  Optically thick energy dominated plasma created in the source of Gamma-Ray
Bursts (GRBs) expands radially with acceleration and forms a shell with
constant width measured in the laboratory frame. When strong Lorentz factor
gradients are present within the shell it is supposed to spread at sufficiently
large radii. There are two possible mechanisms of spreading: hydrodynamical and
thermal ones. We consider both mechanisms evaluating the amount of spreading
that occurs during expansion up to the moment when the expanding shell becomes
transparent for photons. We compute the hydrodynamical spreading of an
ultrarelativistically expanding shell. In the case of thermal spreading we
compute the velocity spread as a function of two parameters: comoving
temperature and bulk Lorentz factor of relativistic Maxwellian distribution.
Based on this result we determine the value of thermal spreading of
relativistically expanding shell. We found that thermal spreading is negligible
for typical GRB parameters. Instead hydrodynamical spreading appears to be
significant, with the shell width reaching $\sim10^{10}$ cm for total energy
$E=10^{54}$ erg and baryonic loading $B=10^{-2}$. Within the fireshell model
such spreading will result in the duration of Proper Gamma-Ray Bursts up to
several seconds.
