Patent Number: 
Section: claims

1. An apparatus comprising:a pressure vessel comprising an upper vessel section and a lower vessel section connected by a mid-flange;a nuclear reactor core comprising fissile material disposed in the lower vessel section;upper internals disposed entirely in the lower vessel section above the nuclear reactor core, the upper internals including at least internal control rod drive mechanisms (CRDMs) with CRDM motors mounted on a suspended support assembly disposed below the mid-flange;a hollow cylindrical central riser disposed in the upper vessel section;a hollow cylindrical section disposed in the lower vessel section and surrounding the nuclear reactor core; anda riser transition element connecting with the hollow cylindrical central riser and the hollow cylindrical section to form a continuous hollow cylindrical flow separator between a columnar hot leg flowing inside the continuous hollow cylindrical flow separator and an annular cold leg flowing in a downcomer annulus surrounding the continuous hollow cylindrical flow separator;wherein the riser transition element is welded to the mid-flange of the pressure vessel and to the suspended support assembly of the upper internals to suspend the upper internals both from and below the mid-flange. 2. The apparatus of claim 1, wherein the hollow cylindrical section disposed in the lower vessel section and surrounding the nuclear reactor core comprises a core basket or shroud. 3. The apparatus of claim 1, wherein the riser transition element comprises:an annular main body; andgussets extending outward and upward from the annular main body and welded to the mid-flange of the pressure vessel. 4. The apparatus of claim 3, wherein each gusset comprises:a first section extending laterally away from the annular main body;a second section extending upward and laterally away from the first section; anda third section extending laterally away from the second section. 5. The apparatus of claim 4, wherein the third section is thicker than the second section. 6. The apparatus of claim 3, wherein the gussets include lifting lugs via which the mid-flange, the riser transition element, and the upper internals can be lifted out of the lower vessel section. 7. An apparatus comprising:a pressure vessel comprising an upper vessel section and a lower vessel section connected by a mid-flange;a nuclear reactor core comprising fissile material disposed in the lower vessel section;upper internals disposed in the lower vessel section above the nuclear reactor core, the upper internals including at least internal control rod drive mechanisms (CRDMs) with CRDM motors mounted on a suspended support assembly;a hollow cylindrical central riser disposed in the upper vessel section;a hollow cylindrical section disposed in the lower vessel section and surrounding the nuclear reactor core; anda riser transition element including an annular main body and gussets extending outward and upward from the annular main body and welded to the mid-flange of the pressure vessel, the riser transition element connecting with the hollow cylindrical central riser and the hollow cylindrical section to form a continuous hollow cylindrical flow separator between a columnar hot leg flowing inside the continuous hollow cylindrical flow separator and an annular cold leg flowing in a downcomer annulus surrounding the continuous hollow cylindrical flow separator,wherein the riser transition element is welded to the mid-flange of the pressure vessel and to the suspended support assembly of the upper internals to suspend the upper internals from the mid-flange, andwherein each gusset includes a first section extending laterally away from the annular main body, a second section extending upward and laterally away from the first section, and a third section extending laterally away from the second section, and the third section is thicker than the second section. 8. The apparatus of claim 7, wherein the hollow cylindrical section disposed in the lower vessel section and surrounding the nuclear reactor core comprises a core basket or shroud. 9. The apparatus of claim 7, wherein the gussets include lifting lugs via which the assembly comprising the mid-flange, the riser transition element, and the upper internals can be lifted out of the lower vessel section.