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[en] Highlights: • A frequency-domain model for a SCWR-M reactor core was developed. • Stability maps for SCWR-M core were constructed. • Sensitivities of several parameters were studied for system stability boundaries. • A time-domain model was developed and applied to nonlinear stability analysis. • A more reasonable system decay ratio was redefined. • Subcritical bifurcation phenomenon in SCWR-M core was studied. - Abstract: The supercritical water reactor (SCWR) is one of the most prominent Generation IV reactors due to its high efficiencies. However, the stability issues, which are mainly caused by the great changes of thermodynamic properties and transport properties of supercritical water near the pseudo-critical temperature, are a challenge to the system safety and must be studied carefully. This paper is focused on 1-D dynamic stability analysis of mixed-spectrum SCWR (SCWR-M) reactor core. To this end, a frequency-domain model has been developed for linear stability analysis, and marginal stability boundaries under both the fixed inlet flow boundary conditions and the fixed external pressures boundary conditions are generated, which indicate that the system normal operational condition is in stable regions. Parametric sensitivity studies in frequency domain have been carried out. Increasing the wall thermal conductivity and mass flows can increase system stability. The system is more stable if the thermal zone has a lower power fraction. System with the designed non-uniform axial power distribution is also more stable than with the uniform distribution. A time-domain model has also been developed for nonlinear analysis, and the system marginal stability boundaries calculated by this method is consistent with those by frequency domain method. The existence of transitional stable region has been observed. A more reasonable definition for system logarithmic decay ratio has been achieved. The SCWR-M core has a subcritical bifurcation characteristic under fixed external pressures boundary conditions, thus its dynamic behaviors are not only related to systematic parameters, but also to the amplitudes of perturbations.
[en] Radiation-induced grafting of styrene onto chitin and chitosan powder was performed at room temperature. The effect of various conditions such as absorbed dose, solvent and oxygen on grafting was investigated. The grafting yield increased with the increase in absorbed dose. At the same dose, the grafting yield of styrene on chitosan was higher than that on chitin. The grafting reaction was promoted in the presence of methanol, and oxygen delayed the grafting reaction but did not inhibit it completely. In order to study the mechanism of grafting reaction and analyze the grafted samples, the grafted products were extracted first by benzene, then they were hydrolyzed in the presence of acid and separated by thin layer chromatography (TLC). Three different kinds of polystyrene (PS), i.e. PS grafted onto chitin, PS embedded in chitin and PS in grafting solution were gained. The variation of their molecular weight (MW) and width index of molecular weight distribution (Mw/Mn) determined by gel permeation chromatography (GPC) at different grafting conditions is discussed
[en] The slight radiation-crosslinked CMC-Na as a substitute for hydrogel was prepared by gamma irradiation below gelation dose. The effects of various parameters such as absorbed dose, concentration of inorganic salts, pH, swelling temperature and swelling time on the swelling ratio in water were investigated in detail. This kind of slight crosslinked CMC-Na showed good water absorption below 60 deg. C, whereas, it became solution when heated up to 70 deg. C. Such CMC-Na gel is different from the true gel that is insoluble in boiled water; nevertheless, it can be used as hydrogel at room temperature and produced at low dose. Due to its low cost, it might be useful for its application in agriculture or others
[en] We introduce a new concept of sparsity for the stochastic elliptic operator −div(a(x,ω)∇(⋅)), which reflects the compactness of its inverse operator in the stochastic direction and allows for spatially heterogeneous stochastic structure. This new concept of sparsity motivates a heterogeneous stochastic finite element method (HSFEM) framework for linear elliptic equations, which discretizes the equations using the heterogeneous coupling of spatial basis with local stochastic basis to exploit the local stochastic structure of the solution space. We also provide a sampling method to construct the local stochastic basis for this framework using the randomized range finding techniques. The resulting HSFEM involves two stages and suits the multi-query setting: in the offline stage, the local stochastic structure of the solution space is identified; in the online stage, the equation can be efficiently solved for multiple forcing functions. An online error estimation and correction procedure through Monte Carlo sampling is given. Numerical results for several problems with high dimensional stochastic input are presented to demonstrate the efficiency of the HSFEM in the online stage
[en] Highlights: • A frequency-domain model for supercritical flow stability analysis was developed. • The stability maps for parallel-channel heat exchanging system were constructed. • The sensitivities of several parameters were studied for system stability boundary. • A time-domain model was developed and applied to nonlinear stability analysis. • The importance of the second eigenvalue has been studied. - Abstract: The flow in the core of supercritical water reactors (SCWRs) experiences drastic change in its thermodynamic properties and transport properties near the pseudo-critical temperature, thus the core flow may be susceptible to density wave oscillation instability, which is a challenge to the system safety and must be studied carefully. This work studies the stability characteristics of parallel-channel systems with heat exchanging, the prototype of which is originated from the thermal-spectrum zone assemblies of a newly designed mixed-spectrum SCWR (SCWR-M). A frequency-domain model has been developed for linear stability analysis, and