depleted uranium is, in these essential respects, the same as the transuranic constituents of TRU waste. The specific ways in which uranium or the transuranic radionuclides in TRU waste might affect people will, of course, depend on the chemical form of the waste, the packaging, and the disposal method.

There is one nominal difference between TRU waste and depleted uranium. TRU waste consists of elements with atomic numbers greater than or equal to 93 — that is of elements with atomic numbers greater than uranium, whose atomic number is 92. But this is a difference of nomenclature; it has no bearing upon health and environmental issues.

A. Properties of Depleted Uranium

1. Specific Activity

The radioactivity per unit weight (called specific activity) of depleted uranium metal is dominated by its principal constituent, uranium-238. It also depends somewhat on the exact extent to which uranium-235, and hence also uranium-234, have been separated and passed into the enriched uranium stream. It may vary from about 360 nanocuries/gram to about 450 nanocuries/gram. Even assuming that only uranium-238 remains, the specific
activity would be still about 340 nanocuries/gram which is 3.4 times higher than that defining transuranic waste.

The specific activity of other chemical forms is somewhat lower than for uranium metal, because when radioactive uranium is chemically bound with non-radioactive isotopes of elements like oxygen and fluorine, its specific activity is correspondingly lower. Table 1 shows the specific activity of four forms of depleted uranium. For convenience we have assumed a single reference value of 360 nanocuries/gram for the specific activity of uranium metal which is the lowest practical value in the range cited above.

Table 1 also shows, for reference, the minimum specific activity of transuranic waste as defined by regulations and the radioactivity of ore containing 0.2 percent uranium.

Table 1: Specific Activities of Various Chemical Forms of Depleted Uranium, TRU Waste and 0.2% Uranium Ore
Chemical form Specific activity, nCi/g
uranium metal (U) 360
uranium oxide (U3O8) 300
uranium tetrafluoride (UF4) 270
uranium hexafluoride (UF6) 240
transuranic activity in TRU waste 100 (see note 2)
0.2% uranium ore 4 (see note 3)
Notes for Table 1:

  1. Specific activities of the four forms of uranium have been rounded to two significant figures, and that of uranium ore to one significant figure.
  2. The minimum limit of 100 nanocuries/gram of transuranic elements for waste to be classified as TRU waste includes only those isotopes of transuranic elements with half-lives greater than 20 years. The most common isotope in TRU waste that is eliminated from the counting in this way is plutonium-241, which has a half-life of 14.4 years. However the decay product of plutonium-241, americium-241 is included in TRU waste because it has a half-life of about 432 years. All these uranium isotopes we are dealing with in these comments have half-lives far longer than 20 years.
  3. The specific activity of 0.2 percent uranium ore shown includes all
    decay products of uranium-238 up to and including radium-226, assuming they
    are in secular equilibrium with uranium-238. Radon-222, and its decay
    products are not included.

It is clear from Table 1 that depleted uranium is comparable in specific activity to transuranic waste. This conclusion is independent of the chemical form of the depleted uranium. Note that depleted uranium is far more radioactive than uranium ore because the ore is mixed with large quantities of non-radioactive materials. Thus, putting depleted uranium in mines is in no way like replacing the original material that was mined out of the ground. Rather it is analogous to putting TRU waste in the ground.

2. Mode of decay, energy of decay, and half-life

The main radionuclide of concern in most TRU waste is plutonium-239. Other radionuclides that are present in significant quantities are plutonium-240, plutonium-238, neptunium-237, and americium-241. The predominant mode of decay of all of these radionuclides is alpha decay. That is also the case with all three uranium isotopes (uranium-238, uranium-234, and uranium-235) present in depleted uranium. In all these cases, the emitted alpha particles have energies between 4 and 6 MeV, so that the total energy
deposited in tissue per picocurie of radioactive material in the body is the same. Thus, once a unit of radioactivity of TRU waste or of depleted uranium is in the body, the amount of radiation dose per unit of time is approximately the same.

Table 2 shows the principal characteristics of concern of the main radionuclides in TRU waste and in depleted uranium. Note that the decay products of uranium-238 build up over hundreds of thousands of years, and we have ignored these for the sake of argument in these comments.3

Table 2: Properties of Uranium Isotopes and Selected Transuranium Elements
Isotope Main decay mode Alpha particle energy, MeV Half-life, years Comments
Uranium Isotopes:
uranium-238 alpha 4.1 4.46 billion
uranium-234 alpha 4.8 245,000
neptunium-237 alpha 4.8 2.14 million
plutonium-238 alpha 5.5 87.7
plutonium-239 alpha 5.1 24,110
plutonium-240 alpha 5.1 6,537
plutonium-241 beta see note 2 14.4 not included in TRU waste definition
americium-241 alpha 5.5 432 strong gamma emitter

  1. All energies rounded to two significant figures. The alpha emitting radionuclides emit alpha particles with more than one characteristic energy, with each energy level being produced with a known probability. The alpha particle energy shown is an approximate average of these particles energies, weighted by the emission probability.
  2. Plutonium-241 is not included in the definition of TRU waste since it has a half-life of less than 20 years. Its beta particle energy is 0.021 MeV.

Chemical forms of depleted uranium

As has been shown, regardless of its chemical form, depleted uranium is always more radioactive than the lowest specific activity defining TRU waste. However, some chemical forms are more reactive than others. For example UF6 is very reactive. In the presence of moist air, UF6 reacts with the water in the air to form uranyl fluoride particulates (UO2 F2 ) and hydrogen fluoride fumes (HF) both of which are very toxic. In water hydrogen fluoride becomes a very corrosive acid. Anhydrous hydrogen fluoride is also corrosive. Other chemical forms are less reactive, the oxide forms of uranium being the most physically and chemically stable. DOE
should consider disposal of depleted uranium as an oxide (U3 08 or UO2 ) under 40 CFR 191 rules.

C. Conclusions

We recognize that in comparing uranium and plutonium on the basis of equal masses of these two elements in pure form, plutonium is far more radioactive than uranium. This is because the half-lives of the isotopes of plutonium shown in Table 2 are far shorter than those of uranium. This means that a given mass of uranium decays more slowly — that is, it gives off fewer alpha particles per unit of time. However, the far lower specific activity of uranium isotopes in depleted uranium relative to pure plutonium is irrelevant to this discussion because we are examining the properties of materials on the basis of the radioactivity per unit mass of waste to be disposed of. TRU waste is defined as that containing more than 100 nanocuries/gram. By weight it consists mainly of non-radioactive materials mixed with small masses of transuranic elements. By contrast, depleted uranium would be disposed of in nearly pure form, with only the possible addition of and element such as oxygen. As a result, TRU waste is comparable to depleted uranium. DOE TRU waste is slated for disposal in a deep geological repository.

Therefore we believe that depleted uranium should be put in the same category as TRU waste so far as its characteristics for waste management and disposal are concerned. Therefore in the PEIS the DOE should:

  • reexamine the waste classification of depleted uranium based on the explicit recognition of the scientific facts discussed above and,
  • add one alternative consisting of converting depleted uranium to the oxide form (U3 08 or UO2 ), and disposing it of in a geologic repository under 40 CFR 191 rules. The proposed PEIS would be fundamentally flawed and
    deficient if it omitted this option.

The implementation plan for the PEIS should include these two items.