  We study interacting Majorana fermions in two dimensions as a low-energy
effective model of a vortex lattice in two-dimensional time-reversal-invariant
topological superconductors. For that purpose, we implement ab-initio quantum
Monte Carlo simulation to the Majorana fermion system in which the
path-integral measure is given by a semi-positive Pfaffian. We discuss
spontaneous breaking of time-reversal symmetry at finite temperature.
