Patent Number: 
Section: claims

1. A method, comprising:activating a heating system prior to withdrawing one or more control rods associated with an initialization of a reactor core to achieve reactor criticality, wherein the reactor core is at least partially surrounded by a primary coolant that feeds into a riser located in a nuclear reactor vessel;introducing heat to the primary coolant, in response to activating the heating system, at one or more insertion points located within the riser at an elevation above the reactor core;circulating the primary coolant through the reactor core while the primary coolant is heated to an initial operating temperature; andwithdrawing, at least partially, the one or more control rods from the reactor core after the primary coolant is heated to the initial operating temperature. 2. The method of claim 1, further comprising deactivating the heating system, wherein the one or more control rods are withdrawn from the reactor core after the heating system is deactivated. 3. The method of claim 2, further comprising reactivating the heating system to control a reactor operating pressure after the reactor core has achieved criticality. 4. The method of claim 2, wherein the heating system is deactivated after the primary coolant has achieved an operating temperature associated with a low power steady state condition of the reactor core. 5. The method of claim 1, wherein the heating system comprises one or more electrical heaters located above the reactor core, and wherein the heaters are configured to generate the heat introduced into the primary coolant. 6. The method of claim 1, wherein the heating system introduces the heat to the primary coolant while the one or more control rods are completely inserted in the reactor core. 7. The method of claim 1, wherein the reactor core is located in the nuclear reactor vessel, wherein the heating system comprises one or more heaters located external to the nuclear reactor vessel, and wherein the one or more heaters are configured to heat liquid. 8. The method of claim 7, wherein the heating system further comprises one or more nozzles operatively connected to the one or more heaters, and wherein the one or more nozzles introduce the heat via the heated liquid inserted directly into the primary coolant. 9. The method of claim 1, wherein the heating system comprises one or more nozzles that introduce the heat via heated liquid inserted directly into the primary coolant located within the riser. 10. The method of claim 9, wherein the heating system comprises one or more heaters located in an upper head of 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 upper head via one or more extraction lines. 11. The method of claim 1, further comprising removing heat from the primary coolant with a heat exchanger, wherein the primary coolant is circulated through the reactor core in response to a difference in liquid density of the primary coolant in the riser and at the heat exchanger prior to withdrawing the one or more control rods from the reactor core. 12. The method of claim 11, wherein the heat exchanger is located above the elevation where the heat is introduced into the riser. 13. A method, comprising:activating a heating system prior to initializing a reactor core, wherein the reactor core is at least partially surrounded by a primary coolant that feeds into a riser located in a nuclear reactor vessel, and wherein at least a portion of the riser is located at an elevation that is above the reactor core;introducing heat to the primary coolant, in response to activating the heating system, at one or more insertion points located within the portion of the riser;circulating the primary coolant through the reactor core while the primary coolant is heated to an initial operating temperature; andinitializing the reactor core after the primary coolant is heated to the initial operating temperature, to achieve reactor criticality. 14. The method of claim 13, wherein the reactor core is initialized by at least partially withdrawing one or more control rods from the reactor core. 15. The method of claim 14, wherein the heat is introduced to the primary coolant prior to withdrawing the one or more control rods from the reactor core. 16. The method of claim 13, further comprising:heating the primary coolant to an initial operating temperature prior to initializing the reactor core; anddeactivating the heating system after the primary coolant is heated to the initial operating temperature. 17. The method of claim 16, wherein the reactor core is initialized after deactivating the heating system. 18. The method of claim 16, wherein the reactor core is initialized prior to deactivating the heating system. 19. The method of claim 13, further comprising removing heat from the primary coolant by a heat sink to create a natural circulation of the primary coolant through the reactor core, wherein the elevation that the heat is introduced into the riser is located between the heat sink and the reactor core. 20. The method of claim 13, wherein the nuclear reactor vessel comprises a pressurized reactor vessel, wherein the reactor core is located in the pressurized reactor vessel, and wherein the heating system comprises one or more heaters located external to the pressurized reactor vessel.