Abstract: A reactor stud hole plug unit having a mandrel with a central stem having a threaded upper portion, a longitudinal passageway extending therethrough and a mandrel plate attached to the lower end. The mandrel plate has threads on its periphery which engage internal threads on a stud hole in a nuclear reactor, and an annular stop is located at the upper end of the threads on the mandrel plate which engages a shoulder on the cavity when the mandrel plate is in position in a stud hole. A compression plate with a central opening having a diameter slightly larger than the diameter of the upper portion of the mandrel stem fits over the mandrel stem, and an annular sealing ring is positioned around the periphery of the mandrel plate above the annular shoulder and in contact with the compression plate when the compression plate is positioned over the mandrel stem.