The thermal safety characteristics of the system after a small break loss of coolant accident (SB-LOCA) in a helium xenon gas-cooled small reactor will determine whether the reactor will experience a meltdown and the duration of the meltdown. At present, research on SB-LOCA both domestically and internationally is mostly focused on water-cooled reactors, lacking relevant research on helium xenon gas-cooled reactors (He-Xe GCR). Therefore, it is of great significance to research the thermal safety characteristics of He-Xe GCR after a breach loss of coolant accident. This article improves the gas turbine model based on the original SB-LOCA analysis program for pressurized water reactors. And added a calculation model for the characteristics of helium xenon mixed gas, a heat transfer model for helium xenon mixed airflow, and a compressor model. Finally, a Brayton Cycle Reactor System Analysis program (BRESA) was developed for SB-LOCA analysis of He-Xe GCR. Explore the impact of the size and location of fractures on the thermal safety characteristics of the system using BRESA. Research has found that as the size of the rupture gradually increases, the decrease in reactor core power is greater, and the reactor power is shut down earlier. Comparative analysis of the impact of different breach positions on the residual heat removal characteristics reveals that a breach accident on the high-pressure side of the system poses a greater threat to system safety. Therefore, when setting the safety threshold for the Brayton cycle system, the focus should be on the hazards of high-voltage side breaches. This research achievement assists with the safe operation strategy of He-Xe GCR in the future.

The small modular helium -xenon cooled reactor has the advantages of simple and compact system structure, short construction cycle, light weight and small volume, which is very suitable for energy supply in remote area and small nuclear propulsion device. The research on accident characteristics and protection control strategies is very important to ensure the safe and reliable operation of the reactor system and mitigate the consequences of the accident. Therefore, the safety characteristics analysis program of helium -xenon cooled reactor system is developed based on Modelica language, and the transient verification of typical working conditions is carried out. The maximum relative error of the verification results was 7.91 %. Through the program, the unprotected transient conditions and corresponding protection control strategy transient conditions of external load loss accident, partial loss of heat sink accident, main valve accidental closing accident and Brayton unit efficiency step decline accident are simulated. The results show that the loss of all external loads will lead to rotation speed overspeed. The introduction of negative reactivity or bypass control can effectively control the rotation speed rise, but the introduction of negative reactivity is better than bypass control. Under unprotected control, the partial loss of the heat sink, the accidental closing of the main valve and the step down of the Brayton unit efficiency will all lead to the rapid decline of the rotation speed and mass flow rate, and there is a risk of the reactor temperature exceeding the limit. The rotation speed and mass flow rate can be effectively stabilized by load following to prevent the temperature exceeding the limit.

Helium-xenon(He-Xe) gas mixture is a commonly used cooling medium in fourth-generation advanced reactor systems. The thermal conductivity of a passive waste heat system is related to the safety and reliability of the reactor system. Therefore, it is crucial to explore the passive characteristics of the helium-xenon gas mixture. The passive waste heat removal characteristics of a helium-xenon gas mixture are influenced by the flow and heat transfer characteristics of the mixture. However, existing research on the flow and heat transfer characteristics of helium-xenon gas mixture mainly focuses on single tube bundles and plate bundles with fixed flow rates and high Reynolds numbers. However, existing research on the flow and heat transfer characteristics of the He-Xe mixtures mainly focuses on single tube bundles and plate bundles with fixed flow rates and high Reynolds numbers, and most empirical formulas apply to Reynolds numbers greater than 10000. However, the natural circulation flow rate under the influence of gravity varies with the variation of heating power, and in most operating conditions, the Reynolds number is less than 10000. Due to the lack of research on the convective heat transfer characteristics of the He-Xe mixtures at low Re, this paper investigates the flow and heat transfer characteristics of the He-Xe under natural circulation conditions. A three-dimensional model of the natural circulation circuit was established using numerical calculation methods. Comparing the numerical simulation results of natural air circulation with empirical formulas, it was found that the error was less than 20 %. This proves that the SST k-omega turbulence model can be used to study the flow and heat transfer characteristics of He-Xe gas mixtures in natural circulation. On this basis, natural cycle numerical simulations of He-Xe mixtures with different mixing ratios were carried out at a heat flux of 5000 W/m2, as well as natural cycle numerical simulations of 40 g/mol He-Xe gas mixture at different heating powers, to study the effects of heating power and He-Xe mixing ratio on the natural cycle heat transfer characteristics of the He-Xe gas mixture. The existing He-Xe gas mixture flow heat transfer equation was modified, and the error of the modified Nu empirical equation was less than 12 %, which can be used for the development of the passive waste heat export system program for subsequent He-Xe gascooled fast reactors.

