Neptunium (Np) is a man-made actinide metal, grayish in color, which lies on the periodic table of elements between uranium and plutonium. Neptunium was first produced by Edwin M. McMillian and Philip H. Abelson, working at the University of California, Berkeley, in 1940. They produced neptunium-239, an isotope of neptunium with a half-life of about 2.4 days, by bombarding uranium with slow moving neutrons.
Neptunium's most stable isotope, neptunium-237, has a half-life of about 2,144,000 years. It decays into protactinium-233 through alpha decay. Neptunium-237, which is produced in gram quantities as a by-product of the production of plutonium in nuclear reactors, is used in neutron detectors.
Once considered to be completely artificial, extremely small amounts of neptunium are produced naturally in uranium ores through the interaction of atoms of uranium in the ore with neutrons produced by the decay of other atoms of uranium in the ore.
Neptunium is prepared by the reduction of NpF3 with barium or lithium vapor at about 1200oC. Neptunium metal has a silvery appearance, is chemically reactive, and exists in at least three structural modifications: alpha-neptunium, orthorhombic, density 20.25 g/cm3, beta-neptunium (above 280oC), tetragonal, density (313oC) 19.36 g/cm3, gamma-neptunium (above 577oC), cubic, density (600oC) 18.0 g/cm3. Neptunium has four ionic oxidation states in solution: Np+3 (pale purple), analogous to the rare earth ion Pm+3, Np+4 (yellow green); NpO2+ (green blue): and NpO2++ (pale pink). These latter oxygenated species are in contrast to the rare earths which exhibit only simple ions of the (II), (III), and (IV) oxidation states in aqueous solution. The element forms tri- and tetrahalides such as NpF3, NpF4, NpCl4, NpBr3, NpI3, and oxides of the various compositions such as are found in the uranium-oxygen system, including Np3O8 and NpO2. Seventeen isotopes of neptunium are now recognized.
Neptunium is an element produced as a by-product of nuclear power generation. Reprocessing is necessary to separate neptunium from other spent radioactive fuels, something many European and Asian countries do. The 100 nuclear power plants across the United States can produce about 12,000 kilograms of neptunium in a decade.
A full-controlled criticality of the element neptunium was achieved in late September 2002 at Los Alamos National Laboratory's Technical Area 18 using a six kilogram nickel-clad neptunium sphere in combination with approximately 60 kilograms of enriched uranium. Scientists now know it takes about 30 percent less neptunium than previously thought, or about 60 kilograms, to generate a nuclear chain reaction.
The isotope of neptunium used in this first criticality experiment was neptunium-237. The element has other isotopes that are very short lived, but neptunium-237 has an extraordinary long half-life of two million years. The International Atomic Energy Agency approved monitoring neptunium in 1999.
The experiment was conducted using the "Planet" assembly device at the Los Alamos Critical Experiments Facility or LACEF. The neptunium and enriched uranium assembly was constructed at TA-18's Critical Assembly and Storage Area-One, and mounted on the "Planet" device. The actual criticality was controlled remotely to assure the safety and security of the experiment.
The experiment yielded preliminary data that show the critical mass of neptunium is actually less than previously predicted. Following additional experimentation, the data will eventually pinpoint the element's exact critical mass, something that has not been determined before in the United States. Lab scientists used neptunium-237, the most stable of 20 isotopes, or variations, of neptunium with a half-life of about 2 million years.
Prior to this experiment, the critical mass of neptunium was only estimated with computer models from data based on earlier experiments using much smaller amounts of the element in less than optimal configurations. "The results of this experiment are of interest to scientists working in the fields of nuclear safeguards, nuclear nonproliferation and criticality safety," said Steve Clement of the Laboratory's Advanced Nuclear Technology group, part of the Nonproliferation and International Security Division. "While the actual criticality was achieved in about four days, this experiment has been in the works for 12 years, so on many levels, it's a major accomplishment."
Rene Sanchez and David Loaiza, both of Advanced Nuclear Technology, were primarily responsible for the successful criticality, along with a team that included Clement, Robert Kimpland, David Hayes, Peter Jaegers, Charlene Cappiello, Bill Myers, Ken Butterfield, Charles Hollas, Charles Goulding, Joetta Goda, Eric Sorensen and a support team of special nuclear materials custodians and others.
"Fabrication of the sphere was completed about 18 months ago here at Los Alamos," said Sanchez. "Since then we've been in planning, getting permission from the Department of Energy to do the experiment and taking care of security issues. Once all that was in place, it took about four days to do the technical operations of the experiment. It could not have been accomplished without the hard work and determination of the whole team, NIS management and the NNSA Office of Los Alamos Site Operations." The work was done to support the DOE's Criticality Safety Program and the National Nuclear Security Administration's Nonproliferation Program and Emergency Response Program.
So-called "bare" criticality is achieved when sufficient mass of fissile material is present to sustain a nuclear chain reaction without any reflective materials. The neptunium criticality was achieved in a "low power" state, where the overall radioactivity is kept relatively low, at about 300 millirem per hour, and no significant heat or fission byproducts are created.
Since the neptunium sphere alone was not of sufficient mass to sustain the nuclear reaction, it was placed in the center of several thin nested shells of enriched uranium configured in an upper and lower half, with the neptunium sphere located in the lower section. The uranium in this case helps drive the system aiding in the neptunium's ability to sustain the chain reaction. The two halves of the assembly were placed in the "Planet" device, one half above the other, a safe distance apart. During the experiment, measurements are taken as the lower section is raised and brought closer and closer to the upper section until sufficient mass of fissile materials is present and criticality is achieved. Since the reactivity of enriched uranium is well established the critical mass of neptunium can be readily calculated from the experiment's resultant data.
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