近年来，随着能源需求急剧增加以及环境污染日益严重，核能作为一种低碳清洁能源，正发挥越来越重要的作用。自然循环熔盐高温堆作为第四代核能系统中的一种概念设计堆型，能够在百万千瓦级功率下依靠自然循环运行，具有广泛的应用前景。目前，该堆型的研究还处于概念设计阶段，有待进一步研究。球床堆芯和板翅式中间换热器是自然循环熔盐高温堆内阻力最大的两个部分，均包含数量庞大且窄小的流道结构，故本文基于CFD商用软件Fluent建立球床多孔介质模型和换热器多孔介质模型。利用SANA实验数据和HTR-10基准例题验证了球床多孔介质模型的可靠性和准确性；利用商用软件Aspen EDR对换热器多孔介质模型进行了验证。对以FLiBe为冷却剂且横向流过球床的堆芯进行优化设计。首先，通过对径向流动与通常的轴向流动进行比较，权衡燃料球尺寸对堆芯压降以及燃料球中心最高温度的影响，确定了燃料球直径。其次，建立了描述径向流球床流动特性的偏微分方程，确定内孔板开孔率和内通道尺寸是影响流量分布的主要因素。最后，利用Fluent建立的球床多孔介质模型，分析了堆芯孔板开孔率对堆芯流动、燃料球中心最高温度以及堆芯压降的影响，优化确定了内、外孔板开孔率。按照该开孔率，计算得到燃料球中心最高温度以及堆芯压降。对板翅式中间换热器进行优化设计研究。首先，利用ANSYS Workbench平台中的实验设计方法结合Fluent数值模拟结果构建响应面，基于多目标遗传算法对翅片尺寸进行了优化。其次，利用MATLAB软件建立了基于ε-NTU方法的板翅式换热器设计程序，以优化后的翅片参数对换热器进一步优化。最后，将优化后的中间换热器布置到一回路中，利用Fluent建立的换热器多孔介质模型，对自然循环球床熔盐堆一回路（除堆芯外）的热工水力进行了计算分析。结合堆芯孔板开孔率和中间换热器优化设计结果，利用Fluent作为主要的计算软件，构建了基于多孔介质模型的自然循环熔盐高温堆一回路模拟计算平台，在该平台上开展了满功率自然循环运行的稳态热工水力模拟。并基于点堆中子动力学方程编制了熔盐高温堆反应堆物理计算程序，将其嵌入到自然循环熔盐高温堆一回路模拟计算平台中，开展了自然循环熔盐高温堆多种瞬态模拟计算。通过计算表明优化设计的堆芯及换热器，可以在保证安全的前提下更好地满足全功率自然循环要求。

In recent years, with the rapid increase of energy demand and increasingly serious environmental pollution, nuclear energy is playing an increasingly important role as a low-carbon and clean energy. As a kind of conceptual design reactor in the fourth generation nuclear energy system, natural circulation molten salt high-temperature reactor can run by natural circulation under the power of one million kilowatts, which has a broad application prospect. At present, the research on natural circulation molten salt high-temperature reactor is still in the preliminary design stage, which needs to be further studied.The pebble bed core and the plate-fin intermediate heat exchanger are the two components with the highest resistance in the natural circulation molten salt high-temperature reactor, both of which contain a large number of narrow flow passage structures. Therefore, this paper established the pebble bed porous media model and heat exchanger porous medium model using the CFD commercial software Fluent. The reliability and accuracy of the pebble bed porous media model were verified by SANA experimental data and HTR-10 reference examples. The commercial software Aspen EDR was used to verify the heat exchanger porous media model.The reactor core with FLiBe as coolant flowing horizontally through the pebble bed was optimized. Firstly, by comparing the radial flow with the normal axial flow, the diameter of the fuel ball was determined for the tradeoff between the influence of the fuel ball size on the core pressure drop and the maximum temperature of the fuel ball center. Secondly, a partial differential equation describing the flow characteristics of the radial flow pebble bed was established, and determining the porosity of the inner orifice plates and the size of the inner channel are the main factors affecting the flow distribution. Finally, using the pebble bed porous media model established by Fluent, the influence of the core orifice porosity on the core flow, the maximum temperature of the fuel ball center and the core pressure drop was analyzed, and the porosity of the inner and outer orifice plates was optimized and determined. According to this porosity, the maximum temperature of the fuel ball center and the core pressure drop are calculated.The intermediate heat exchanger of natural circulating molten salt high temperature reactor was optimized. Firstly, the experimental design method in ANSYS Workbench platform with Fluent numerical simulation results were used to construct the response surface, and the fin size was optimized based on the multi-objective genetic algorithm. Secondly, a plate-fin heat exchanger design program based on the ε-NTU method was established using MATLAB software to further optimize the heat exchanger with the optimized fin parameters. Finally, the optimized intermediate heat exchanger was arranged in the primary circuit, using the heat exchanger porous medium model established by Fluent to calculate and analyze the thermal hydraulics of the primary circuit (except the core) of the natural circulation pebble bed molten salt reactor.Combined with the optimization design results of the porosity of the core orifice plate and the intermediate heat exchanger, Fluent was used as the main calculation software to construct the primary circuit simulation calculation platform of the natural circulation molten salt high-temperature reactor based on the porous medium model, on which the steady-state thermal hydraulic simulation of natural circulation operation at full power was carried out. Based on the point reactor neutron kinetics equation, the physical calculation program of the molten salt high-temperature reactor was developed, which was embedded into the primary circuit simulation platform, and various transient simulation calculations of the natural circulation molten salt high-temperature reactor were carried out. The results showed that the optimized design of the reactor core and heat exchanger could better meet the full power natural circulation requirements under the premise of ensuring safety.