AbstractNatural uranium dioxide specimens of Shippingport PWR-l blanket-rod geometry are exposed in the Materials Testing Reactor (flux 2×1014n/cm2−sec) and discharged periodically (every three weeks) for measurements in the Reactivity Measurement Facility (RMF). The time-integrated thermal and epithermal fluxes are measured during each exposure cycle, and together with the MTR Daily Power Logs, give the complete exposure history. Measurements in the RMF are used to determine an experimental value forη/η0(η0is the preirradiation value) which may be compared with the theoreticalη/η0calculated for the measured exposure history using appropriate neutron-interaction parameters. In the theoretical calculations, the thermal absorption cross section of stable fission products is taken to be 50 barns per fission. Although the experimental and theoretical results are derived completely independently, agreement within 1% inη/η0is found for the behavior following all cycles of irradiation comprising exposures from zero to 15,600 Mwd/ton.