produce Mo-99 in a reactor that is also fueled with HEU but is in the process of being converted to LEU.

ESA has also received HEU from Russia, and some of this HEU has been used to fuel three European reactors: the High Flux Reactor of the Institut Laue-Langevin, which is located in Grenoble, France; the Orpheus Reactor, which is located in Saclay, France; and the FRM II Reactor, which is located in Garching, Germany. (See http://www.francenuc.org/en_sources/sources_unat_e.htm for a discussion of HEU use in France.) None of these reactors is used to produce Mo-99. ESA does not publicly disclose the sources of HEU used for the manufacture of targets for medical isotope production. Most of this HEU is probably of U.S. origin, but some may also be of U.K. origin.

The primary concern with civilian use of HEU for applications such as Mo-99 production is its attractiveness for use in improvised nuclear devices by terrorists or rogue states. The amount of HEU required to achieve a sustained nuclear chain reaction (referred to as the critical mass) depends on the enrichment of U-235 as well as the design of the device. The IAEA defines a significant quantity of a nuclear material to be the approximate quantity of material from which the possibility of manufacturing a nuclear explosive device (i.e., a device that can achieve a prompt critical mass) cannot be excluded. The IAEA significant quantity for HEU is 25 kg. The HEU-based weapon used on Hiroshima, Japan, in August 1945 contained 64 kg of HEU having an average enrichment of about 80 percent. However, a well-designed nuclear explosive device could be made with less than 25 kg of HEU. The Atomic Energy Act gives the U.S. government the authority to regulate uranium that is enriched in U-235 (and also U-233) above natural abundances. Such materials are referred to as special nuclear materials. The U.S. government requires stepped-up security for facilities that handle greater than 5 kg of HEU.

As U-235 enrichment decreases, more uranium is required to achieve a prompt critical mass. It is difficult but not impossible to achieve a prompt critical mass with LEU.

  • LEU targets have been developed and demonstrated for use in the reactors and target processing facilities that produce significant quantities of medical isotopes to serve U.S. needs for such isotopes.

  • Sufficient quantities of medical isotopes are available from low enriched uranium targets and fuel to meet U.S. needs.

  • The average anticipated total cost increase from production of medical isotopes in such facilities without the use of HEU is less than 10 percent.

During the negotiations between the National Academies and the sponsoring organization within DOE (the National Nuclear Security Adminis-



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