  The first solids that form as a cooling white dwarf (WD) starts to
crystallize are expected to be greatly enriched in actinides. This is because
the melting points of WD matter scale as $Z^{5/3}$ and actinides have the
largest charge $Z$. We estimate that the solids may be so enriched in actinides
that they could support a fission chain reaction. This reaction could ignite
carbon burning and lead to the explosion of an isolated WD in a thermonuclear
supernova (SN Ia). Our mechanism could potentially explain SN Ia with
sub-Chandrasekhar ejecta masses and short delay times.
