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 from each of a plurality of stored 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, and calculating an operating margin to the given operating limit based on the determined time time-dependent average bias value and time-dependent uncertainty value so as to satisfy the risk-tolerance level of the evaluated plant.