The paper discusses the phenomena and processes occurring in the industrial, power and research uraniumgraphite nuclear reactors at heat removal trouble, clad damage and technological channel dewatering. The authors describe the accidental situations leading to build-up of the fragments of irradiated nuclear fuel in graphite stack of the Magnox, HTR, AGR reactors and B and N reactors in Hanford. The processes occurring at serious fuel element bridging which consisting of metal uranium are best documented. Moreover, the paper analyzes the possible chemical compounds of metal uranium with fuel element case, technological channel material, steam gas mixture, water and graphite stack. The authors adduce data on qualitative composition of compounds that could contain accidental release of nuclear fuel in the graphite stack. Such data were obtained by thermodynamic method based on the principle of the entropy maximum. It was shown that at reaching the melting temperature of metal uranium various solid and gaseous chemical compounds such as oxides, hydrates, carbides and others could form, but their concentration and amount depend on temperature inside of the graphite stack. The paper also shows the mathematical model of dewatering and steam lock of technological channel of the B and N type reactors. Aforementioned model follows the non-stationary heat transfer equation with Dirichlet and Neumann boundary conditions. Solution algorithm of this equation is realized in Ansys Fluent and Matlab. It was clearly shown that UO2 is the main product that build-up in the graphite stack at the heat removal trouble, clad damage and technological channel dewatering. Furthermore, it is possible to form the gaseous products such as CO, CH4 and H2, and it is unlikely to form the compound of metal uranium and hydrogen (tritium). The results could be used in choosing the way of treatment of irradiated graphite in particular at the disassembling graphite stack.