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Theoretical and Computational Works on Oxide Nuclear Fuel Materials
Published in Masato Kato, Masahiko Machida, Materials Science and Fuel Technologies of Uranium and Plutonium Mixed Oxide, 2023
Nuclear fuels display different evolutional behaviors related to fission products and crystalline defects produced under irradiation conditions. Thus, the calculation of the release rate of fission gases, swelling rate of the cladding cavity, etc. is an important topic in the studies tackling nuclear fuel performance modeling. In such situations, the most crucial modeling issue is the determination of rates like the change rates of reactant or product concentrations in chemical reactions [69]. This means that the rate theory is originated from the chemical reaction rate theory on the basis. Generally, the chemical reaction rate can be given by microscopic calculations using the molecular orbital calculations of the reactant and product molecules in the liquid and gas phases. The rate theory can then be extended to more complex situations in nuclear fuels by similarly giving the rate constants based on the microscopic scheme and other calculation schemes. The rate theory is so mature that the kinetic theory of gases (collision theory) and quantum mechanics (transition state theory) are used to give the rate constants in complex reaction behaviors. In these rate constant determinations, one of the most crucial points is understanding their process evolutions in detail. Nowadays, advanced high-precision microscopic calculations can also be applied to obtain the constants.
Interfacial Catalysis at Oil/Water Interfaces
Published in Alexander G. Vdlkdv, Interfacial Catalysis, 2002
The rates of most organic reactions increase with increasing temperature as expected from the transition-state theory. Therefore, increased temperature is likely to be considered for PTC systems that have slow organic phase reactions. However, in PTC reactions the effect of temperature is complicated by the thermal stability of the catalyst. Quaternary ammonium and other onium salts usually decompose at high temperatures 120-150∘C under neutral conditions and at lower temperatures 50-70∘C in the presence of concentrated NaOH(aq). The stability of complex formation of polyether catalyst with salts decreases with increasing temperature and thus reduces the catalytic activity of polyethers [54]. Microwave irradiation is a good method of choice for heating in a PTC reaction [55]. Under microwave irradiation, in the PTC reaction of o - and p-chloronitronenzene with ethanol in the presence of NaOH(aq), a 144- to 240 -fold increase in the reaction rate was observed due to the enormous increase in the reactivity of ethoxide ion resulting from the dehydration effect of the irradiation [56].
Mechanisms of N2 Formation from Armchair Configurations with Different Dinitrogen Active Sites During Coal Pyrolysis
Published in Combustion Science and Technology, 2023
Tingting Jiao, Pengzheng Shi, Wenguang Du, Shoujun Liu, Ju Shangguan
According to the transition state theory, the elementary reaction with the highest activation energy (rate-determining step) determines the overall rate of the reaction. The rate-determining steps of R1 and R2 during pyrolysis are IM4 → IM5（ΔE = 273.26 kJ·mol −1）and IM6 → IM7（ΔE = 273.13 kJ·mol −1）. Both of which are the conversion of the dinitrogen six-membered ring to the dinitrogen four-membered ring and the energy barriers of the two reactions are comparable.