Abstract: A pressure-retaining disk receives a plurality of sensors, the disk of the pressure-retaining disk having a plurality of sensor-receiving bores for receiving the sensors. These bores are connected by a passage. The disk is part of an apparatus that sealingly closes an entry or opening to a pressure vessel. The apparatus or sensor assembly allows for passing a plurality of sensors at least partially into the vessel through the opening in the vessel. The apparatus includes means for mounting the disk to the vessel, such means can include a tubular body through which the sensors pass. The pressure-retaining disk is connected to one end of the mounting means such that pressure of the vessel is retained. The sensors pass through the bores in the pressure-retaining disk and the bores or opening in the mounting means.