Detailed energy spectra have been calculated with the codes CASMO-4 and BOXER for neutron reaction rates in individual regions of a Westinghouse SVEA-96+ BWR fuel assembly immersed in water at room temperature, as investigated in Core 1A of the LWR-PROTEUS Phase I experimental program. First, results in 39 energy groups from the two codes were compared for 123 regions to characterise the lattice heterogeneity and to identify spectral trends qualitatively. Second, the various regions were classified into 4-5 broad spectral categories for quantitative comparisons. Third, results were collapsed to 2 groups, and then finally to 1 group, in order to obtain the principal neutron balance components, as also to derive and compare integral parameters such as the multiplication factor and average number of neutrons emitted per fission. These detailed spectral heterogeneity studies have led to the interpretation of certain trends in the prediction of experimental pin power distribution maps, which were not recognised earlier, providing a clear physical explanation for the observed discrepancies. [All rights reserved Elsevier]