December 2006 Issue - Volume 2, Number 6

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The Nuclear Fuel Cycle: Environmental Aspects

Rodney C. Ewing – Guest Editors

Overview

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2006 Topics

Overview

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2006 Topics

Thematic Articles

The Nuclear Fuel Cycle: A Role for Mineralogy and Geochemistry

By Rodney C. Ewing

As the world faces the consequences of global warming caused by the use of fossil fuels, there has been a resurgence of interest in nuclear power. However, there is no “silver bullet,” and each energy-producing system produces waste. This issue of Elements shows the importance of mineralogy and geochemistry in the safe management and disposal of the different types of waste generated by the nuclear fuel cycle.

Uranium Mill Tailings: Geochemistry, Mineralogy, and Environmental Impact

By Abdesselam Abdelouas*

Worldwide, the mining of uranium has generated 938 × 106 m3 of mill tailings. The radioactivity of these tailings depends on the grade of ore mined and varies from less than 1 Bq/g to more than 100 Bq/g. The most common mode of disposal is near-surface impoundment in the vicinity of the mine or mill. The principal radiation risks from uranium tailings are gamma radiation, essentially from radium decay; windblown radioactive dust dispersal; and radon gas and its radioactive progeny, which are known to cause lung cancer. Uranium mill tailings are also often associated with elevated concentrations of highly toxic heavy metals, which are a major source of surface and groundwater contamination. Due to their high sulfide content (a few to tens of wt%), tailings may acidify groundwater, accelerating the release of radioactive and hazardous elements.

Spent Nuclear Fuel

By Jordi Bruno, Rodney C. Ewing

The primary waste form resulting from nuclear energy production is spent nuclear fuel (SNF). There are a number of different types of fuel, but they are predominantly uranium based, mainly UO2 or, in some cases, metallic U. The UO2 in SNF is a redox-sensitive semiconductor consisting of a fine-grained (5–10 µm), polycrystalline aggregate containing fission-product and transuranium elements in concentrations of 4 to 6 atomic percent. The challenge is to predict the long-term behavior of UO2 under a range of redox conditions. Experimental results and observations from natural systems, such as the Oklo natural reactors, have been used to assess the long-term performance of SNF.

Uranium Mineralogy and Neptunium Mobility

By Peter C. Burns, Amanda L. Klingensmith

Spent nuclear fuel, mainly UO2, is chemically unstable under oxidizing conditions. Alteration occurs by oxidation and hydration and can be rapid and substantial. Uranyl minerals, containing U6+, form when commercial spent fuel is altered in a moist, oxidizing environment. During the alteration and dissolution of the UO2, fission-product radionuclides and transuranium elements are released. Uranyl minerals that form locally as alteration products in a geologic repository may incorporate many of these radionuclides, thereby immobilizing them for lengthy periods and thus improving the long-term performance of the repository. Here we discuss one very important radionuclide, 237Np (half-life = 2.14 million years), and focus on mineralogical studies that probe the potential impact of uranyl minerals on neptunium mobility.

Nuclear Waste Glasses – How Durable?

By Bernd Grambow

High-level nuclear wastes (HLW) are the liquid effluents that result from the reprocessing of spent nuclear fuel. These wastes are typically solidified in a glass for final disposal in deep geologic formations. At present, there is no geologic repository receiving these vitrified wastes. A primary issue in nuclear waste management is whether there can be societal, regulatory, and political confidence that the radiotoxic constituents of HLW can be safely disposed of for hundreds of thousands of years. If a glass waste form, placed at a depth of hundreds of meters, is stable and essentially insoluble in groundwater, it would be almost impossible for radioactivity to reach the environment. This paper summarizes the state of knowledge of glass performance in a geologic repository and examines the question of whether the long-term stability of the glass and radionuclide retention can be assured.

Ceramic Waste Forms for Actinides

By Gregory R. Lumpkin

The concept of nuclear waste forms based on minerals that contain actinides has led to the development of polyphase and special-purpose crystalline ceramics. These ceramics are considered by many to be attractive media for the long-term storage of actinides in geological repositories. The available data show that monazite, pyrochlore, zircon, and zirconolite are all highly durable in both natural and synthetic aqueous systems at low temperatures. In comparison, perovskite is prone to dissolution and conversion to anatase and other secondary alteration products. The titanate and silicate phases of interest become metamict (amorphous) as a result of irradiation. Several compounds, including monazite, cubic zirconia, and the defect fluorite structure types with Zr on the B site, exhibit the attractive property of radiation “resistance.” These results, together with other materials properties, are discussed briefly with respect to criteria for waste form performance.