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On October 17, the Los Alamos National Laboratory (LANL) in New Mexico released details of an experiment designed to draw attention to a possible new source of fissile material - neptunium - for use in nuclear explosive devices. Neptunium (atomic number 93, lying between uranium and plutonium on the periodic table) is an artificial element produced in nuclear reactors. A laboratory press release explained the nature and purpose of the experiment:
"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.' 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 press release further explained that the neptunium used in the experiment achieved only 'bare' criticality, which occurs "when sufficient mass of fissile material is present to sustain a nuclear chain reaction without any reflective materials." In this case, 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."
On October 20, quoted in the Albuquerque Journal, Steve Clement noted: "There is a lot of neptunium out there in the world... We want to be able to account for that material and track that material to be sure that it does not fall into the wrong hands."
Neptunium can be separated from other radioactive spent fuels during reprocessing, a common commercial practice in the civilian nuclear sector in Europe and Asia.
Reports: Neptunium criticality achieved, Los Alamos National Laboratory Press Release, October 17; LANL adds new element to nukes, Albuquerque Journal, October 20; Common nuclear waste element could be bomb threat, USA Today, October 21.
© 2002 The Acronym Institute.