The effective resonance absorption of coated particles, which are embedded in a graphite matrix, is studied. The effect of a random arrangement of particles on the resonance integral is examined using the radial distribution function derived from the Percus-Yevick equation. A differential equation is proposed to obtain the neutron-beam current from a source particle in a medium in which the distribution function of coated particles is specified. By the use of the neutron beam current and the distribution function as the weight, the fuel-to-fuel collision probability is defined. This collision probability is applied to a RICM-type resonance integral code. The depression of the resonance integral of 238U due to grain structure amounts 5% in a design study of the multi-purpose high-temperature gas-cooled reactor (HTGCR) at the Japan Atomic Energy Research Institute. The applicabilities of the spherical cell model and of the collision probability in the high-dilution approximation of Lane et al. are tested. These simple procedures give satisfactory results for the treatment of microscopic heterogeneity in the range of the HTGCR design.