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The steam generator tube rupture (SGTR) accident is one of the design basis accidents of lead-cooled fast reactor (LFR). In the event of such an accident, a subcooled water jet is generated in the lead pool of primary circuit, which forms a complicated distribution of the liquid metal-gas two-phase flow. The detection of the two-phase interface and migration of steam bubbles is an significant part of reactor safety analysis. Due to the opacity, corrosiveness, high temperature of liquid metal, and the current limitations of measurement methods involving liquid metal-gas two-phase flow, a diamond-like carbon (DLC) coated wire-mesh sensor (WMS) based on electrical principles was proposed in this paper. The sensor could achieve the image of the liquid metal-gas two-phase distribution inside the flow channel, while preventing the corrosion failure of the sensor. Based on the COMSOL software, a numerical model of the DLC coated WMS was established. Selecting the materials of two phases as liquid GaInSn alloy and air, the simulation of the electric field in the liquid metal environment containing a single bubble was performed, and an electric signal value matrix representing the liquid metal-gas two-phase was output. The signal matrix was further normalized through the linear relation to obtain the two-phase distribution image, which verified the application feasibility of the coated WMS in liquid metal. In addition, a prototype of the DLC coated WMS was manufactured, and the numerical simulation results were experimentally validated in order to demonstrate the rationality of the numerical model. Furthermore, the influence of different structural parameters on the measurement accuracy of coated WMS was studied in detail by changing the transverse interval, axial interval, electrode diameter, coat thickness, and the size of air bubble. The simulation results show that the electric field distribution in the liquid metal-gas two-phase environment differs significantly from that in conventional fluid medias such as water-gas due to the high relative permittivity of liquid metal. On the other hand, considering the resolution of the WMS and the phenomenon of electric signal crosstalk, the transverse interval of electrode should be selected between 2-3 mm, and the axial interval should be selected between 1.5-2 mm to achieve high measurement accuracy. The coat thickness and electrode diameter have little influence on the measurement accuracy, but to reduce the intrusion effect of the WMS, the electrode diameter should be selected between 0.4-0.5 mm and the coat thickness should not exceed half of the electrode radius. This study could guide the practical application of DLC coated WMS in the liquid metal-gas two-phase environment and provide a technical basis for the analysis of multi-phase distribution under SGTR accidents.

The power system of heat pipe cooled nuclear reactor can be divided into dynamic conversion system and static conversion system. Thermoelectric generator (TEG) is a typical static conversion device, which can directly convert heat energy into electric energy. The combination of heat pipe cooled nuclear reactor and TEGs can bring many advantages, such as compact structure, high inherent safety, no auxiliary mechanical equipment and low noise. However, the maximum core temperature could exceed 1 000 K, and ordinary thermoelectric generators can not operate in such high temperature. To solve this problem, the use of segmented thermoelectric generator (STEG) in the power system of heat pipe cooled nuclear reactor is a method worthy of research. STEG can play an important role in the heat pipe cooled nuclear reactor. But there is not enough literature to study the application of STEG in heat pipe cooled nuclear reactor. STEG usually splices two or three thermoelectric materials with the highest power generation efficiency in different temperature ranges. In this way, STEG can obtain a high thermoelectric conversion efficiency in a wide temperature range (usually above 600 K). Therefore, a kind of STEG for heat pipe cooled nuclear reactor was designed. In this paper, a conceptual design of a heat pipe cooled nuclear reactor with STEGs was proposed. The single STEG and single channel model were explored by COMSOL software. A three-dimensional finite element model of STEG’s performance in heat pipe cooled nuclear reactor was established. STEG was designed to compose of bismuth telluride and skutterudite, and its geometric structure and performances were optimized. Numerical simulation was carried out under steady-state conditions to determine the optimal STEG’s geometry. Then, the optimized STEG was connected with the heat pipe to form a single channel model for simulation to explore its performances. When the temperature difference reaches 550 K, the maximum STEGs’ output power reaches 0.374 W, and the maximum conversion efficiency is 15.75%. In single channel model, when the heating power is 300 W, the maximum STEGs’ output power reaches 47.29 W, and the maximum conversion efficiency is 15.63%. The simulation also show that the maximum stress of single STEG and single channel model are 270 MPa and 547.5 MPa, respectively. And the maximum stress appears on the copper conductor near the hot side. This paper could provide a preliminary basis for the numerical simulation of the combination of STEG and heat pipe cooled nuclear reactor. This paper can also be used as a reference for the thermoelectric conversion of heat pipe cooled nuclear reactors.

