  Reactor antineutrino experiment are used to study neutrino oscillation,
search for signatures of nonstandard neutrino interaction, and monitor reactor
operation for safeguard application. Reactor simulation is an important source
of uncertainties for a reactor neutrino experiment. Commercial code is used for
reactor simulation to evaluate fission fraction in Daya Bay neutrino
experiment, but the source code doesn't open to our researcher results from
commercial secret. In this study, The open source code DRAGON was improved to
calculate the fission rates of the four most important isotopes in fissions,
$^{235}$U,$^{238}$U,$^{239}$Pu and $^{241}$Pu, and then was validated for PWRs
using the Takahama-3 benchmark. The fission fraction results are consistent
with those of MIT's results. Then, fission fraction of Daya Bay reactor core
was calculated by using improved DRAGON code, and the fission fraction
calculated by DRAGON agreed well with these calculated by SCIENCE. The average
deviation less than 5\% for all the four isotopes. The correlation coefficient
matrix between $^{235}$U,$^{238}$U,$^{239}$Pu and $^{241}$Pu were also studied
using DRAGON, and then the uncertainty of the antineutrino flux by the fission
fraction was calculated by using the correlation coefficient matrix. The
uncertainty of the antineutrino flux by the fission fraction simulation is
0.6\% per core for Daya Bay antineutrino experiment. The uncertainties source
of fission fraction calculation need further to be studied in the future.
