  We investigate the volume growth of ionized regions around UV photon sources
with the WENO algorithm, which is an effective solver of photon kinetics in the
phase space described by the radiative transfer equation. We show that the
volume growth rate, either of isolated ionized regions or of clustered regions
in merging, generally consists of three phases: fast or relativistic growth
phase at the early stage, slow growth phase at the later stage, and a
transition phase between the fast and slow phases. We also show that the volume
growth of ionized regions around clustered sources with intensity $\dot{E}_i$
($i=1, 2, ...$) would have the same behavior as a single source with intensity
$\dot{E}=\sum_i\dot{E}_i$, if all the distances between nearest neighbor
sources $i$ and $j$ are smaller than $c(t^i_c+t^j_c)$, $t^i_c$ being the time
scale $t_c$ of source $i$. Therefore, a tightly clustered UV photon sources
would lead to a slow growth of ionized volume. This effect would be important
for studying the redshift-dependence of 21cm signals from the reionization
epoch.
