A HP/LP seal arrangement is provided for a guide tube and thimble extending outwardly from the core of a nuclear reactor to a seal table where the guide tube is welded to the seal table to provide a high pressure seal relative thereto. A tubular seal fitting is disposed in alignment with the guide tube with the thimble extending therethrough on the low pressure side of the seal table.A first high pressure seal couples one end of the fitting to an end of the guide tube to prevent leakage from within the guide tube. Inwardly facing threading is disposed adjacent the other and outer end of the seal fitting. A nut is disposed about the thimble and it is provided with outwardly facing threading in mating engagement with the fitting threading.The fitting has a seal seat spaced longitudinally inwardly from the threading and facing the fitting outer end and extending annularly about the inner surface of the fitting. A deformable ring seal is integral with the nut and is disposed about the thimble and between the nut body and the fitting seal seat. The ring seal has respective sealing surfaces in engagement with the radially outwardly facing surface of the thimble, the radially inwardly facing surface of the fitting and the fitting seal seat to provide a low pressure seal between the fitting and the thimble. A second releasable high pressure seal couples the thimble to the outer end portion of the guide tube.