Patent Number: 
Section: claims

1. A method of operating a nuclear power plant that includes a pressurized water reactor (PWR), a steam generator, a turbine, an electric generator, and a condenser, the method comprising:operating the PWR to heat primary coolant flowing through a nuclear reactor core comprising fissile material immersed in the primary coolant water;operating the steam generator to convert secondary coolant feedwater to steam using primary coolant water heated by the operating PWR;operating the turbine by flowing steam from the steam generator through the turbine and then through the condenser;driving the electric generator using the turbine to generate electricity;conveying the generated electricity to an electrical switchyard; andresponsive to a station blackout, transitioning the nuclear power plant to an island mode over a transition time interval by transition operations including:at the beginning of the transition time interval, disconnecting the electric generator from the electrical switchyard and opening a bypass valve to convey bypass steam flow from the steam generator to the condenser wherein the bypass steam flow does not flow through the turbine;after opening the bypass valve, gradually closing the bypass valve over the transition time interval; andgradually reducing the thermal power output of the PWR over the transition time interval,wherein the transition time interval comprises at least a plurality of minutes. 2. The method of claim 1 further comprising:concurrently with conveying the generated electricity to the electrical switchyard, also conveying the generated electricity to an electrical power system of the nuclear power plant; andin the transitioning, continuing to convey the generated electricity to the electrical power system of the nuclear power plant after the disconnecting of the electric generator from the electrical switchyard. 3. The method of claim 2 wherein the gradual reducing comprises gradually reducing the thermal power output of the PWR over the transition time interval such that the thermal power output of the operating PWR at the end of the transition time interval is 20% or less of the thermal power output of the operating PWR before the transition time interval. 4. The method of claim 2 wherein the gradual closing of the bypass valve over the transition time interval comprises:controlling the bypass valve over the transition time interval to convey bypass steam flow from the steam generator to the condenser that is sufficient to enable said gradual reducing of the thermal power output of the PWR over the transition time interval without approaching steam generator or pressurizer safety valve setpoints and while continuing to convey the generated electricity to the electrical power system of the nuclear power plant after the disconnecting of the electric generator from the electrical switchyard. 5. The method of claim 1 wherein the transition operations further include:at the beginning of the transition time interval, opening a supplemental bypass valve to convey supplemental bypass steam flow from the steam generator to a feedwater system of the PWR wherein the supplemental bypass steam flow does not flow through the turbine and does not flow through the condenser. 6. The method of claim 5 wherein the transition operations further include: after opening the supplemental bypass valve, gradually closing the supplemental bypass valve over the transition time interval. 7. The method of claim 1 wherein the transition operations do not include venting the steam from the steam generator to atmosphere. 8. The method of claim 1 wherein the operating of the steam generator includes:disposing the steam generator inside a pressure vessel of the PWR; andflowing the secondary coolant feedwater into a feedwater inlet of the PWR and through the steam generator where primary coolant water heated by the operating PWR heats the secondary coolant feedwater to convert the secondary coolant feedwater to steam that exits a steam outlet of the PWR. 9. The method of claim 1 wherein the gradual closing of the bypass valve over the transition time interval results in the bypass valve being fully closed at the end of the transition time interval. 10. A method operating in conjunction with a nuclear power plant comprising a pressurized water reactor (PWR) operating to heat primary coolant water, a steam generator using the heated primary coolant water to convert secondary coolant feedwater to steam, a turbine driven by steam from the steam generator and operatively connected with an electric generator, and a condenser connected with the turbine to condense steam after flowing through the turbine, the method comprising:transitioning the nuclear power plant to an island mode over a transition time interval by transition operations including:responsive to detecting a station blackout, electrically isolating the nuclear power plant and opening a bypass valve to convey bypass steam flow from the steam generator to the condenser without flowing through the turbine;after opening the bypass valve, gradually closing the bypass valve over the transition time interval; andgradually reducing the thermal power output of the PWR over the transition time interval,wherein the transition time interval comprises at least a plurality of minutes. 11. The method of claim 10 wherein the transition operations further include:responsive to detecting the station blackout, opening a supplemental bypass valve to convey supplemental bypass steam flow from the steam generator to a feedwater system supplying the secondary coolant feedwater to the steam generator;wherein the supplemental bypass steam flow does not flow through the turbine and does not flow through the condenser. 12. The method of claim 10 wherein the transition operations do not include venting steam from the steam generator to atmosphere. 13. The method of claim 10 wherein the transition operations further include conveying electricity generated by the generator to an electrical power system of the nuclear power plant during the transition time interval, the method further comprising:after the transition time interval, continuing to convey electricity generated by the generator to an electrical power system of the nuclear power plant. 14. The method of claim 10 wherein the gradual reducing of the thermal power output of the PWR over the transition time interval comprises:gradually reducing the thermal power output of the PWR over the transition time interval to a level that is 20% or less of the thermal power output of the PWR prior to detecting the station blackout.