  Very massive 140-260 Msun stars can die as highly-energetic pair-instability
supernovae (PI SNe) with energies of up to 100 times those of core-collapse SNe
that can completely destroy the star, leaving no compact remnant behind. These
explosions can synthesize $0.1-30$ Msun of radioactive Ni56, which can cause
them to rebrighten at later times when photons due to Ni56 decay diffuse out of
the ejecta. However, heat from the decay of such large masses of Ni56 could
also drive important dynamical effects deep in the ejecta that are capable of
mixing elements and affecting the observational signatures of these events. We
have now investigated the dynamical effect of Ni56 heating on PI SN ejecta with
high-resolution two-dimensional hydrodynamic simulations performed with the
CASTRO code. We find that expansion of the hot Ni56 bubble forms a shell at the
base of the silicon layer of the ejecta about 200 days after the explosion but
that no hydrodynamical instabilities develop that would mix Ni56 with the
Si/O-rich ejecta. However, while the dynamical effects of Ni56 heating may be
weak they could affect the observational signatures of some PI SNe by diverting
decay energy into internal expansion of the ejecta at the expense of
rebrightening at later times.
