Neptunium, Radioactive

Exposure Summary

Neptunium was the first synthetic transuranium element of the actinide series discovered. Neptunium-239 (half-life = 2.4 days) was first produced in 1940 at Berkeley, CA by the bombardment of uranium-238 with cyclotron-produced neutrons. Seventeen isotopes of neptunium are known and all are radioactive. Neptunium-237 is obtained in gram quantities as the by-product from nuclear reactors in the production of plutonium. The longest lived isotope is Np-237; it is an alpha-emitter with a half-life of 2.14 million years. Neptunium is a by-product of plutonium production activities. Neptunium is present in spent nuclear fuel, high-level radioactive wastes resulting from the processing of spent nuclear fuel, and radioactive wastes associated with operations of reactors and fuel reprocessing plants. A small amount of neptunium would have been generated by atmospheric nuclear weapon testing, which ceased worldwide by 1980. The amount of neptunium in soil from past nuclear testing is on the order of 0.0001 pCi/g. Releases of neptunium from weapons production facilities have cause localized contamination. There are no major commercial uses of neptunium. Trace quantities of neptunium are found in nature associated with uranium ores. Neptunium compounds are ionic and would not be volatile and would exist solely in the particulate phase in the ambient atmosphere. Particulate-phase neptunium compounds will be removed from the atmosphere by wet or dry deposition. In soil, neptunium is generally more mobile than other transuranic elements such as plutonium, americium, and curium, moving with percolating water to lower soil layers. Neptunium compounds bind to soil particles, and bind more tightly with clay soils as compared with sandy soils. Neptunium is readily taken up by plants, with plant concentrations similar to soil concentrations. Neptunium compounds are ionic and would not volatilize from moist or dry soil surfaces. Neptunium has 4 valence states in water: Np3+; Np4+; NpO+; and (NpO)2+. Neptunium forms tri- and tetrahalide compounds such as NpF3, NpF4, NpCl4, NpBr3, NpI3, and oxides of various compositions such as Np3O8 and NpO2. Since neptunium has only been produced in limited quantities and it has few uses outside of research activities, exposure to neptunium compounds would be limited to individuals involved in scientific research using neptunium or at plutonium production or nuclear waste facilities. (SRC)
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