The physical behavior of burnable poison fuel pins, containing 0.1, 0.5, and 1.0 wt% Gd2O3 in 1.5 wt% UO2 pellets, has been studied through the measurements of reactivity change, coolant void reactivity, local power distribution, and thermal neutron flux distribution including fine structure, using a heavy-water-moderated, cluster-type fuel lattice. A new technique for utilizing a burnable poison has been developed using a gadolinium absorber rod inserted into the center of the cluster-type fuel assembly. Its physical behavior has been studied through the measurements of accompanying reactivity change, coolant void reactivity, local power distribution, and thermal neutron flux distribution. When the Gd2O3 content of the fuel pellets is more than 0.5 wt%, the reactivity effect is reduced largely due to the saturation of the thermal neutron self-shielding effect in the poisoned fuel pin. A gadolinium absorber rod inserted in the center of the fuel assembly, although it causes a small increase in local power peaking, is effective in the control of the initial excess reactivity and favorably affects the coolant void reactivity. An accurate calculation by the WIMS-D code requires division of the fuel pellet region into more than five mesh intervals owing to the enhancement of the thermal neutron self-shielding effect due to absorption by the gadolinium in the poisoned fuel pins.