referring to fig4 , there is shown a pressurizer 22 for a pressurized water nuclear power plant system . the pressurizer 22 comprises a pressure vessel having a vertically oriented cylindrical shell 32 , a first or upper hemispherical head portion 34 and a second or lower hemispherical head portion 36 . a cylindrical skirt 38 extends downwardly from the lower head portion 36 and has a flange 40 fastened thereto by welding or other means to form a support structure for the vessel . the upper head portion 34 has a manhole or man - way 42 for servicing the interior of the vessel , one or more nozzles 44 , respectively , in fluid communication with a safety valve ( not shown ) and a spray nozzle 46 disposed therein . the spray nozzle 46 is in fluid communication with a supply of relatively cool primary fluid and has means associated therewith ( not shown ), which controls the flow of the relatively cool fluid to the pressurizer . a plurality of nozzles 48 are vertically disposed in the lower head 36 and a plurality of straight tubular electrical immersion heating elements 50 extend through the nozzles 48 and into the pressurizer 22 . the heating elements 50 have a metal sheath covering the outer surface thereof and seal welds are formed between the metal sheaths and the nozzles 48 . to support the heating elements in the pressurizer , a single or a plurality of support plates 52 are disposed transversely in the lower portion thereof . the support plate ( s ) 52 have a plurality of holes 54 which receive the heating elements 50 . a combined inlet and outlet nozzle 24 , commonly referred to as a surge nozzle is centrally disposed in the lower head 36 and places the pressurizer in fluid communication with the primary fluid system of the pressurized water nuclear reactor power plant . as previously mentioned , the surge nozzles of pressurizers include the thermal sleeves or liners previously discussed with regard to fig2 and 3 , which are employed to reduce the effect of thermal transients on the fatigue of the nozzle . in accordance with this invention , the thermal sleeve is attached to the bore of nozzle by means of a mechanical attachment . the attachment means of this invention allows for the sleeve to fully expand in the longitudinal direction as well as radially , which is necessary to address thermal transients experienced by the surge nozzle . the sleeve attachment is accomplished by the inclusion of annular grooves in the nozzle bore , which receive supporting keys to provide axial support for the sleeve . to prevent rotational movement , slots are provided in the lower end of the sleeve which receive keys machined in the bore of the nozzle 24 . fig5 illustrates a sectional view of surge nozzle 24 having a thermal sleeve 26 lining the interior surface thereof , which is attached to the nozzle in accordance with this invention . the thermal sleeve 26 is supported at the top end of the nozzle 24 by radial keys 58 . the radial keys fit through openings 60 in the thermal sleeve 26 , and are welded around their perimeter 62 on the bore of the thermal sleeve 26 . preferably , on one side the radial keys 58 have enlarged heads that are captured by the inner surface of the thermal sleeve around the opening 60 and at the other end are received into an annular groove 66 machined in the cladding 64 that lines the surface of a nozzle 24 . the lower end of the thermal sleeve includes axially extending slots 68 . small keys 70 , machined in the bore of the nozzle 24 , are received in the slots 68 , to maintain centering of the thermal sleeve lower end during transient conditions . flow in the crevice region 72 behind the thermal sleeve 26 is restricted by small clearances between the sleeve and the nozzle bore at both the upper end raised cladding surface and at the lower end . thus , an improved thermal sleeve attachment to the interior surface of the nozzle is provided that can accommodate thermal growth without adding substantial stress to the nozzle . while specific embodiments of the invention have been described in detail , it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure . accordingly , the particular embodiments disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof .