  Unitarity corrections to several electromagnetic processes in collisions of
relativistic heavy nuclei are considered. They are due to the unitarity
requirement for the S-matrix and correspond to the exchange of light-by-light
scattering block between colliding nuclei. We obtain improved results for the
corrections to e+e- and mu+mu- pair production, as well as new results for
unitarity corrections to the production of photons via virtual Compton and
virtual Delbruck scattering. These corrections can be numerically large; e.g.,
the mu+mu- pair production cross section is reduced by about 50% and the
nuclear bremsstrahlung by about 15 to 20%.
