Abstract: An electrical penetration assembly is disclosed for providing a vapor and pressure seal between a conductor passing through two freestanding walls of a nuclear reactor containment which are subject to relative movement due to a seismic event. Penetration nozzles or tubes are placed through the walls of the containment when it is constructed. The penetration assembly is inserted into the nozzles and comprises a first subassembly fixed and sealed to a first penetration nozzle in the first wall and a second penetration subassembly in sliding or rolling engagement and sealed to a second penetration nozzle in the second wall. The sliding or rolling seal in the second nozzle allows the unrestrained reaction of the conductor to relative movement between the two freestanding walls during a seismic event without damage to the conductor or the penetration assembly. The penetration assembly also prevents the possibility of damage to either the conductor or the penetration assembly due to thermally induced stresses.