This paper describes an extension of the method of Nordheim-Scalettar to the case of rods partially inserted in a bare reactor. In this study, the axial flux harmonics are introduced. It is assumed that the extrapolation distances of the rods for thermal neutrons still have the same value whatever their insertion length may be. This extrapolation distance can be calculated especially from data relative to rods of infinite length. Calculation methods for the determination of the efficiency of rod assemblies and the distribution of thermal neutron flux are described. It should be noted that the determination of the flux distribution may require the use of many more harmonics than the determination of rod efficiency, since the results converge more rapidly in the latter case. Significant examples are given. The calculation method has been programmed to be used with a BULL Gamma AET Computer.