Due to the advantages of long service life, compact structure, strong environmental adaptability and high inherent safety, the heat pipe reactor power supply system is worth studying. The NUSTER2.0 dual-mode heat pipe reactor non-nuclear prototype built by Xi’an Jiaotong University adopts the integrated design of “simulated reactor core-high temperature heat pipe-Stirling generator-thermoelectric power generation”. The combination of the dynamic module coupled with Stirling and the static module coupled with thermoelectric power generation device can meet the diversified demands of energy. In this paper, the steady-state experiment of NUSTER2.0 was carried out to verify the design, and the system coupling simulation of the prototype was carried out by COMSOL multiphysics to analyze its operation and mechanical characteristics. In the 15 kW thermal power steady-state experiment, the generating power of the dynamic module is 2 019 W, and the static module is 493.2 W. The overall thermoelectric conversion efficiency is 16.7%, which verifies the engineering feasibility of the design. The system simulation model is coupled with the thermal resistance network model of high temperature heat pipe, the equivalent thermal conductivity model of thermoelectric power generation device and the Stirling adiabatic analysis program. The heat source of fuel rod and the heat sink of cooling water were simulated by heat flux boundary and convective boundary respectively. Under 15 kW thermal power, the steady-state simulation results are in good agreement with the experimental results on both the heat pipe temperature distribution and the main operating parameters of the dynamic and static modules, while the maximum relative error is 6.73%, which verifies that the coupled model is reliable. The variable thermal power steady-state simulation determines that the limit due to Stirling is 16.5 kW, and predicts that the static module will reach the peak efficiency near 25 kW. The system start-up transient simulation obtains the start-up characteristics under the 3.5-10.15 kW stepped power platform. The prototype start-up strategy based on the combination of power platform cold start and thermal insulation restart was also proposed. The thermal-mechanical coupling simulation indicates that the stress danger zone of the system is the coupling between the heat pipe and assembly matrix. Under 15 kW thermal power, the maximum stress at the coupling of heat pipe-core matrix and heat pipe-collector matrix are 160 MPa and 97 MPa, respectively. In the experiment, the self-adjusting function of the Stirling insulation ring can significantly relieve the 366 MPa stress between it and the Stirling matrix. The thermal expansion rate of the reactor core is 1.21%, which will cause negative feedback on the reactivity. The simulation results provide the numerical basis for the operation safety analysis and further experimental design of the NUSTER2.0 prototype.

The third generation reactors have characteristics of safety and efficiency, and the C-shaped heat exchanger is the main equipment in the design of the third generation reactor, which is widely used in the design of the passive residual heat removal system. It is of great significance on reactor safety to study on the flow and heat transfer process of Cshaped heat exchanger. To study the coupling flow and heat transfer characteristics of its primary and secondary side, and also to obtain flow distribution mechanism in heat exchange pipes for optimizing structure design, a fullsize physical model of Cshaped heat exchanger was established in this paper. It included inletoutlethousings, inletoutlet pipes and 500 heat transfer pipes as the primary side, and the heat transfer model of the secondary side was also established. Numerical simulation studies under different thermal and geometric conditions were conducted by the software FLUENT to obtain the laws of temperature, speed, pressure and flow distribution between the heat transfer tubes in primary side. At the same time, the influence of inlet flow, inlet temperature, inlet position to the flow distribution and flow resistance distribution was analyzed, and the affecting mechanism of floating force and inertial force on flow distribution was also analyzed. The calculation results show that the current design of the heat transfer tube is uneven in flow distribution, the flow of inner side is low, the flow of outer side is high, and the inner layer has the phenomenon of reverse flow. The uneven flow distribution in the current design is mainly caused by thermosiphon (density difference causing gravity pressure difference). This phenomenon is more obvious when inlet flow is low or the inlet temperature is high. During the range of the thermal parameters in this paper, the phenomenon of the thermosiphon is always dominated in the influencing mechanisms of flow distribution. Three inlet pipe positions of 0° (existing design), 90° and 180° were compared and it is found that the position of the inlet pipeline is not obvious for flow unevenness. When the flow is low, the flow distribution unevenness of the existing design (overentering and downout) is optimal on the contrary. In further study, it is possible to explore measures to reduce flow distribution unevenness. The measures include changing the connection of the heat transfer tubes and the headers, changing the distribution of the heat transfer coefficient of the primary and secondary side, and changing the installation method of the inlet headers, etc.

