The positive scram effect (PSE) during the first seconds of the Chernobyl accident following the activation of the scram command has been investigated by using the French CRONOS three-dimensional code under different hypotheses on the axial shape of the initial power distribution. Assuming an initial power shape similar to the information recorded by the SKALA monitoring system and relevant to the core condition -2 min before the reactivity accident, the results of the present work well confirm the first seconds of the simulation annexed to the INSAG-7 report. But, these results cannot explain the signals of too high power and too short period registered by all the lateral ionization chambers 3 s after the scram command activation. The present work shows that the PSE can reproduce those alarms under the hypothesis of a further power shape deformation in the lower part of the core.