Abstract To obtain the change law of oxidation characteristics of unloaded coal at different burial depths, the experimental coal samples were loaded and unloaded with various degrees of stress according to the in situ stress characteristics of the Changcun coal mine in China. Through low-field nuclear magnetic resonance (LNMR) and temperature-programmed experiments, the change law of oxidation characteristic parameters and pore structure parameters of unloaded coal under different stresseswas tested. The main conclusions are obtained through the analysis of the experimental results. ①With increasing burial depth, the oxidation reaction products of unloaded coal under different stresses regularly changed. ② With increasing burial depth, the oxidation characteristics of unloaded coal at different burial depths gradually increased and gradually slowed down after 1200-1600 m, and the concept of the "critical depth" of unloaded coal oxidation characteristics was proposed. ③ With increasing burial depth, the porosity of the unloaded coal body gradually decreased, the number of micropores increased, and the number of small pores, mesopores and macropores gradually decreased. It is further concluded that the difference in oxidation characteristics of unloaded coal at different depths was caused by the change in the number of micropores.
Understanding microstructural changes in nuclear fuels under different irradiation conditions is important as it directly affects thermal, oxidation and mechanical properties and thereby reactor safety and longevity. This work focuses on the effect of temperature on the evolution of extended defects in uranium dioxide. In-situ transmission electron microscopy (TEM) annealing of proton irradiated UO2 was performed at different temperatures: 900 °C, 1100 °C and 1300 °C, 1 hour for each temperature. Microstructural evolution in terms of dislocation loops and voids were captured using in-situ TEM during annealing at different temperatures. Post-annealing at each temperature, detailed characterization using relrod dark field, bright field, and underfocus-overfocus imaging in TEM were used to identify faulted loops, perfect dislocation loops, and voids, respectively. Dislocation loops showed migration, disappearance, coalescence and loop-line interactions which significantly contributed to the recovery process during annealing at temperatures of 1100 °C and 1300 °C. Small voids were observed at 900 °C (diameter ~ 0.5-1.5 nm) and grew rapidly during 1300 °C annealing (diameter ~ 1-3 nm). Void growth was attributed to vacancy absorption, Ostwald ripening and void coalescence mechanisms. Void nucleation was unique to in-situ TEM annealing due to free surface effect as ex-situ annealing confirmed no voids at 1300 °C. The dislocation loop and voids behavior during in-situ TEM annealing were compared with rate theory and cluster dynamics prediction.
This study explores the influence of a repeated mining process on an upper coal pillar in a close coal seam group. The pillar's breaking and instability processes are emphasized, and the influence of fracture development on the oxidation and spontaneous combustion of coal pillars is revealed. A numerical simulation is used to simulate the dynamic evolution characteristics of stress, displacement of the upper coal pillar, and the numerical results elucidate that the mining of the lower adjacent coal seam is a pressure relief process for the upper coal pillar. The theoretical length of the fracture along the strike of the upper coal pillar is also obtained for the upper coal pillar.
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