Under the joint ENEL-EURATOM program for the utilization of plutonium in thermal reactors, the power distribution calculation methods used by ENEL were subjected to extensive investigation aimed at ascertaining their adequacy in handling the various aspects of UO2and PuO2cores.Two categories of codes were verified by comparison with experimental data:Codes that permit the determination of the macroscopic power distribution of the whole core as a function of irradiation; namely, FLARE and CONDOR. The calculated data were found to compare favorably with the experimental observations.Codes that permit the assessment of the effect of local nonuniformities on power distribution over the fuel lifetime, namely, the BURNY code in the two-group, in the four-group (1 thermal and 3 fast), and in the five-group (2 thermal and 3 fast) versions gave a satisfactory agreement with the actual power distribution while the239Pu and235U fission rate ratios were evaluated better by the two-thermal-group version. Since allowance was made for the spectrum softening due to the water gap, the GAM-THERMOS-SQUID system achieved the same accuracy as the two-thermal-group BURNY, but it was much more time consuming. However, the two-thermal-group version of the BURNY code tended to overestimate the effect of the water gap on the corner rod slightly, and the one-group version rather undere stimated it.