marginal stability boundaries under several conditions are generated, which indicate that the system normal operational condition is in an absolute stable region. Decreasing the wall thermal conductivity can improve system stability while increasing mass flow is beneficial for the system stability. The system is not very sensitive to the axial power distributions. A one-dimensional time-domain model has also been developed for nonlinear analysis, and several transients with mass flow perturbations are calculated. The system marginal stability boundaries calculated by using frequency-domain and time-domain methods are in good agreement with each other. The existence of transitional stable region has been observed. A special case of parallel-channel systems with heat exchanging has been studied and achieved the conclusion that the second eigenvalue should be considered when studying the stability characteristics of complicated systems by using frequency-domain methods
[en] Sodium carboxymethyl cellulose (CMC) is a kind of degraded polymer under γ-irradiation. However, in this work, it has been found that CMC crosslinks partially to form hydrogel by radiation technique at more than 20% CMC aqueous solution. The gel fraction increases with the dose. The crosslinking reaction of CMC is promoted in the presence of N2 or N2O due to the increase of free radicals on CMC backbone, but gel fraction of CMC hydrogel is not high (<40%). Some important values related to this kind of new CMC hydrogel synthesized under different conditions, such as radiation yield of crosslinking G(x), gelation dose Rg, number average molecular weight of network Mc were calculated according to the Charlesby-Pinner equation. The results indicated that although crosslinked CMC hydrogel could be prepared by radiation method, the rate of radiation degradation of CMC was faster than that of radiation crosslinking due to the character of CMC itself. Swelling dynamics of CMC hydrogel and its swelling behavior at different conditions, such as acidic, basic, inorganic salt as well as temperature were also investigated. Strong acidity, strong basicity, small amount of inorganic salts and lower temperature can reduce swelling ratio
[en] Sodium carboxymethyl cellulose (CMC) is a kind of degraded polymer under irradiation. However, in this work, it can be found that CMC crosslink partially to form hydrogel by radiation technique at more than 20% CMC aqueous solution. The gel fraction increases with the dose. The crosslinking reaction of CMC is promoted in the presence of N2 or N2O due to the increasing of free radicals on CMC backbone, but gel fraction of CMC hydrogel isn't high (less than 40%). Some important values related to this kind of new CMC hydrogel synthesized at different conditions, such as radiation yield of crosslinking G(x), gelation dose Rg, number average molecular weight of network Mc is calculated according to Charlesky-Pinner equation. The results indicated that although crosslinked CMC hydrogel can be prepared by radiation method, the rate of radiation degradation of CMC is faster than that of radiation crosslinking due to the character of CMC itself. Swelling dynamics of CMC hydrogel and its swelling behavior at different conditions, such as acidic, basic, inorganic salt as well as temperature is investigated, too. Strong acidity, strong basicity, small amount of inorganic salts and lower temperature can reduce swelling ratio. Because of no toxicity and good biocompatibility of natural polymer, CMC based hydrogel will have wide application in biomedical materials. (author)
[en] Experimental advanced superconducting tokamak (EAST) is a new experiment equipment for doing controlled nuclear fusion research in China and ion cyclotron wave resonant heating (ICRH) is one of the most important methods to heat plasma in this equipment to achieve the high efficiency of wave heating, the antenna phase need be well controlled. A phase control system was designed to effectively regulate the phase relationship of the ICRH system, and the good wave heating was achieved. (authors)
[en] A series of Mn/ZSM-5 catalysts for the selective catalytic reduction of NO with NH3 was prepared by precipitation method at different calcination temperature. X-ray diffraction, X-ray photoelectron spectroscopy, H2 temperature-programmed reduction and N2 adsorption/desorption technologies were conducted to explore the effects of calcination temperature on the physical and chemical properties of Mn/ZSM-5 catalysts. Results suggested that when calcined at lower temperatures (<500 °C) MnOx existed in the form of Mn3O4 and amorphous MnO2 on the catalyst surface. However, when calcined at 600 °C Mn2O3 species which is unfavorable for the SCR process were formed and became the major phase at 700 °C. On the other hand, with the increase of calcination temperatures, the surface Mn concentration and the specific surface area of catalysts both decreased. The catalytic activity test indicated that the Mn/ZSM-5 catalyst calcined at 300 °C demonstrated the best performance for NO removal, with almost 100% NO conversion in the range of 150–390 °C. According to the characterization results, the enrichment of surface Mn, surface Mn3O4 and amorphous MnO2 species may account for its superior catalytic activity.
[en] Pd-Ce/γ-Al2O3-TiO2 catalysts were prepared by combined sol.gel and impregnation methods. Transmission electron microscopy, X-ray diffraction, H2-temperature-programmed reduction, O2-temperature-programmed desorption, and ethanol oxidation experiments were conducted to determine the properties of the catalysts. Addition of an optimal amount of Ce improved the performance of the Pd/γ-Al2O3-TiO2 catalyst in promoting the complete oxidation of ethanol. The catalyst with 1% Ce exhibited the highest activity, and catalyzed complete oxidation of ethanol at 175 .deg. C; its selectivity to CO2 reached 87%. Characterization results show that addition of appropriate amount of Ce could enrich the PdO species, and weaken the Pd-O bonds, thus enhancing oxidation ability of the catalyst. Meanwhile, the introduction of CeO2 could make PdO better dispersed on γ-Al2O3-TiO2, which is beneficial for the improvement of the catalytic oxidation activity