Patent Number: 
Section: claims

1. A nuclear reactor comprising:a cylindrical pressure vessel including an upper vessel section and a lower vessel section;a nuclear reactor core comprising fissile material disposed in the cylindrical pressure vessel immersed in primary coolant water;a cylindrical central riser disposed in the cylindrical pressure vessel above the nuclear reactor core, the cylindrical central riser including an upper riser section disposed in the upper pressure vessel section and a lower riser section disposed in the lower pressure vessel section, the upper riser section having a smaller diameter than the lower riser section; anda riser cone including a frustoconical body having a lower end engaging the lower riser section, a sealing ring engaging the upper riser section, and a spring compressed between the sealing ring and an upper end of the frustoconical body. 2. The nuclear reactor of claim 1 wherein the lower riser section includes one of a core basket and a shroud surrounding the nuclear reactor core. 3. The nuclear reactor of claim 2 wherein the lower riser section further comprises a riser transition weldment disposed on the one of a core basket and a shroud, the riser transition weldment including interface features engaging a lower end of the riser cone. 4. The nuclear reactor of claim 1 wherein the spring is a nested wave spring. 5. The nuclear reactor of claim 1 wherein the riser cone includes a flow path for reactor coolant bypass flow.