1. A method of determining a core design for a nuclear reactor, comprising the steps of: defining a set of limits applicable to determining a core design, the set of limits including a target hot excess reactivity constraint to be satisfied over a given core energy cycle and a given desired control blade definition that is set for the cycle; generating a reference core design based on the defined limits, the reference core design including an initial loading pattern of current fresh fuel bundles arranged in a plurality of fuel locations within the reference core design and the given control blade definition set for the reference core design; selecting a unique subset of fresh fuel bundles to be evaluated in the reference core design; and performing a first iteration to determine an accepted core design, the first iteration comprising replacing, at each fuel location, at least one of the current fresh fuel bundles with at least one of the selected fresh fuel bundles; simulating reactor operation on the reference core design to produce a plurality of outputs, each output corresponding to the reference core design containing one or more of the selected fresh fuel bundles; and ranking the outputs based on the defined set of limits, the highest ranked output representing an accepted core design for the nuclear reactor that satisfies the target hot excess reactivity constraint with the given desired control blade definition.