Patent Number: 
Section: claims

1. A stable startup system for a nuclear reactor comprising:a pressurized reactor vessel;a reactor core housed in the reactor vessel, wherein the reactor core is submerged in a primary coolant of the nuclear reactor, and wherein the reactor core is located beneath a riser;a heat sink configured to remove heat from the primary coolant after the primary coolant has passed through the riser of the reactor vessel; andan electrical heater configured to add heat to the primary coolant contained within the reactor vessel prior to an initialization of the reactor core to achieve reactor criticality, wherein the heat is added by the electrical heater into the riser at an insertion point that is below the heat sink to cause a density difference in the primary coolant that drives the primary coolant through the reactor core which occurs only by the use of natural circulation prior to the initialization of the reactor core, and wherein the insertion point is above the reactor core. 2. The system according to claim 1, wherein the heat sink comprises a heat exchanger of a secondary cooling system. 3. The system according to claim 1, wherein the heat sink is located within the reactor vessel at an elevation above the reactor core. 4. The system according to claim 1, wherein the elevation of the electrical heater is above the reactor core. 5. The system according to claim 1, wherein the electrical heater is located at least partially within a shroud that surrounds the reactor core. 6. The system according to claim 1, wherein the electrical heater is further configured to control pressure within the reactor vessel after the initialization of the reactor core. 7. A nuclear reactor module comprising:a pressurized reactor vessel containing primary coolant;a reactor core submerged in the primary coolant;a riser located above the reactor core;one or more electrical heaters configured to increase a temperature of the primary coolant of the nuclear reactor module prior to an initialization of the reactor core by adding heat to the primary coolant at an insertion point is located within the riser, wherein the insertion point is located above the reactor core;a heat exchanger configured to remove heat from the primary coolant after the primary coolant has passed through the riser, wherein the heat exchanger is located within the reactor vessel at an elevation that is above the insertion point within the riser that the heat is added, and wherein a difference in liquid density of the primary coolant within the riser and at the heat exchanger results in circulation of the primary coolant through the reactor core only by the use of natural circulation prior to the initialization of the reactor core;means for deactivating the one or more electrical heaters; andmeans for initializing the reactor core to achieve criticality. 8. The nuclear reactor module according to claim 7, wherein the one or more electrical heaters heat the primary coolant to an operating temperature that provides for the natural circulation of the primary coolant from the heat exchange to the one or more electrical heaters and through the reactor core. 9. The nuclear reactor module according to claim 8, wherein the reactor core is caused to go critical after the primary coolant reaches the operating temperature. 10. The nuclear reactor module according to claim 7, wherein the insertion point at which the one or more electrical heaters add the heat to the primary coolant is located below the heat exchanger. 11. The nuclear reactor module according to claim 7, wherein the one or more electrical heaters are further configured to control a pressure in the reactor vessel after the reactor core has gone critical. 12. A method of startup for a nuclear reactor comprising:activating a heating system to increase a temperature of a primary coolant of the nuclear reactor prior to an initialization of a reactor core located within a pressurized reactor vessel, wherein the heating system comprises one or more electrical heaters that add heat to the primary coolant at an insertion point that is located within a riser, and wherein the insertion point is located above the reactor core;removing heat from the primary coolant with a heat exchanger located within the reactor vessel at an elevation that is above the insertion point where the heat is added to the primary coolant in the riser,circulating the primary coolant through the reactor core, wherein a difference in liquid density of the primary coolant in the riser and at the heat exchanger results in circulation of the primary coolant through the reactor core only by the use of natural circulation prior to the initialization of the reactor core;deactivating the heating system; andinitializing the reactor core to achieve criticality. 13. The method according to claim 12, wherein the one or more electrical heaters are located within the reactor vessel. 14. The method according to claim 12, further comprising reactivating the heating system to control an operating pressure of the nuclear reactor after the rector core has achieved criticality. 15. The method according to claim 12, further comprising monitoring the temperature of the primary coolant, 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. 16. The method according to claim 12, wherein the one or more electrical heaters are located within the reactor vessel at an elevation that is above the reactor core. 17. The method according to claim 12, wherein the reactor core is initialized to achieve criticality after the heating system is deactivated. 18. The nuclear reactor module according to claim 7, wherein the reactor core is initialized after the one or more electrical heaters are deactivated.