Spent fuel reprocessing is one of the most critical links in the process of closed nuclear fuel cycle process. Carrying out computer simulation research on this process and establishing its dynamic simulation model can carry out process analysis and optimization with less human and material resources and in a more general scope. In this paper, by analyzing the mass transfer process of the dissolution reaction between spent fuel pellets and nitric acid and the mass transfer process of solute in the solution, a dynamic dissolution model of spent fuel pellets dissolution process was obtained based on mass balance calculation. The model includes three parts: chemical reaction module, solute transfer module and solution flow field analysis module, which are respectively used to calculate the mass transfer caused by chemical reaction, the mass transfer caused by the convection and diffusion of liquid in the liquid phase and the analysis of the flow state of fluid in the solution system. The turbulent flow state of the dissolved liquid in the dissolver was simulated by Comsol software, and the flow field cloud diagram of the dissolved liquid flow was obtained, which provided the analysis data of the liquid phase flow state for the established dynamic dissolution model of the spent fuel pellet dissolution process. The numerical simulation program for solving the real-time simulation model of spent fuel pellet dissolution process was established by using Matlab software, and the feeding dynamic simulation of spent fuel dissolution process in continuous dissolver was carried out under the process conditions of 90 ℃, nitric acid feed concentration of 6 mol/L, feed flow rate of 11 L/min and return flow of 35 L/min. The change trend of the simulation results is basically consistent with the actual process, and the dissolution process needs about 2 000 minutes to reach dynamic equilibrium. After equilibrium, the concentration of nitric acid in the solution discharge is 2.78 mol/L and the concentration of uranium is 253.23 g/L. The relative error of material balance calculation of uranium in the whole dissolution process is generally in an acceptable range. When it reaches equilibrium, the relative error of overall mass balance calculation is only 2.57%. The simulation results show that the dynamic dissolution simulation model of the rotary continuous dissolver established in this study can better describe the actual dissolution behavior of the spent fuel in the dissolver, and provide simulation data support for the spent fuel dissolution process in the actual production process.

The 2 K cryogenic system is a key subsystem of the new generation superconducting accelerator, and the 2 K heat exchanger is the key equipment. With the increasing scale of 2 K cryogenic system, the demand for high-flow 2 K heat exchanger becomes more and more urgent, and the current application of the tube and fin type, coil and tube structure of the heat exchanger is gradually replaced by a more efficient and compact platefin heat exchanger in high flow conditions, but the platefin heat exchanger has more structural parameters, and it is difficult to take into account the heat transfer efficiency, pressure drop and volume size in the design process. In this paper, based on the basic differential equations of heat transfer and pressure drop and the related flow heat transfer correlation equation, the design calculation of a 50 g/s flow rate 2 K heat exchanger supporting a 1 000 W@2 K cryogenic system was carried out using the distributed parameter method. Five parameters affecting the performance of the heat exchanger were analyzed and the comprehensive optimization index of the heat exchanger was proposed using the weighting coefficient method. Based on the proposed optimization indexes, a genetic algorithm was used to optimize the heat exchanger with multiple foci on high heat transfer efficiency, low pressure drop and small size according to the engineering needs, and a series of optimization solutions were obtained, from which the solution focusing on heat transfer efficiency was selected as the preliminary design result. Based on the parameter of this result, a threedimensional model of the heat exchanger heat transfer channel was established, and CFD numerical simulation was used for simulation calculation. The simulation results were in good agreement with the design results, verifying the accuracy of the preliminary design results, and the performance of the heat exchanger under different operating conditions was analyzed for this optimization result. The optimization result shows that the heat exchanger is selected with perforated plate type fins to cold-hot-cold (CHC) structure for the arrangement, it has 10 layers of CHC structure, its length is 760 mm, fin height is 4 mm, fin spacing is 7.9 mm, heat transfer efficiency of the heat exchanger is 84.4%, pressure drop is 222.16 Pa, and the volume is 0.022 1 m3. A variety of fin types of heat exchangers will be studied in the future to meet the demand of heat exchanger use.