Abstract: A process for controlling operation of a nuclear reactor in which first control means is operated to cause reactor power to rise to a level at which a pellet-clad-mechanical-interaction begins to take place between a cladding and pellets of a fuel element. After interrupting the operation of the first control means, second control means is operated to cause the reactor power to rise to a preset level, the second control means being capable of effecting finer control of the reactor power than the first control means. When the reactor power deviates from the preset level with the progress of the reactor operation in the preset level, the second control means is operated so as to maintain the reactor power at the preset level.