  The spontaneous fission lifetime of 264Fm has been studied within nuclear
density functional theory by minimizing the collective action integral for
fission in a two-dimensional quadrupole collective space representing
elongation and triaxiality. The collective potential and inertia tensor are
obtained self-consistently using the Skyrme energy density functional and
density-dependent pairing interaction. The resulting spontaneous fission
lifetimes are compared with the static result obtained with the minimum-energy
pathway. We show that fission pathways strongly depend on assumptions
underlying collective inertia. With the non-perturbative mass parameters, the
dynamic fission pathway becomes strongly triaxial and it approaches the static
fission valley. On the other hand, when the standard perturbative cranking
inertia tensor is used, axial symmetry is restored along the path to fission;
an effect that is an artifact of the approximation used.
