  This paper presents a detailed study of the electron degeneracy and nonlinear
screening effects which play a crucial role in the validity of Salpeter's
weak-screening model. The limitations of that model are investigated and an
improved one is proposed which can take into account nonlinear screening
effects. Its application to the solar pp reaction derives an accurate screening
enhancement factor and provides a very reliable estimation of the associated
neutrino flux uncertanties.
