Nuclear Fuel Selection

Material Science Volume 2 of 2
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FUEL MATERIALS DOE-HDBK-1017/2-93 Plant Materials As noted above, interest in thorium as a contributor to the world's useful energy supply is based on its transmutability into the fissile isotope ²³³U. The ease with which this property can be utilized depends on the impact of the nuclear characteristics of thorium on the various reactor systems in which it might be placed and also on the ability to fabricate thorium into suitable fuel elements and, after irradiation, to separate chemically the resultant uranium. The nuclear characteristics of thorium are briefly discussed below by comparing them with ²³⁸U as a point of reference. First, a higher fissile material loading requirement exists for initial criticality for a given reactor system and fissile fuel when thorium is used than is the case for an otherwise comparable system using ²³⁸U. Second, on the basis of nuclear performance, the interval between refueling for comparable thermal reactor systems can be longer when thorium is the fertile fuel. However, for a given reactor system, fuel element integrity may be the limiting factor in the depletion levels that can be achieved. Third, ²³³Pa (protactinium), which occurs in the transmutation chain for the conversion of thorium to ²³³U, acts as a power history dependent neutron poison in a thorium-fueled nuclear reactor. There is no isotope with comparable properties present in a ²³⁸U fuel system. Fourth, for comparable reactor systems, the one using a thorium-base fuel will have a larger negative feedback on neutron multiplication with increased fuel temperature (Doppler coefficient) than will a ²³⁸U-fueled reactor. Fifth, for comparable reactor configurations, a ²³²Th/²³³U fuel system will have a greater stability relative to xenon-induced power oscillations than will a ²³⁸U/²³⁵U fuel system. The stability is also enhanced by the larger Doppler coefficient for the ²³²Th/²³³U fuel system. And sixth, the effective value of b for ²³²Th/²³³U systems is about half that of ²³⁵U-fueled reactors and about the same as for plutonium-fueled reactors. A small value of b means that the reactor is more responsive to reactivity changes. In conclusion, the nuclear properties of thorium can be a source of vast energy production. As demonstrated by the Light Water Breeder Reactor Program, this production can be achieved in nuclear reactors utilizing proven light water reactor technology. Nuclear Fuel Selection The nuclear properties of a material must be the first consideration in the selection of a suitable nuclear fuel. Principle properties are those bearing on neutron economy: absorption and fission cross sections, the reactions and products that result, neutron production, and the energy released. These are properties of a specific nuclide, such as ²³²Th, and its product during breeding, ²³³U. To assess these properties in the performance of the bulk fuel, the density value, or frequency of occurrence per unit volume, of the specific nuclide must be used. MS-05 Page 8 Rev. 0

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