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An analytical Detailed Loop Model (DLM) has been developed to analyze the performance of solar thermosiphon water heaters with heat exchangers in storage tanks. The model has been used to study the performance of thermosiphons as a function of heat exchanger characteristics, heat transfer fluids, flow resistances, tank stratification, and tank elevation relative to the collector. The results indicate that good performance can be attained with these systems compared to thermosiphons without heat exchangers.

An analytical Detailed Loop Model (DLM) has been developed to analyze the performance of solar thermosiphon water heaters with heat exchangers in storage tanks. The model has been used to study the performance of thermosiphons as a function of heat exchanger characteristics, heat transfer fluids, flow resistances, tank stratification, and tank elevation relative to the collector. The results indicate that good performance can be attained with these systems compared to thermosiphons without heat exchangers.

ABSTRACTRadiation damage has been studied in natural rock salt from various localities, including potential repository sites. In the 100 to 300 C range the damage consists of point defects, primarily F-centers, and colloidal metal sodium particles. With increasing dose the F-centers grow to a saturation level, reached at 107 –108 rad, that decreases with increasing temperature to a negligible level at 300 C. Colloid concentration vs. irradiation-time curves follow nucleation and growth curves accurately described by C tn, or C(dose)n, relations at large irradiation times. For fourteen samples,n = 1.85± 0.18 but the values of C vary by a factor of more than 103. The constant C is related to the sample strain, the impurity and void content, dose rate, and possibly other factors. The currently available data indicate that rock salt adjacent to radioactive waste canisters, at a temperature of 150 C, will contain between 0.01 and 10 mole percent of sodium metal when the total dose reaches 1010 rad.

ABSTRACTRadiation damage has been studied in natural rock salt from various localities, including potential repository sites. In the 100 to 300 C range the damage consists of point defects, primarily F-centers, and colloidal metal sodium particles. With increasing dose the F-centers grow to a saturation level, reached at 107 –108 rad, that decreases with increasing temperature to a negligible level at 300 C. Colloid concentration vs. irradiation-time curves follow nucleation and growth curves accurately described by C tn, or C(dose)n, relations at large irradiation times. For fourteen samples,n = 1.85± 0.18 but the values of C vary by a factor of more than 103. The constant C is related to the sample strain, the impurity and void content, dose rate, and possibly other factors. The currently available data indicate that rock salt adjacent to radioactive waste canisters, at a temperature of 150 C, will contain between 0.01 and 10 mole percent of sodium metal when the total dose reaches 1010 rad.

The heat deposition in a blanket is concentrated near the first wall. Uniform liquid-metal velocity in a self-cooled blanket is unattractive, because it leads to low mixed-mean temperature rise through the blanket and reduced power conversion efficiency. The objective of MHD flow control is to use the electromagnetic forces to produce a non-uniform velocity distribution which gives a uniform temperature distribution over the thickness of the blanket. Three methods of MHD flow control are presented here and the MHD pressure drops corresponding to the three methods are compared. One of the methods, although successful at achieving nonuniform velocity profiles, permits a large circulation of electric current which produces a high pressure drop. The analytical results do not indicate a clear choice between the other two methods. The analytical results do point to possible difference in heat transfer performance with the two methods.

The heat deposition in a blanket is concentrated near the first wall. Uniform liquid-metal velocity in a self-cooled blanket is unattractive, because it leads to low mixed-mean temperature rise through the blanket and reduced power conversion efficiency. The objective of MHD flow control is to use the electromagnetic forces to produce a non-uniform velocity distribution which gives a uniform temperature distribution over the thickness of the blanket. Three methods of MHD flow control are presented here and the MHD pressure drops corresponding to the three methods are compared. One of the methods, although successful at achieving nonuniform velocity profiles, permits a large circulation of electric current which produces a high pressure drop. The analytical results do not indicate a clear choice between the other two methods. The analytical results do point to possible difference in heat transfer performance with the two methods.

Abstract Lignin is considered as a renewable and sustainable resource for producing value-added aromatic chemicals and functional carbon materials. Herein, we develop a one-step catalyst-free depolymerization strategy to convert lignin into aryl monomers and carbon nanospheres simultaneously. Importantly, microwave-assisted depolymerization (MAD) in conjunction with dichloromethane (CH2Cl2) vapors is developed. The total mass yield of guaiacols reached the highest amount of 225.1 mg/g at 600 °C, and the highest yields of phenols (49.0 mg/g) and aromatic hydrocarbons (155.1 mg/g) were obtained at 700 °C. Hydrogen radicals and hydrogen chloride (HCl) are in-situ formed from CH2Cl2, significantly decreasing the activation barrier and reforming pyrolysis vapors to promote the formation of aryl monomers. Interestingly, uniform carbon nanospheres with an average size of 140 nm were produced as co-products at 700 °C. The microwave “hot-spots”, allied with the continuous surface erosion and the decrease in surface energy of lignin-derived carbon precursors by CH2Cl2 vapor, can be considered the driving force for the ultimate formation of carbon nanospheres. The CH2Cl2/MAD system produces aryl monomers (26.8 wt% yield) and carbon nanospheres (36.6 wt% yield) at 700 °C. We provide a facile, intriguing and scalable approach to convert lignin to valuable aryl monomers and sustainable carbon materials that can be applied in the chemistry, energy and environmental fields.

Abstract Lignin is considered as a renewable and sustainable resource for producing value-added aromatic chemicals and functional carbon materials. Herein, we develop a one-step catalyst-free depolymerization strategy to convert lignin into aryl monomers and carbon nanospheres simultaneously. Importantly, microwave-assisted depolymerization (MAD) in conjunction with dichloromethane (CH2Cl2) vapors is developed. The total mass yield of guaiacols reached the highest amount of 225.1 mg/g at 600 °C, and the highest yields of phenols (49.0 mg/g) and aromatic hydrocarbons (155.1 mg/g) were obtained at 700 °C. Hydrogen radicals and hydrogen chloride (HCl) are in-situ formed from CH2Cl2, significantly decreasing the activation barrier and reforming pyrolysis vapors to promote the formation of aryl monomers. Interestingly, uniform carbon nanospheres with an average size of 140 nm were produced as co-products at 700 °C. The microwave “hot-spots”, allied with the continuous surface erosion and the decrease in surface energy of lignin-derived carbon precursors by CH2Cl2 vapor, can be considered the driving force for the ultimate formation of carbon nanospheres. The CH2Cl2/MAD system produces aryl monomers (26.8 wt% yield) and carbon nanospheres (36.6 wt% yield) at 700 °C. We provide a facile, intriguing and scalable approach to convert lignin to valuable aryl monomers and sustainable carbon materials that can be applied in the chemistry, energy and environmental fields.

In 1960, Hansen analyzed the problem of assembling fissionable material in the presence of a weak neutron source. Using point kinetics, he derived the weak source condition and analyzed the consequences of delayed initiation during ramp reactivity additions. Although not clearly stated in Hansen`s work, the neutron source strength that appears in the weak source condition actually corresponds to the equivalent, fundamental-mode source. In this work, they describe the concept of an equivalent, fundamental-mode source and they derive a deterministic expression for a factor, g*, that converts any arbitrary source distribution to an equivalent, fundamental-mode source. They also demonstrate a simplified method for calculating g* in subcritical systems. And finally, they present a new experimental method that can be employed to measure the equivalent, fundamental-mode source strength in a multiplying assembly. They demonstrate the method on the zero-power, XIX-1 assembly at the Fast Critical Assembly (FCA) Facility, Japan Atomic Energy Research Institute (JAERI).