Patent Number: 
Section: claims

1. A method for determining a critical effective k at an off-rated core state of a nuclear power plant, the method comprising:determining for the off-rated core state a control rod density, a percent core power, a gadolinium reactivity worth, a doppler reactivity worth, and a xenon reactivity worth responsive to a control rod pattern, a reactor power plan including the off-rated core state, and a reference effective k;calculating a change in an effective k from the reference effective k at the off-rated core state responsive to two or more parameters at the off-rated core state selected from the group consisting of the control rod density, the percent core power, the gadolinium reactivity worth, the doppler reactivity worth, and the xenon reactivity worth; andgenerating the critical effective k at the off-rated core state responsive to the change in the effective k from the reference effective k at the off-rated core state. 2. The method of claim 1, wherein the calculating a change in the effective k step includes determining a correlation quantifying the change in the effective k from the reference effective k at the off-rated core state based on at least one of a percent core power at the off-rated core state, a change in the gadolinium reactivity worth, a change in the doppler reactivity worth, a change in the xenon reactivity worth, and a change in the control rod density. 3. The method of claim 2, wherein the determining the correlation includes at least one of,determining at the off-rated core state a change in the doppler reactivity worth from a reference doppler reactivity worth associated with the reference effective k;determining at the off-rated core state a change in the xenon reactivity worth from a reference xenon reactivity worth associated with the reference effective k;determining at the off-rated core state a change in the gadolinium reactivity worth from a reference gadolinium reactivity worth associated with the reference effective k; anddetermining at the off-rated core state a change in the control rod density from a reference control rod density associated with the reference effective k. 4. The method of claim 2, wherein the determining the correlation includes identifying a coefficient for each correlation as a function of an exposure at the off-rated core state, a plant type, and a type of off-rated plant operation. 5. The method of claim 4, wherein the identifying a coefficient includes identifying an empirical correlation polynomial including a summary set of each correlation and coefficient reflecting the change in the effective k from the reference effective k at the off-rated core state, wherein the empirical correlation polynomial provides a model for changes to effective k for various off-rated core states of a nuclear core. 6. The method of claim 5, wherein the identifying the empirical correlation polynomial includes selecting a subset of the correlations within the empirical correlation polynomial based on the off-rated core state, a predetermined plant type, and a type of off-rated plant operation associated with the off-rated core state, wherein the determining the effective k change is responsive to the selected subset of correlations. 7. The method of claim 6 wherein selecting a subset of correlations includes creating a separate model for calculating the change in the effective k from the reference k for each off-rated core state based on the type of off-rated plant operation and for a startup off-rated plant operation, based also on the predetermined plant type. 8. The method of claim 6, further comprising:measuring an effective k during an off-rated core state;comparing the determined critical effective k to the measured effective k; andselecting a plant type from the group consisting of xenon driven and gadolinium driven in response to the comparing,wherein selecting the subset of correlations is responsive to the selected plant type. 9. The method of claim 8 wherein the nuclear power plant is a first nuclear power plant and wherein selecting the power plant type for the first nuclear power plant is performed during a core modeling of the first nuclear power plant, further comprising a second nuclear power plant, wherein selecting a power plant type for the second nuclear power plant is during a core modeling of the second nuclear power plant. 10. The method of claim 6 wherein selecting the subset of correlations is responsive to a type of off-rated operation selected from the group consisting of beginning of cycle, in cycle start-up, power maneuver down power, power maneuver up power, rod exchange sequence down power, and rod exchange sequence up power. 11. The method of claim 6 wherein the off-rated plant operation associated with the off-rated core state is a beginning of cycle and the predetermined plant type is xenon driven, the selected subset of correlations are responsive to the set of parameters consisting of the change in the control rod density, the percent core power, and the change in xenon reactivity worth. 12. The method of claim 6 wherein the off-rated plant operation associated with the off-rated core state is a beginning of cycle and the predetermined plant type is a xenon driven plant type, the selected subset of correlations are responsive to the set of parameters consisting of the change in the control rod density, the percent core power, the change in the gadolinium reactivity worth, and the change in the doppler reactivity worth. 13. The method of claim 6 wherein the off-rated operation type is a down power maneuver, the selected subset of correlations responsive to the set of parameters consisting of the change in the control rod density, the percent core power, the change in the xenon reactivity worth, a change in the doppler reactivity worth, a change in the gadolinium reactivity worth. 14. The method of claim 6 wherein the off-rated operation type is an up power maneuver, the selected subset of correlations responsive to the set of parameters consisting of the change in the control rod density, the percent core power, the change in the xenon radioactivity worth, the change in the gadolinium reactivity worth, and the change in the doppler reactivity worth. 