  In this work we present an accurate parameterization of the anti-neutrino
flux produced by the isotopes 235U, 239Pu and 241Pu in nuclear reactors. We
determine the coefficients of this parameterization, as well as their
covariance matrix, by performing a fit to spectra inferred from experimentally
measured beta spectra. Subsequently we show that flux shape uncertainties play
only a minor role in the KamLAND experiment, however, we find that future
reactor neutrino experiments to measure the mixing angle $\theta_{13}$ are
sensitive to the fine details of the reactor neutrino spectra. Finally, we
investigate the possibility to determine the isotopic composition in nuclear
reactors through an anti-neutrino measurement. We find that with a 3 month
exposure of a one ton detector the isotope fractions and the thermal reactor
power can be determined at a few percent accuracy, which may open the
possibility of an application for safeguard or non-proliferation objectives.
