Patent Number: 
Section: claims

1. A method comprising:depressurizing a nuclear reactor that includes at least:a pressure vessel including an upper vessel section and a lower vessel section connected by a mid-flange and containing primary coolant,a nuclear reactor core disposed in the lower vessel section and immersed in the primary coolant, andupper internals suspended from the mid-flange of the pressure vessel and disposed below the mid-flange, the upper internals including at least internal control rod drive mechanisms (CRDMs) with CRDM motors immersed in the primary coolant and control rod guide frames disposed between the CRDM motors and the nuclear reactor core;disconnecting and removing the upper vessel section from the remainder of the pressure vessel while leaving the mid-flange in place on the lower vessel section with the upper internals remaining suspended from and disposed below the mid-flange;removing the mid-flange from the lower vessel section with the upper internals remaining suspended from the mid-flange and disposed in the lower vessel section;replacing fuel of the nuclear reactor core;placing the mid-flange back onto the lower vessel section with the upper internals remaining suspended from the mid-flange; andplacing the upper vessel section back onto the remainder of the pressure vessel and re-connecting the upper vessel section with the remainder of the pressure vessel,wherein the nuclear reactor further includes a central riser disposed in the upper vessel section and riser cone connecting the central riser with the lower vessel section, the riser cone remaining disposed with the remainder of the pressure vessel upon completion of the disconnecting and removing of the upper vessel section, and the method further comprises:removing the riser cone prior to removing the mid-flange from the lower vessel section with the upper internals remaining suspended from the mid-flange. 2. The method of claim 1 wherein the disconnecting comprises disconnecting fasteners that extend through a flange assembly including the mid-flange, a flange on the upper vessel section, and a flange on the lower vessel section. 3. The method of claim 2 wherein the disconnecting also disconnects the upper vessel section from the remainder of the pressure vessel and disconnects the mid-flange from the lower vessel section, the disconnecting and removing of the upper vessel section leaving the mid-flange in place on the lower vessel section due to weight of the mid-flange and the upper internals that remain suspended from the mid-flange. 4. The method of claim 2 wherein the fasteners include bolts that remain in a parked position at least partially inserted into the flange on the upper vessel section after completion of the disconnecting and removing of the upper vessel section. 5. The method of claim 2 wherein the fasteners include bolts that remain in a parked position at least partially inserted into one of (1) the flange on the upper vessel section and (2) the flange on the lower vessel section after completion of the disconnecting and removing of the upper vessel section. 6. The method of claim 1 wherein the nuclear reactor further includes a steam generator disposed inside and secured with the upper vessel section, and the steam generator remains disposed inside and secured with the upper vessel section during and upon completion of the disconnecting and removing of the upper vessel section. 7. The method of claim 1 wherein the mid-flange includes electrical connections through the mid-flange to the upper internals, and the method further comprises:prior to the removing of the mid-flange, disconnecting cables external to the pressure vessel from the connections through the mid-flange to the upper internals. 8. The method of claim 7 wherein cables internal to the pressure vessel running from the connections through the mid-flange to the upper internals are left in place during the removing of the mid-flange. 9. A method comprising:depressurizing a nuclear reactor that includes at least:a pressure vessel including an upper vessel section and a lower vessel section connected by a flange assembly and containing primary coolant wherein the flange assembly includes at least a flange on the upper vessel section and a flange on the lower vessel section,a nuclear reactor core disposed in the lower vessel section and immersed in the primary coolant,a mid-flange supported by the lower vessel section, andupper internals suspended from the mid-flange of the pressure vessel and disposed below the mid-flange, the upper internals including at least internal control rod drive mechanisms (CRDMs) with CRDM motors immersed in the primary coolant and control rod guide frames disposed between the CRDM motors and the nuclear reactor core;disconnecting the upper vessel section from the lower vessel section and lifting the upper vessel section away from the lower vessel section with the mid-flange remaining supported by the lower vessel section and the upper internals remaining suspended from and disposed below the mid-flange;lifting the mid-flange away from the lower vessel section with the upper internals remaining suspended from the mid-flange so as to lift both the mid-flange and the suspended upper internals away from the lower vessel section;replacing fuel of the nuclear reactor core disposed in the lower vessel section; lowering the mid-flange back onto the lower vessel section with the upper internals remaining suspended from the mid-flange;reconnecting the upper vessel section to the lower vessel section; and re-pressurizing the nuclear reactor,wherein the nuclear reactor further includes a central riser disposed in the upper vessel section and riser cone connecting the central riser with the lower vessel section, the riser cone remaining disposed with the lower vessel section upon completion of the disconnecting and lifting of the upper vessel section away from the lower vessel section, the method further comprising:removing the riser cone prior to lifting the mid-flange away from the lower vessel section with the upper internals remaining suspended from the mid-flange. 10. The method of claim 9 wherein the mid-flange connects the upper and lower vessel sections. 11. The method of claim 10 wherein the mid-flange includes vessel penetrations connecting with the upper internals and the method further comprises:before lifting the mid-flange away from the lower vessel section, disconnecting external connections to the vessel penetrations of the mid-flange. 12. The method of claim 11 wherein the disconnecting of the upper vessel section from the lower vessel section also disconnects the mid-flange from the lower vessel section, the mid-flange remaining supported by the lower vessel section due to weight of the mid-flange and weight of the upper internals that remain suspended from the mid-flange. 13. The method of claim 9 wherein the nuclear reactor further includes a steam generator disposed inside and secured with the upper vessel section, and the steam generator remains disposed inside and secured with the upper vessel section during and upon completion of the disconnecting and lifting of the upper vessel section away from the lower vessel section.