  In this paper we explore the possibility that isolated CO-WDs with mass
smaller than the Chandrasekhar limit may undergo nuclear runaway and SNa
explosion, triggered by the energy produced by under-barrier pycno-nuclear
reactions between carbon and light elements. Such reactions would be due to
left over impurities of the light elements, which would remain inactive until
the WDs transit from the liquid to the solid state. We devise a simple
formulation for the coulombian potential and the local density in a ionic
lattice affected by impurities and introduce it in the known rates of
pycno-nuclear reactions for multi-component plasmas. Our semi-analytical
results indicate that the energy generated by these pycno-nuclear reactions
exceeds the WD luminosity and provides enough energy to elementary cells of
matter to balance the energy cost for C-ignition at much younger ages than the
age of the Universe, even for WDs with masses as low as $\simeq 0.85\,
M_\odot$. A thermonuclear runaway may thus be triggered in isolated WDs. The
explosion would occur from few hundred thousand to a few million years after
the WD formation in the mass interval $0.85 - 1.2\, M_\odot$.
