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Disarmament DocumentationBack to Disarmament Documentation Los Alamos Press Release on Neptunium Experiment, October 17'Neptunium criticality achieved', Los Alamos National Laboratory (LLNL) News Release, October 17. A full-controlled criticality of the element neptunium was achieved in late September 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. The experiment was conducted using the "Planet" assembly criticality 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 mounted on the "Planet" device. The actual criticality was controlled remotely to assure the safety and security of the experiment. The experiment has 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. Neptunium is an element produced as a by-product of nuclear power generation. 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." Neptunium is a man-made actinide metal, grayish in color, which lies on the periodic table of elements between uranium and plutonium. 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 monitoringneptunium in 1999. 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." "After working together as a team for so many years doing something and wondering 'Is it ever going to happen?' it was such a great feeling to actually bring it together," said Loaiza. "The people here at TA-18 are just amazing, excellent people." 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. "LACEF is the only operational general purpose critical mass laboratory in the Western Hemisphere. It is the only facility in the United States where this kind of experiment could be done," said Clement. The team plans to continue with further experiments using a variety of reflecting materials. This will enable the scientists to determine the critical mass of neptunium under a variety of conditions, as that mass can vary depending on the amount of neutrons reflected or absorbed by materials surrounding the nuclear material. Los Alamos National Laboratory is operated by the University of California for the National Nuclear Security Administration (NNSA) of the US Department of Energy and works in partnership with NNSA's Sandia and Lawrence Livermore national laboratories to support NNSA in its mission. Los Alamos enhances global security by ensuring the safety and reliability of the US nuclear stockpile, developing technologies to reduce threats from weapons of mass destruction, and solving problems related to energy, environment, infrastructure, health and national security concerns. Source: Los Alamos National Laboratory, http://www.lanl.org. © 2002 The Acronym Institute. |