Apparatus and a method for retracting in-core instrument thimble tubes from the reactor core prior to refueling a nuclear reactor with top mounted instrumentation. The apparatus includes a penetration flange interposed between the head flange and the reactor vessel flange through which the instrumentation cabling passes. The penetration flange is connected to the upper internals and can be raised relative thereto to retract instrumentation thimbles from the core prior to removal of the upper internals from the reactor vessel for refueling. The penetration flange is removed from the vessel with the upper internals.