In this paper, a numerical simulation method is used to study the process of multiple droplets impact on the wall of a liquid droplet radiator in a space environment. The effects of droplet spacing, droplet velocity, wall properties and other factors on the evolution of droplet impact on the wall are analyzed. The results show that the critical Reynolds number of droplets without splashing decreases and then remains unchanged as the droplet spacing of silicone oil increases. Thin liquid film promotes droplet splashing, while thick liquid film inhibits droplet splashing. The larger the initial incident velocity of the droplets, the larger the number of splashing droplets and the smaller their diameters. The critical Reynolds number of silicone oil droplets decreases monotonously with the increase of the inclination angle between the wall and the vertical direction. Finally, a 15° frustum of a cone model is chosen to simulate the actual working process of the liquid droplet radiator to verify the conclusions. The results of the study can provide guidance for the optimal design of droplet collectors.

Space power nuclear systems require reactors to have characteristics such as compactness,miniaturization and efficiency.The combination of helium xenon mixture gas and the Brayton cycle is a good choice.Existing research on helium-xenon gas cooled small reactors mostly focuses on the system level,with insufficient consideration for the refinement of individual components.The core is the most critical part of the entire Brayton cycle system,and the thermal safety characteristics of the core are the most important component of the safety characteristics of the reactor system.Therefore,mastering the thermal safety characteristics of the core is particularly important.There are not only two forms of heat transfer inside the core:convective heat transfer and heat conduction,but also radiative heat dissipation.In this paper,the three-dimensional sim-ulation method was adopted to establish a 1:1 model of the helium-xenon gas cooled small reactor core and analyzed the radiation heat dissipation characteristics inside the core.The Monte Carlo method was used to verify the proposed method of calculating radiation angle coefficients using three-dimensional simulation.The relative error between the two calculation results is less than 1％,proving the accuracy of the three-dimensional simulation method.On this basis,a study was conducted on the radiation heat dissipation characteristics of fuel rods in the core of helium-xenon gas cooled small reactors.The sensitivity analysis on geometric parameters such as fuel rod aspect ratio,fuel rod surface temperature and fuel rod length was conducted,and paid attention to their impact on the radiation heat dissipation characteristics of fuel rods.The results show that as the aspect ratio between fuel rods increases,the radiation angle coefficient also increases.When the length of the fuel rod is less than 100 mm,the length of the fuel rod has a significant impact on the radiation angle coefficient.When the length of the fuel rod is greater than 100 mm,the radiation angle coefficient is not affected by the length of the fuel rod.Finally,a universal empirical formula for the radiation angle coefficient of fuel rods in the core of a helium-xenon gas cooled small reactor was estab-lished based on the relationship between the radiation angle coefficient and the radial aspect ratio of fuel rods,the arrangement of fuel rods,and the length of fuel rods.The calculation relative error is less than 8.5％.This paper aims to study the radiation and heat dissipation characteristics of fuel rods and core radiation,understand the laws of internal radiation and heat dissipation,and provides technical support for subsequent research on core thermal safety characteristics.

