  We derive thermodynamic Bethe ansatz equations for a model describing an
$U\to\infty$ Anderson impurity embedded in a BCS superconductor. The equations
are solved analytically in the zero-temperature limit, T=0. It is shown that
the impurities depress superconductivity in the Kondo limit, however at T=0 the
system remains in the superconducting state for any impurity concentration. In
the mixed-valence regime, an impurity contribution to the density of states of
the system near the Fermi level overcompensates a Cooper pairs weakening, and
superconductivity is enhanced.
