AbstractThis article is devoted to an extension of Galanin’s heterogeneous theory to the tridimensional problems usually met in studies of small reactors. Clearly, this method is also applicable to bidimensional problems. Here also, it brings in several improvements compared with the parent theory, especially concerning the radial reflector, which can here be of finite dimensions. Significant numerical examples are given in the final section. The codes used—for the IBM 7090 computer—lead to times of calculation which are reasonable for small reactors. They may also be used for much bigger reactors provided that they are used to treat bidimensional problems only.