Within the framework of the state independent pairing-plus-octupole model in the cluster representation, heavy nuclei in the actinide region are shown to undergo phase transitions of the second kind, between an asymmetric phase and a symmetric one, as the excitation energy is raised. For a description of the nucleus at high excitation energies, we have made use of the Hartree-Fock-Gorkov approximation. The valence states responsible for the fission process are restricted within one major shell in each cluster, because the vicinity of the transition points is of interest. The situation is complicated, since there also occurs the superconducting-normal phase transition. The calculated transition points are in qualitative agreement with experiment, although more detailed and systematic experiments are required.