A simple method has been developed for treating large cylindrical empty and rodded void regions in high-temperature gas-cooled reactors with each existing diffusion code. The cavity is treated as a diffusion region with zero reaction cross sections. Only diffusion constants, found by an optimization process, are used. Verification of this model is done by comparing results with transport solutions for identical problems. Very good agreement is attained when anisotropic diffusion is foreseen in the diffusion code. Even with isotropic diffusion, however, eigenvalues and rod reactivities proved to be acceptable. This treatment yields usual diffusion running times and allows three-dimensional calculations.