  We have developed a Hauser-Feshbach fission fragment decay model, HF$^3$D,
which can be applied to the statistical decay of more than 500 primary fission
fragment pairs (1,000 nuclides) produced by the neutron induced fission of
$^{235}$U. The fission fragment yield $Y(A)$ and the total kinetic energy TKE
are model inputs, and we estimate them from available experimental data for the
$^{235}$U(n$\rm_{th}$,f) system. The model parameters in the statistical decay
calculation are adjusted to reproduce some fission observables, such as the
neutron emission multiplicity $\overline{\nu}$, its distribution $P(\nu)$, and
the mass dependence $\overline\nu(A)$. The calculated fission product yield and
isomeric ratio are compared with experimental data. We show that the calculated
independent fission product yield $Y_I(A)$ at the thermal energy reproduces the
experimental data well, while the calculated isomeric ratios tend to be lower
than the Madland-England model prediction. The model is extended to higher
incident neutron energies up to the second chance fission threshold. We
demonstrate for the first time that most of the isomeric ratios stay constant,
although the production of isomeric state itself changes as the incident energy
increases.
