Two-particle correlation from a relativistic fluid with a first order phase transition

We numerically calculate two-particle correlation functions of CERN-SPS 158 A GeV Pb+Pb central collisions based on a (3+1)-dimensional relativistic hydrodynamics with a first order phase transition. We analyze the pair momentum dependence of pion source sizes extracted from the Yano-Koonin-Podgoretski\u{\i} (YKP) parametrization which is expected to give the source sizes directly. We find that, even in the case of the first order phase transition, the collective expansion and surface dominant freeze-out of the fluid naturally lead to the opaque source for which the interpretation of the temporal source parameter as the emission duration breaks down.