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 l
ight 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 $0.85 - 1.2, M_odot$.
