In the method of the active surfaces a whole of nuclear fuel rods is described by a surface possessing suitable properties characterized in terms of 'heterogeneous parameters'. This study gives a calculus allowing the theoretical determination of the heterogeneous parameters concerning a cylindrical active surface. The field in which an active surface is equivalent to fuel rods is discussed with particular references to 5 different fuel arrangements. A method for reaching the heterogeneous parameters from measurements in the sub-critical assembly CACTUS is described. This method takes into account the particular energy spectrum and locations of the neutron supply sources, The values of the heterogeneous parameters theoretically obtained are in the error limits of the experimental results.