Heterogeneity effects of powdered or slurry fuel is studied through reactivity calculations of tiny cells consisting of a small fuel lump surrounded by water. The kind of fuel is slightly-enriched uranium dioxide which is immersed in water. The cells are made smaller keeping constant the volume ratio of water to fuel; the homogeneous system is regarded as the limit of grain size 0. Reaction rates are calculated by solving the ultra-fine energy group transport equations by the collision probability method. The collision probabilities are calculated using the extended version of the RABBLE code for the fast energy groups and that of the THERMOS code for the thermal energy groups as applicable to spherical cells. The infinite-medium multiplication factor and its four-factors, and their fractional changes from the homogeneous system are obtained. The results of calculations show that substitution of homogeneous system for heterogeneous ones underestimates the reactivity some 2% for as small as 2-mm-diam. fuel elements, and this underestimation mainly comes from the change of the resonance escape probability. (author).