  It has been proposed to determine the mass hierarchy of neutrinos by
exploiting the beat between the oscillation frequencies corresponding to the
two neutrino mass squared differences. JUNO is based on this concept and uses a
large liquid scintillator detector at a distance of 53km from a powerful
nuclear reactor complex. We argue that the micro-structure present in
antineutrino fluxes from nuclear reactors makes it essential to experimentally
determine a reference spectrum with an energy resolution very similar to the
one of JUNO.
