  Recent measurements of the positron energy spectrum obtained from inverse
beta decay interactions of reactor electron antineutrinos show an excess in the
4 to 6 MeV region relative to current predictions. First-principle calculations
of fission and beta decay processes within a typical pressurized water reactor
core identify prominent fission daughter isotopes as a possible origin for this
excess. These calculations also predict percent-level substructure in the
antineutrino spectrum due to Coulomb effects in beta decay. Precise measurement
of this substructure can constrain nuclear reactor physics. The substructure
can be a systematic uncertainty for measurements utilizing the detailed
spectral shape.
