The purpose of this technical note is to introduce a definition for breeding gain and other performance parameters for nuclear reactors and their associated fuel cycle. The newly proposed performance parameters have a more general nature than expressions currently in common use. Since the performance parameters require a weighting scheme, which expresses how individual isotopes contribute to the overall fuel cycle performance (breeding, transmutation, or otherwise) of the reactor, expressions are derived for the isotope weight factors. In this technical note, weighting schemes are introduced for a breeding fuel cycle and a transmutation fuel cycle, and the proposed definitions are applied to specific example calculations of a pressurized water reactor mixed oxide irradiation, a breeder reactor cycle, and a transmutation reactor cycle. It will be shown by an example that a net destruction of transuranic material will not always lead to a reduction of the decay heat released from the spent nuclear fuel. This effect is due to the buildup during irradiation of isotopes with a high decay heat release. With the general performance parameters defined in the present work, it is possible to more fully characterize (advanced) nuclear fuel cycles, incorporating long-term radioactivity in a straightforward manner.
