Critical and Thermal Investigation of Curium and Other Minor Actinides for Safe Storage and Disposal
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Curium poses special problems in the chemical preparation of spent nuclear fuel for transmutation. Once separated from the other minor actinides, the seven curium isotopes in spent fuel can lead to nuclear fission with the subsequent release of a large amount of radiation. The investigation takes into account the fact that the isotropic concentrations of the fuel may change with varying fuel burn up, initial fuel enrichment, and age of the spent fuel. A neutron criticality code was used to determine the effective neutron multiplication factor for varying quantities of curium held within cylindrical containers or stored in a spherical shape. A sphere has minimal surface area for a given volume of curium and its shape minimizes the loss of neutrons through the surface. This is considered the “conservative” or worst case shape. The second shape analyzed was a square cylinder in which the diameter and height are equal. This is the conservative case of a cylinder since it minimizes surface area for a given volume. Both “bare” and “shielded” masses of curium were considered. A bare mass is surrounded by air with no container walls. A shielded mass involves curium placed in a 1/8 inch thick steel wall container. To estimate the extreme or “conservative” case, we also studied the impact of placing curium in water, a significant neutron moderator. Recommendations are made on the maximum amount of curium that can be safely stored or handled before encountering nuclear criticality.
Several isotopes of curium also generate a significant amount of heat by radioactive decay. For spent nuclear fuel, 2.6 watts of heat may be generated for every gram of curium. This will present special problems when curium is separated from the other minor actinides. If kilogram-quantities of curium are stored in a container, for example, the curium may heat to an equilibrium temperature that exceeds the melting temperature of this actinide. Sustained fission can be avoided by preventing the accumulation of a critical mass of curium. The heat generation of curium presents even more restriction on the mass of curium that can safely be contained in one location.
Other minor actinides, including plutonium and americium, and a variety of mixtures of both may also pose similar problems in the treatment of nuclear waste. It will be necessary to investigate the properties of these materials, in a similar fashion to curium, in order to ensure safe handling and storage.
Criticality (Nuclear engineering); Curium; Radioactive wastes — Storage; Radioactive wastes — Transmutation; Spent reactor fuels
Materials Science and Engineering | Nuclear | Nuclear Engineering | Oil, Gas, and Energy
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Critical and Thermal Investigation of Curium and Other Minor Actinides for Safe Storage and Disposal.