The rapid development of a new generation of X-ray radiation sources providing ultrashort (from atto- to femtoseconds) pulses creates unique possibilities for generating high energy density states of matter. Instruments, like free-electron lasers (FELs) produce pulses of very high intensity and allow to extend the optical studies of radiation induced phase transitions of solids. The excitation of solid materials with x-ray femtosecond pulses offers a number of advantages over irradiation with femtosecond optical lasers. First of all the energy deposition process is not influenced by optical nonlinearities i.e. multiphoton absorption and free carrier absorption. Moreover the absorption depth can be varied over many orders of magnitude. E.g. for silicon it changes from a few nanometres up to hundreds of microns. Therefore, ultrashort X-ray pulses allow the preparation of well-defined excitation conditions in variable sample volumes and thus to study the energy transport processes. Single shot irradiations of the Si flat mirror were performed at SACLA FEL facilities in the range of 5.5 – 12 keV photon energies, at normal and grazing incidence angles. Observed radiation induced structural modification of materials is related to melting of silicon and its resolidification and a have threshold nature. The experimental damage thresholds are the highest in case of the irradiations below the critical angles. In these cases the energy density of the radiation absorbed at the sample’s surface can reach above a melting threshold (approx. 1eV/atom) without any structural modification. This may be explained by the transport of the energy out of the excitation volume (limited to the absorption skin depth) by hot electrons on the time scales shorter than the one typical for the electron-phonon coupling (~2 ps for Si). Modelling of the energy transport by ballistic electrons has been performed by means of the PENELOPE simulation code.