  Recently, three successful antineutrino experiments (Daya Bay, Double Chooz,
and RENO) measured the neutrino mixing angle theta_{13}; however, significant
discrepancies were found, both in the absolute flux and spectral shape. Much
effort has been expended investigating the possible reasons for the
discrepancies. In this study, Monte Carlo-based sampling was used to evaluate
the fission fraction uncertainties. We found that fission cross-section
uncertainties are an important source of uncertainty for $^{235}$U, $^{239}$Pu,
and $^{241}$Pu, but for $^{238}$U, elastic and inelastic cross-sections are
more important. Among uncertainty related to manufacturing parameters, fuel
density is the main uncertainty; however, the total manufacturing uncertainty
was very small. The uncertainties induced by burnup were evaluated through the
atomic density uncertainty of the four isotopes. The total fission fraction
uncertainties from reactor simulation were 0.83%, 2.24%, 1.79%, and 2.59%
for235U, 238U, 239Pu, and 241Pu, respectively, at the middle of the fuel cycle.
The total fission fraction uncertainty was smaller than the previously derived
value of 5\%. These results are helpful for studying the reactor antineutrino
anomaly and precisely measuring the antineutrino spectrum in the Daya Bay
antineutrino experiment.