Although Helium–Xenon mixture cooled small reactor has been widely concerned, there is no mature thermal calculation software that can be applied to Helium–Xenon mixture cooled small reactor in China. In this paper, a semi empirical formula used to predict the physical properties of binary inert gas mixtures is used to establish a physical property calculation model of Helium–Xenon mixture gas mixtures, and the flow and heat transfer calculation model, turbine model and compressor model are modified and improved, so as to achieve the calculation of the Brayton cycle system. The function of the original system analysis program is expanded and improved, and the simulation program for he xenon mixed gas reactor system is developed. At the same time, the closed Brayton cycle system is established by using the system analysis program developed in this paper, and the calculation results are compared with the design values. The maximum relative error is 6.9%. The steady-state operation results of the model are in good agreement with the design operation parameters, which proves the applicability of the improved simulation model. The steady-state response characteristics analysis, the introduction of positive and negative reactivity accidents and the calculation of transient conditions such as step load drop of the reactor were carried out. The overall trend is the same as the literature survey results. The results show that the system analysis program developed in this paper is suitable for the analysis of the Brayton cycle system of Helium–Xenon mixture cooled small reactor.

Helium-Xenon (He-Xe) gas mixtures are characterized by stable physical properties, easy compression, and excellent thermodynamic performance, making them suitable for thermal cycle coolant for underwater reactors. Due to the unattended deep sea, and harsh and complex operating environment, the residual heat removal characteristics are directly related to reactors' safety characteristics and stable and reliable operation ability. Therefore, studying the He-Xe gas mixture residual heat removal characteristics can provide an essential guarantee for reactor safety analysis. This article optimizes the physical property calculation model of the He-Xe gas mixture in the Brayton Cycle Reactor System Analysis Program (BRESA) and obtains a system property analysis program suitable for calculating the natural circulation of the He-Xe gas mixture. And a natural circulation system for the He-Xe mixed gas cooling reactor was constructed using the modified BRESA. Numerical calculations and theoretical analysis were conducted on the He-Xe gas mixture's natural circulation flow process and residual heat removal characteristics. The study of the transient factors of temperature, flow rate, and other related parameters in the system circuit found that when the reactor is shut down, relying on the natural circulation system can effectively output the residual heat of the reactor core to ensure reactor safety. The relevant research results have important theoretical and scientific significance for the safety assessment and evaluation of the Brayton cycle reactor system.

The layout of nuclear-powered spacecraft is very different from the layout of nuclear reactors used on Earth, so the design principle is also different. Taking nuclear-powered spacecraft as the object, this paper studies the coverage of the shadow area formed by the shield and distribution of neutron fluence and photon dose in the shadow area. In this paper, a nuclear power spacecraft model with a width of 6 m and a length of 25 m is built. The Monte Carlo method is used to calculate the transport of neutrons and photons, and the distribution of neutrons and photons in space is obtained. The coverage of the shadow area formed by shields with different widths is obtained, and the main factors affecting the width and angle of the shadow area are summarized. When the diameter of the shield decreases, the width of the shadow area also decreases, and the angle of the shadow area decreases, which greatly reduces the area of the shadow area. The distribution of neutron fluence and photon dose in the shadow area is obtained. At the end of the truss, the maximum value of the photon dose is 1.03 x 10-4 Gy center dot s-1, and the average value is 6.42 x 10-5 Gy center dot s-1. The maximum value of the neutron fluence rate is 1.01 x 104 n center dot cm-2 center dot s-1, and the average value is 6.18 x 103 n center dot cm-2 center dot s-1. And the influence of spacecraft structure on radiation is analyzed. The truss and droplet emitters have a greater impact on photons, and the heat sink and droplet emitters have a greater impact on neutrons. Based on the above work, a local shield scheme is proposed to reduce the mass. After being blocked by local shield, the photon dose is reduced to 1/10 and the neutron fluence rate is reduced to 1/6.

