The methodology used to prepare the source for neutron fluence calculation at the reactor pressure vessel is examined, and its effect on the calculated cavity dosimeter reaction rate is evaluated. Different source distributions for the Three Mile Island Unit 1 and Davis-Besse reactors and a simulated low low-leakage loading pattern are analyzed based on different levels of homogenization, different isotopic averaging approaches, contribution of U-238, use of the LEPRICON C factor formulation, and the SAILOR spectrum. Fuel isotopics can significantly affect the source distributions (through the fission spectrum), thereby leading to uncertainties of approximately 7% in the calculated cavity dosimetry reaction rates. Higher uncertainties (>10%) are expected due to both the C factor and fission spectrum when the low low-leakage fuel designs are utilized. [References: 15]
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