Rayleigh-taylor overturn in supernova core collapse

We perform a two dimensional radiation diffusion coupled to hydrodynamics calculation of nonspherical instabilities in the collapsed core from a massive star. The core properties are taken from a one dimensional collapse calculated with detailed microphysics and neutrino transport. The shocked outer core [0.7 M/sub sun/< or approx. =M(r)1.3 M/sub sun/] is found to contain three subregions. The innermost is doubly diffusive (''neutron fingers'') unstable, the center subregion is dynamically unstable, and the outermost is stable. We find that the unstable part of the outer core overturns in approx.5 ms without disturbing the inner unshocked core of M< or approx. =0.7 M/sub sun/. This overturned outer core expands as a piston, creating an outgoing shock wave which may help power envelope ejection. This outer core overturn would seem to be a generic feature of core collapse which has heretofore been neglected