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Numerous electrochemistry reactions require the precise calculation of the ion solvation energy. Despite the significant progress in the first-principles calculations for crystals and defect formation energies for solids, the liquid system free energy calculations still face many challenges. Ion solvation free energies can be calculated via different semiempirical ways, e.g., using implicit solvent models or cluster of explicit molecule models; however, systematically improving these models is difficult due to their lack of a solid theoretical base. A theoretically sound approach for calculating the free energy is to use thermodynamic integration. Nevertheless, owing to the difficulties of self-consistent convergence in first-principles calculations for unphysical atomic configurations, the computational alchemy approach has not been widely used for first-principles calculations. This study proposes a general approach to use first-principles computational alchemy for calculating the ion solvation energy. This approach is also applicable for other small molecules. The calculated ion solvation free energies for Li+, Na+, K+, Be2+, Mg2+, and Ca2+ are close to the experimental results, and the standard deviation due to molecular dynamics fluctuations is within 0.06 eV. [ABSTRACT FROM AUTHOR]

Small reactors have the characteristics of modular, short construction period and flexible deployment, which can serve as a clean distributed energy source not only for supplying power but also for meeting various needs such as seawater desalination, heating supply for both residence and industry. The double cladding of an annular fuel rod can significantly improve its heat transfer capacity. This characteristic is helpful for designing reactors with smaller core volume, higher core safety and better economy. It is believed that the application of annular fuel in small reactors can fully show its advantages, and meet the requirements of China’s nuclear energy development strategy. Based on low-temperature nuclear heating reactors, a scheme design of annular fuel assemblies for small reactors has been carried out, which mainly considered the influence of factors such as the height to diameter ratio of the core and the array of fuel rods. Physical analysis of different fuel assemblies design schemes were conducted by using the CMS code, and annular fuel assembly with a 11×11 array of fuel rods was ultimately designed. The annular fuel assembly used for small reactors mainly consists of bottom nozzle, top nozzle, fuel rods, guide tubes, and spacer grids, the primary structure design of annular fuel assembly was conducted based on results of the mechanical analysis. The feasibility of annular fuel assemblies applied to different types of small reactors was demonstrated by reactor physics analysis, thermal-hydraulic analysis and safety analysis. The radial power distribution factor, enthalpy rise factor, hot channel factor, moderator temperature coefficient, Doppler temperature coefficient and other parameters were analyzed by using the SIMULATE5 code, which meet the design requirements. The thermal-hydraulic performance, including distribution of velocity field, pressure field, and temperature field of annular fuel assemblies for low-temperature heating reactors was evaluated by using the CFX program, including distribution of velocity field, pressure field, and temperature field. For the small marine reactor, thermal-hydraulic analysis was carried out by using SAAF code, and the changes of fuel rod radial temperature distribution, heat flux, minimum DNBR and other parameters were obtained, the results show that annular fuel has a high safety margin. For cores loaded with different fuel assemblies, small break loss of coolant accidents was analyzed by using the RELAP5 code, the results show that the safety margin of annular fuel used for small reactor is large. In conclusion, annular fuel rod has a completely new structure design with superior heat transfer performance. The relevant analysis results indicate that annular fuel assembly has good prospect when applied in small reactors, it can further improve the safety and economy of small reactors.

Annular fuel is a new type of pressurized water reactor fuel with inner and outer dual coolant channels. Selecting the actual reactor and annular fuel pilot assembly to form a mixed core to carry out irradiation test is one of the important links in the research and development of annular fuel. Through the operation test in the actual reactor, the structural design and thermal safety performance of the fuel assembly can be fully verified, laying the foundation for the subsequent construction of the annular fuel reactor and engineering applications. The annular fuel mixing core has open flow channels, and the cross flow characteristics have an important impact on the thermal safety of the original core. Therefore, a 3×3 annular fuel mixed core was established based on computational fluid dynamics method. Taking the local resistance characteristics of the two assemblies, the cross velocity field of the mixed core, the flow deviation between inlet and outlet of each assembly in the mixed core, and the flow deviation of each grid section as the evaluation indicators, the cross-flow characteristics of the annular fuel mixed core were analyzed in detail. The results show that the smaller equivalent diameter of the annular fuel outer channel has a retarding effect on the cross flow of the mixed core, so that the deflection effect of the original core mixing vane on the coolant is greatly weakened at the interface of the mixed core assemblies. It effectively eliminates the cross flow caused by the mixing vane. Meanwhile, under the mixed core condition, the coolant flow rate of the original core fuel is only in the 5th, 7th, and 9th grid sections, which is smaller than that of the annular fuel, and the rest of the grid sections are larger than the annular fuel. The maximum relative deviation of the flow rate of each spacer grid section in the axial direction is 12%, the average relative deviation of the flow rate in each spacer grid section is about 9%, and the relative deviation of the flow rate between inlet and outlet of each assembly is less than 1.8%. The coolant flow distribution of the mixed core rod bundle is uniform and there is no significant cross flow.

