When a semiconductor or a dielectric is irradiated with ultrashort intense X-ray pulse, several processes occur: first the photoabsorption brings the electron subsystem out of equilibrium, bringing valence or deeper shells electrons into high energy states of the conduction band. Then, secondary electron cascading promotes further electrons of the valence to conduction band increasing their number there. These electrons also influence the atomic motion, modifying the interatomic forces. This process is known as a nonthermal melting. It can turn a material into a new phase state on ultrashort timescales. Recently developed hybrid model for treating all of these processes with different computational tools was reported in [N. Medvedev et al, New J. Phys. 15, 015016 (2013)]. Based on this model, we present here further investigations of nonthermal processes occurring in diamond under irradiation with a FLASH pulse of 10 fs FWHM and 92 eV photon energy. It is shown that the diamond turns into graphite under such irradiation, independently whether constant pressure or constant volume modeling is performed. However, for the latter case, the time of the nonthermal phase transition is longer (few tens of fs for P=const vs few hundreds of fs for V=const) and the damage threshold is slightly higher (0.69 eV/atom vs 0.74 eV/atom, correspondingly).