  The thermal structure of the post-shock region of a young supernova remnant
(SNR) is heavily affected by two main physical effects, the back-reaction of
accelerated cosmic rays (CRs) and the Rayleigh-Taylor (RT) instabilities
developing at the contact discontinuity between the ejecta and the shocked
interstellar medium (ISM). Here, we investigate the role played by both
physical mechanisms in the evolution of SNRs through detailed 3D MHD modeling.
Our model describes the expansion of the remnant through a magnetized ISM,
including consistently the initial ejecta clumping and the effects on shock
dynamics due to back-reaction of accelerated CRs. We discuss the role of the
initial ejecta clumpiness in developing strong instabilities at the contact
discontinuity which may extend upstream to the main shock and beyond.
