Patent Number: 
Section: claims

1. A method of determining an operating margin to a given operating limit in a nuclear reactor, comprising:accessing operational plant data during a current operating cycle from an on-line nuclear reactor plant to be evaluated,simulating reactor operation off-line using the operational plant data to generate simulation results including predicted dependent variable data representative of the given operating limit,normalizing the predicted dependent variable data for evaluation with normalized historical dependent variable data stored from each of a plurality of operating cycles of nuclear reactor plants having a similar plant configuration to the reactor plant being evaluated,determining a time-dependent average bias value for the predicted dependent variable data using the normalized historical dependent variable data,determining a time-dependent uncertainty value for the predicted dependent variable data using the normalized historical dependent variable data,obtaining a risk-tolerance level for the plant being evaluated related to a probability of an event not occurring during a given period in the current operating cycle, andcalculating an operating margin to the given operating limit based on the determined time-dependent average bias value and time-dependent uncertainty value so as to satisfy the risk-tolerance level of the evaluated plant. 2. The method of claim 1, wherein accessing operational data further includesretrieving plant operating conditions representing independent variables and monitored thermal limit data representing actual dependent variable data at one or more exposure points in the current operating cycle, andstoring the independent variables and actual dependent variable data in a database. 3. The method of claim 2, wherein the independent variables are selected from the group consisting of reactor power level, core flow rate, rod pattern, control blade sequence, mechanical conditions, cycle exposure, enrichment properties, and gadolinium properties. 4. The method of claim 1, wherein the given operating limit is one of a thermal limit selected from the group consisting of a Maximum Fraction of Limiting Power Density (MFLPD) limit, a Maximum Average Planar Linear Heat Generation Rate (MAPLHGR) limit, a Maximum Fraction of Limiting Critical Power Ratio (MFLCPR) limit, a power-related limit such as an eigenvalue for the evaluated plant, and other industry-standard, power-related plant limits or thermal limits on nuclear fuel. 5. The method of claim 2, wherein simulating reactor operation off-line includescreating a simulator input file which models the plant being evaluated using the independent variables for simulating reactor operation in an off-line simulation program, andexecuting the off-line simulation program to generate the predicted dependent variable data representing the given operating limit. 6. The method of claim 1, whereina bias value for the historical dependent variable data stored from each of the operating cycles at a plurality of exposure points in each of the respective operating cycles has been calculated in advance, the bias value at a given exposure point in a given stored operating cycle representing a difference between the measured and the predicted operating limit at that exposure point for a given historical cycle, anddetermining the time-dependent average bias value for the predicted dependent variable data includes:calibrating the normalized historical dependent variable data to force the bias values for the historical dependent variable data to a current exposure point in the operating cycle of the reactor plant being evaluated, anddetermining the time-dependent average bias value as an average of all the bias values of the normalized historical dependent variable data at each of the exposure points in each of the stored operating cycles, as calibrated from the current exposure point in the operating cycle of the reactor plant being evaluated. 7. The method of claim 6, wherein determining the time-dependent uncertainty value for the predicted dependent variable data includes determining a standard deviation of each of the bias values of the historical dependent variable data at each of the exposure. 8. The method of claim 1, further comprising:determining revised plant operating parameters based on the calculated operating margin for the given operating limit, andchanging plant operational conditions of the on-line reactor based on the revised plant operating parameters. 9. The method of claim 8, wherein the revised plant operating parameters are determined using a method selected from the group consisting of manually and using an optimization algorithm using the calculated operating margin. 10. The method of claim 1, wherein the calculation of the operating margin for the given operating limit is repeated continuously or at a given periodicity. 11. The method of claim 1, wherein the historical dependent variable data is filtered to incorporate historical dependent variable data from similar plant operation styles to the plant being evaluated. 12. The method of claim 1, wherein the historical dependent variable data is correlated by a method selected from the group consisting of least squares and neural networks.