  We report new shell-model calculations of the isospin-symmetry-breaking
correction to superallowed nuclear beta decay. The most important improvement
is the inclusion of core orbitals, which are demonstrated to have a significant
impact on the mismatch in the radial wave functions of the parent and daughter
states. We determine which core orbitals are important to include from an
examination of measured spectroscopic factors in single-nucleon pick-up
reactions. We also examine the new radiative-correction calculation by Marciano
and Sirlin and, by a simple reorganization, show that it is possible to
preserve the conventional separation into a nucleus-independent inner radiative
term and a nucleus-dependent outer term. We tabulate new values for the three
theoretical corrections for twenty superallowed transitions, including the
thirteen well-studied cases. With these new correction terms the corrected Ft
values for the thirteen cases are statistically consistent with one another and
the anomalousness of the 46V result disappears. These new calculations lead to
a lower average Ft value and a higher value of Vud. The sum of squares of the
top-row elements of the CKM matrix now agrees exactly with unitarity.