Annular fuel is a new type of fuel used in pressurized water reactor. The fuel pellets are annular shape and loaded between internal and external cladding. Annular fuel consists of two coolant channels, namely the internal channel and the external channel. The pellets can be cooled by both sides simultaneously. The spacer grid is one of the core components of annular fuel assembly, which has an important influence on flow and heat transfer characteristics. In this paper, the numerical calculation model of 13×13 annular fuel fullspan grid was established for straightthrough cell grid and externallyexpanded cell grid based on computational fluid dynamics (CFD) method. The flow and heat transfer characteristics of the two grids were studied. Due to the symmetrical structure of annular fuel, 1/8 external subchannels were selected to evaluate the uniformity of velocity distribution by calculating the standard deviation (σ) of velocity distribution. Three evaluation factors including swirl factor, cross flow factor and turbulence intensity factor were defined, and the mixing characteristics of the two kinds of grids were analyzed based on central sub-channel. The axial heat transfer characteristics of the two grids were evaluated by calculating the average Nusselt number (Nu) distribution of the angular sub-channel, the side sub-channel and the central sub-channel. The circumferential heat transfer characteristics of fuel rods were evaluated by calculating the circumferential Nu. The resistance characteristics of the spacer grid were evaluated by comparing the axial pressure distribution of annular fuel and the resistance coefficient of the spacer grid. By comparing and calculating the coolant flow distribution ratio (φ) between the external channel and the internal channel, the adjustment effect of two kinds of grids on the flow distribution ratio was evaluated. It shows that externallyexpanded cell grid can form a constricted orifice with throttling function, compared with the straightthrough cell grid. It has a good regulating effect on the resistance characteristics and flow distribution characteristics of annular fuel, and can significantly improve the distribution uniformity of flow and heat transfer parameters of external sub-channels. By adopting the externally-expanded cell grid, the velocity distribution uniformity in the external sub-channel is increased by 54%, the maximum relative deviation of the average Nu of each subchannel is reduced from 65% to 24%, and the maximum relative deviation of the circumferential Nu of the fuel rods is reduced from 35% to 24%, which significantly improves the distribution uniformity of the flow and heat transfer parameters in the subchannels. The externally expanded grid has a more significant mixing effect on the fluid, and the active distance reaches 35Dh, which has an enhanced effect on the subchannel heat transfer. In addition, the resistance coefficient of the external expansion grid is 45 times that of the straightthrough grid, and the flow distribution ratio is only 53% of that of the straight-through grid, which has a good adjustment effect on the resistance characteristics and flow distribution characteristics of the annular fuel.

The internally and externally cooled annular fuel is one of the innovative fuel geometry proposals for advanced PWR, which could provide a substantial increase of power density while maintaining or improving safety margins. The quenching behavior of annular fuel during reflood stage in LOCA is significantly different from that of cylindrical solid fuel, owing to the coupling effect of dual-cooling. However, the experimental study focusing on quenching characteristics in annular fuel geometry is very little. In the present study, a bottom reflood experimental study on quenching behavior in a 3×3 external and internal cooled annular rod bundle was carried out by using self-designed indirect-heating annular rods to set up the experimental apparatus. The typical physical process and the effects of key parameters on quench during reflooding were studied in detail. The results show that the flooding process in annular rod bundle is similar to that of cylindrical solid rod bundle. The quench front propagation and the variation of heat transfer mode of both external and internal surfaces are almost synchronous. However, it is observed that there is a temperature gradient between internal and external sides of annular rods at the same time. The quench front propagation velocity increases with reflood velocity and inlet subcooling, while decreases with the increase of peak cladding temperature and linear power density. In addition, the quench front propagation velocity at the downstream of spacer grid is found to increase under the conditions of low reflood velocity, low subcooling and high cladding temperature.