  We model the radial accretion of radiation on Primordial Black Holes (PBH) by
numerically solving Einstein's equations coupled to an ultrarelativistic ideal
gas with equation of state $p=\rho/3$. We calculate the final mass of a black
hole by the integration of the accreted radiation energy density during the
leptonic era between $t\sim10^{-4}s$ to $t\sim 10^2s$ after the Big Bang. Our
results indicate that small PBHs with initial masses between $10^{-4}$ to
$1M_{\odot}$ may grow up to hundreds of solar masses, and thus can be SMBH
seeds. On the other hand, PBHs formed at $t\sim 1s$ with initial mass between
900 and $\sim 980M_{\odot}$, by the time $t\sim 100s$ show masses of $10^4$ to
$10^6M_{\odot}$ which are masses of seeds or already formed SMBHs. The fact
that we consider only radial flow implies that our results work well as
limiting cases, and it is expected that under more general scenarios the
accretion rates may change significantly. Nevertheless we show that it is
possible that SMBHs can be PBHs that grew due to the accretion of radiation.
