Method and device for restoring the fall-time of at least one control cluster for regulating reactivity in the core of a light-water-cooled nuclear reactor

A method of restoring the drop time of at least one control cluster for adjusting the reactivity in the core of a light water cooled nuclear reactor, in which the core is constructed by juxtaposed fuel assemblies placed in a nucleaar reactor vessel with vertically disposed guide tubes for guiding absorber rods of control clusters, the reactor including upper internal equipment above the core and resting on the top portions of the fuel assemblies of the core via a top plate of the core extending in a horizontal disposition and including a set of vertical guide tubes each used for guiding a control cluster including by a bundle of mutually parallel absorber rods and including a plurality of mutually parallel guide channels, said method comprising:

• operating the reactor for a period of time, subsequently stopping the reactor and providing a cool down period, placing the upper internal equipment in a reactor pool, enlarging the guide channels of at least one guide tube, wherein the enlarging is performed in at least one zone of the continuous guide channels.