15. The method of claim 2 wherein the reactor power plan includes an exposure defining, at least in part, the off-rated core state. 16. The method of claim 1 wherein the reference effective k is selected from the group consisting of a design basis effective k for the beginning of cycle startups and a last recorded rated effective k for the in cycle startups and power maneuvers. 17. The method of claim 1, further comprising determining a core coolant rate for the off-rated core state responsive to the generated critical effective k, the control rod pattern, and the reactor power plan. 18. The method of claim 1 wherein generating the critical effective k at the off-rated core state includes adding the change in the effective k at the off-rated core state to the reference k at the off-rated core state. 19. The method of claim 1, further comprising:determining the control rod density, the percent core power, the gadolinium reactivity worth, the doppler reactivity worth, and the xenon reactivity worth for each of a plurality of off-rated core states responsive to the control rod pattern plan, the reactor power plan including the plurality of off-rated core states, and one or more reference effective k's associated with the plurality of off-rated core states;calculating the change in the effective k from an associated reference effective k at each of the plurality off-rated core states responsive to two or more parameters at the off-rated core states selected from the group consisting of the control rod density, the percent core power, the gadolinium reactivity worth, the doppler reactivity worth, and the xenon reactivity worth; andgenerating the critical effective k at each of the off-rated core states responsive to the associated change in the effective k for each off-rated core state. 20. The method of claim 19, further comprising determining a core coolant rate plan responsive to the generated critical effective k's for each off-rated core state, the control rod pattern, and the reactor power plan. 21. A method for modeling critical effective k's at a plurality of off-rated core states in an off-rated plant operation of a reactor in a nuclear power plant, the method comprising:estimating, for a plurality of off-rated core states, a control rod density, a gadolinium reactivity worth, a Doppler reactivity worth, and a xenon reactivity worth responsive to a control rod pattern, a reactor power plan defining the off-rated core states and one or more reference effective k's;calculating a change in the control rod density, a change in the gadolinium reactivity worth, a change in doppler reactivity worth, and a change in xenon reactivity worth, each change being calculated relative to the corresponding received reference value and associated with each off-rated core state;determining a plurality of correlations affecting a neutron balance within the reactor at each off-rated core state responsive to the percent core power, the change in the gadolinium reactivity worth, the change in the doppler reactivity worth, the change in the xenon reactivity worth, and the change in the control rod density;determining a change in the effective k in response to the determined correlations;determining an estimated critical effective k as a function of the determined change in effective k for each off-rated core state;comparing an actual critical effective k for one or more of the off-rated core states to the estimated critical effective k for the corresponding off-rated core states;selecting a type of plant from the group consisting of a xenon driven plant and a gadolinium driven plant, responsive to the comparing;identifying a plurality of coefficients wherein at least one coefficient is identified for each of the determined correlations as a function of an exposure at the off-rated core state, the selected plant type, and a type for the off-rated plant operation; andselecting a subset of the correlations and coefficients in response to the selected plant type and the type of off-rated plant operation. 22. The method of claim 21, further comprising:receiving the control rod pattern, the reactor power plan defining the plurality of off-rated core states for the off-rated reactor operation, the one or more reference effective k's, a reference control rod density, a reference gadolinium reactivity worth, a reference doppler reactivity worth and a reference xenon reactivity worth;measuring a plurality of parameters during one or more off-rated reactor operations of the reactor including an actual gadolinium reactivity worth, an actual xenon reactivity worth, an actual doppler reactivity worth, an actual control rod density, and an actual percent core power; anddetermining the actual critical effective k for one or more of the off-rated core states responsive to measuring the plurality of parameters. 23. The method of claim 22, further comprising:receiving second control rod pattern, a second reactor power plan defining a second plurality of off-rated core states for a particular type of off-rated reactor operation and the off-rated core states, a second plurality of reference effective k's, a second reference control rod density, a second reference gadolinium reactivity worth, a second reference doppler reactivity worth and a second reference xenon reactivity worth;predicting, for one or more of the second off-rated core states in the second reactor power plan, a change in the effective k from a corresponding second reference effective k, the predicting being responsive to the selected subset of correlations and coefficients; andgenerating, for each one or more predicted off-rated core state, a second critical effective k responsive to each predicted change in effective k from the second reference effective k and one or more of the received second reference control rod density, second reference gadolinium reactivity worth, second reference doppler reactivity worth, and second reference xenon reactivity worth. 24. The method of claim 23, further comprising modeling a core coolant flow rate plan responsive to generating the critical effective k's for each off-rated core state, the second control rod pattern, and the second reactor power plan.