A core management analysis of a 165-MW(electric) plutonium-uranium mixed-oxide-fueled heavy water reactor, Fugen, has been carried out and compared with some of the operational data through the initial cycle and three reload cycles. During these burnup cycles, the Fugen reactor has experienced refueling of 72 plutonium-uranium fuel bundles and 76 UO2 fuel bundles, including four special bundles for irradiating test samples. The core burnup, thermal flux distribution, power distribution, coolant flow distribution, etc., were calculated and compared with measurements. The root-mean-square (rms) error of the calculated thermal flux over the three-dimensional spatial nodes was found to be ∼3% for various core conditions, and the rms error of the calculated core flow distribution was within 2%. The reload patterns have been chosen in such a way that mixed oxide and UO2 fuel bundles can exist in a mixed bed loading with no attendant reactor control problems.