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The thermal-neutron behavior in a highly heterogeneous cluster-type plutonium fuel lattice has been studied through the measurements of the dysprosium reaction-rate distribution in a unit cell covering three plutonium fuel elements, four coolant voids, and two lattice pitches. The study included comparison with the results obtained with UO₂ fuel. A new technique for locating the foils has been developed, resulting in an accurate measurement of the thermal-neutron flux distribution. Depression of the thermal-neutron flux in the fuel region is larger in the plutonium fuel lattice than in the uranium lattice because thermal-neutron absorption in the plutonium fuel is enhanced by the resonances of ²³⁹Pu and ²⁴¹Pu at 0.3 eV. In addition, the 1/v cross section of plutonium is larger than that of uranium. This property of the plutonium fuel appears markedly at 100% void fraction, but less at 0% because this property is weakened by the presence of H₂O coolant. The results of calculations obtained by means of the LAMP-DCA code showed good agreement with experimentally determined data within 5%.