The nuclear power system is one vital choice for interplanetary migration. The control rod and sliding reflector are the two most widely used control schemes with different characteristics for the space nuclear reactor. In the present paper, the control rod scheme and sliding reflector scheme of the 2.6 MWt reactor model are investigated and compared. According to the neutron physics analysis, the core arrangement and the control assembly motion scheme for the reactor start-up process are proposed, along with the reactivity insertion behavior. The results show that the reactivity insertion of the control rod scheme is $18.76, which is lower than the $26.01 of the sliding reflector scheme, and the weight of the former is 79.6 kg larger than the latter. However, the control rod scheme has significant advantages in core power flattening and control accuracy. Upon comprehensive consideration, the control rod scheme is recommended as the control scheme. The research results can lay the foundation for the control system layout scheme and control strategy design.

In order to study the characteristics of small helium-xenon cooled nuclear reactor that only relies on natural circulation to remove residual heat after shutdown, this paper carries out geometric and physical modeling of helium-xenon cooled nuclear reactor, and uses CFD to simulate the process of natural circulation residual heat removal. Variation laws of the hot spot temperature on the surface of fuel cladding and the mass flow of helium-xenon mixed gas under different working conditions are calculated, and the transient characteristics of temperature, flow and other related parameters in the system loop are analyzed. The calculation results show that during the natural circulation residual heat removal process, the overall change trend of the hot spot temperature on the surface of the fuel cladding, the average temperature of the primary coolant and reactor outlet temperature first rises to the highest value and then decreases, and the mass flow of helium-xenon mixed gas shows an exponential decay trend; Increasing the initial mass flow of helium-xenon mixed gas and reducing the residual heat of the reactor core can reduce the maximum hot spot temperature on the surface of the fuel cladding. The relevant research results provide a useful reference for the optimal design of natural circulation residual heat removal of helium-xenon cooled nuclear reactor.

Although helium xenon cooled small reactor has been widely concerned, there is no mature thermal calculation software that can be applied to helium xenon cooled small reactor in China. In this paper, a semi empirical formula used to predict the physical properties of binary inert gas mixtures is used to establish a physical property calculation model of helium xenon gas mixtures, and the flow and heat transfer calculation model, turbine model and compressor model are modified and improved, so as to achieve the calculation of the Brayton cycle system. The function of the original system analysis program is expanded and improved, and the simulation program for he xenon mixed gas reactor system is developed. At the same time, the closed Brayton cycle system is established by using the system analysis program developed in this paper, and the calculation results are compared with the design values. The maximum relative error is 6.9%. The steady-state operation results of the model are in good agreement with the design operation parameters, which proves the applicability of the improved simulation model. The steady-state response characteristics analysis, the introduction of positive and negative reactivity accidents and the calculation of transient conditions such as step load drop of the reactor were carried out. The overall trend is the same as the literature survey results. The results show that the system analysis program developed in this paper is suitable for the analysis of the Brayton cycle system of helium xenon cooled small reactor.

针对阴影屏蔽体的材料选取与仪器仓布局问题,本文以核动力航天器为对象,研究了屏蔽材料与结构材料的辐射特性,计算得到了屏蔽体阴影区域内的辐射场分布.本文利用蒙特卡罗方法在屏蔽阴影区范围内对中子与光子进行输运计算,得到中子与光子在空间中的分布情况.通过得到的不同屏蔽材料与结构材料对具有特定能谱的中子与光子的屏蔽效果,选出了适用于空间反应堆屏蔽体设计的材料,确定了一种满足辐射防护限制要求的屏蔽材料组合方式.研究表明:铅、氢化锆和聚乙烯应用于空间堆的辐射屏蔽能使屏蔽体更加轻质化,掺杂氢化锆粉末的聚乙烯复合型材料的屏蔽效果优于单一材料,反应堆至仪器仓前的辐射分布能为仪器舱内的布局设计提供了参考.