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  The Department of Energy’s National Isotope Development Center (NIDC) on March 31 announced the successful long-distance shipment in the United States of a biologically active compound labeled with the medical radioisotope astatine-211 (At-211). Because previous shipments have included only the “bare” isotope, the NIDC has described the development as “unleashing medical innovation.”
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Doppler effect measurements have been made in two plutonium-fueled fast reactor assemblies at the ZPR-3 reactor. One assembly, Assembly 45A, mocked up a large plutonium-uranium monocarbide fast power breeder reactor having a ²³⁹Pu-to-²³⁸U ratio of 1:7. The second assembly, Assembly 45, had a considerably softer spectrum, produced by replacing 40% of the canned sodium of the previous assembly by graphite. Zone-loading techniques were used to achieve these mockups. Doppler measurements were made with samples containing highly enriched ²³⁹Pu, ²³⁹Pu, and ²³⁸U mixed in a ratio of 1:7, as in a large breeder reactor, and ²³⁸U in two rod diameters. Various check experiments were performed to test the validity of the measurements. The experimental results showed a strong negative Doppler response for ²³⁸U, in good agreement with theoretical estimates. The ²³⁹Pu in the mixed isotope fuel composition gave only a small positive Doppler contribution. A separate measurement for ²³⁵U in the power breeder spectrum was positive and in reasonable agreement with theory. The ²³⁹Pu results were the most interesting. Three degrees of increasing “hardness” of neutron energy spectrum incident upon the ²³⁹Pu Doppler element were generated by Assembly 45, Assembly 45A, and Assembly 45A with natural B₄C surrounding the Doppler element, respectively. The experimental positive ²³⁹Pu Doppler effect reactivity change was much smaller than calculated for the softest spectrum. The agreement between theory and experiment improved significantly, however, as the neutron energy spectrum hardened.