Patent Number: 
Section: claims

1. A stable startup system, comprising:a nuclear reactor vessel;a reactor core housed in the nuclear reactor vessel, wherein the reactor core is submerged in a primary coolant of the nuclear reactor vessel;a riser located at least partially above the reactor core;a heat sink configured to remove heat from the primary coolant after the primary coolant has passed through the riser; anda heating system configured to introduce heat to the primary coolant prior to an initialization of the reactor core, wherein the initialization comprises at least a partial withdrawal of one or more control rods from the reactor core, and wherein the heat is introduced at one or more insertion points located within the riser at an elevation between the heat sink and the reactor core. 2. The stable startup system of claim 1, wherein the heating system comprises:one or more heaters configured to generate heated water from the heat; andone or more nozzles configured to introduce the heated water directly to the primary coolant located within the riser. 3. The stable startup system of claim 2, wherein the one or more heaters are located external to the nuclear reactor vessel, and wherein the one or more nozzles are at least partially located within the riser and are operably connected to the one or more heaters via one or more fluid distribution lines. 4. The stable startup system of claim 2, wherein the one or more heaters are located in a pressurizer system located in an upper head space of the nuclear reactor vessel, wherein the pressurizer system is configured to control a system pressure in the nuclear reactor vessel, and wherein the one or more nozzles are least partially located within the riser and are operably connected to the upper head space via one or more extraction lines. 5. The stable startup system of claim 1, wherein the introduction of heat into the riser causes a density difference between the primary coolant in the riser and in an annulus that drives the primary coolant through the reactor core via natural circulation prior to the initialization of the reactor core. 6. The stable startup system of claim 5, wherein the heat ink comprises a heat exchanger configured to remove at least a portion of the heat from the primary coolant in the annulus, and wherein the annulus is located outside of the riser. 7. The stable startup system of claim 1, wherein the initialization of the reactor core comprises removing control rods from the reactor core to achieve reactor criticality, and wherein the heat is introduced to the primary coolant prior to removing the control rods. 8. The stable startup system of claim 1, wherein the heating system comprises a pressurizer system configured to control pressure within the nuclear reactor vessel after the initialization of the reactor core. 9. The stable startup system of claim 1, wherein the heating system is configured to heat the primary coolant to an operating temperature that provides for circulation of the primary coolant from the riser to the heat sink and through the reactor core, and wherein the operating temperature identifies a coolant temperature associated with a low power steady state condition of an initialized reactor core. 10. The stable startup system of claim 1, wherein the heating system comprises one or more electric heaters configured to generate the heat introduced into the primary coolant. 11. An apparatus, comprising:a nuclear reactor vessel;a reactor core housed in the nuclear reactor vessel, wherein the reactor core is submerged in a primary coolant of the nuclear reactor vessel;a riser located at least partially above the reactor core;means for removing heat from the primary coolant after the primary coolant has passed through the riser; andmeans for introducing heat to the primary coolant prior to an initialization of the reactor core, wherein the initialization comprises at least a partial withdrawal of one or more control rods from the reactor core, and wherein the heat is introduced at one or more insertion points located within the riser at an elevation above the reactor core. 12. The apparatus of claim 11, further comprising one or more control rods configured to initialize the reactor core, wherein the reactor core is initialized after the means for introducing heat is deactivated. 13. The apparatus of claim 12, wherein the means for introducing heat is part of a pressurizer system, and wherein the means for introducing heat is reactivated to control an operating pressure of the nuclear reactor vessel after the reactor core has achieved criticality. 14. The apparatus of claim 12, wherein the means for introducing heat is deactivated after the primary coolant has achieved an operating temperature associated with a low power steady state condition of an initialized reactor core. 15. The apparatus of claim 12, wherein the one or more control rods are configured to be at least partially withdrawn from the reactor core, and wherein the heat is introduced to the primary coolant prior to withdrawing the control rods. 16. The apparatus of claim 12, wherein a difference in liquid density of the primary coolant in the riser and at the means for removing heat results in a circulation of the primary coolant through the reactor core prior to the initialization. 17. The apparatus of claim 11, wherein the means for removing heat is located above the elevation where the heat is introduced into the riser. 18. The apparatus of claim 11, wherein the nuclear reactor vessel comprises a pressurized vessel, wherein the reactor core is located in the pressurized reactor vessel, and wherein the means for introducing heat comprises one or more heaters located external to the pressurized reactor vessel. 19. The apparatus of claim 18, wherein the means for introducing heat further comprises one or more nozzles operatively connected to the one or more heaters, wherein the one or more nozzles are at least partially located within the riser, and wherein the one or more nozzles introduce the heat directly to the primary coolant located within the riser. 20. The apparatus of claim 11, wherein the means for introducing heat comprises one or more nozzles that are at least partially located in the riser and are configured to introduce the heat directly to the primary coolant located within the riser.