This invention report suggests a method for drastically increasing the amount of Pu-241 in isotopic mixtures of plutonium. It is felt that with process experience, as much as 70 percent or more of the potential Pu-241 atoms can be concentrated in the fuel at one time. Such a concentration step would double the amount of production obtainable from Pu-241 by nuclear decay. The process to concentrate the Pu-241 consists of two basic steps: 1. Irradiate plutonium consisting largely of Pu-239 isotope until the Pu-239 has largely been converted to Pu-240 and 241 by thermal neutron fission and absorption events. Most of the fuel value has been taken advantage of at this point. The isotopic mixture will be largely Pu-240 but contain smaller percentages of 241 and 239. The irradiation is terminated at this point and the products are separated. 2. The depleted plutonium isotopes from the first irradiation are refabricated for a second irradiation with a thermal neutron absorber surrounding the depleted plutonium isotope. This element is again irradiated, preferably in an epithermal neutron flux with a peak energy slightly above 1 ev. The Pu-240 has a huge resonant cross section at the 1 ev level and will be most …

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