  We used one-dimensional radiative-transport radiation hydrodynamical (RHD)
simulations to investigate the formation of clumping in freely-expanding
supernova ejecta due to the radioactive heating from the Ni56 -> Co56 -> Fe56
decay sequence. The heating gives rise to an inflated Nickel bubble, which
induces a forward shock that compresses the outer ambient gas into a shell. The
radiative energy deposited by the radioactivity leaks out across the shock by
radiative diffusion, and we investigate its effect on the evolution of the
ejecta structure. Compared to the hydrodynamical adiabatic approximation with
gamma =4/3, the preshock gas becomes accelerated by the radiation outflow. The
shock is thus weakened and the shell becomes broader and less dense. The
thickness of the shell takes up <~ 4 % of the radius of the bubble, and the
structure of the shell can be approximately described by a self-similar
solution. We compared the properties of the shell components with those of the
ejecta clumps indicated by our previous hydrodynamical simulations for the
later interaction of clumps with the outer supernova remnant. The high density
contrast across the shell, chi ~ 100, is compatible with that of ejecta clumps
as indicated for Tycho's knots, but there is insufficient dense gas to cause a
pronounced protrusion on the outline of a core collapse supernova remnant, like
the bullets in the Vela remnant.
