  One of the challenges that must be overcome in order to determine the
neutrino mass hierarchy using reactor neutrinos is the theoretical uncertainty
in the unoscillated reactor neutrino spectrum: this is one of the reasons why,
recently, it was proposed to add a near detector to the JUNO experiment. A
model-independent treatment of the spectrum uncertainty will be discussed, as
well as the effect that it will have on the final result. Moreover, since the
neutrino spectrum depends on the chemical composition of the fuel, the spectra
at the near and far detectors will be different, because they will receive
neutrinos from different cores. Taking into account the time evolution of the
chemical composition of the fuel in the reactor core, it is possible to
reconstruct the far detector spectrum from the near detector data. We will show
that the method used to reconstruct the spectrum can affect sensitivity to the
mass hierarchy, however if the near detector is large enough the difference
will be negligible.
