熔盐堆非能动余热排出基础研究

Since there are no solid fuel elements in the molten salt reactor (MSR), the liquid fuel will be stored in the fuel-salt drain tank after the reactor shutting-down, which makes the decay heat cannot be removed in the same way as the water-cooled reactor, so it is necessary to investigate a new method for removing the decay heat and other scientific problems involved according to the characteristic of liquid-fuel reactor. In this project a special heat-exchanging element will be introduced in the fuel-salt drain tank for removing the decay heat, and the element is expected to isolate the fuel- salt from the cooling medium well, to transfer the heat by natural circulation, and to present an appropriate temperature difference for heat transfer. The problems about heat output of the element, thermal stratification and so on will be investigated aiming to prevent the fuel-salt temperature from being too high, and at the same time the fuel-salt can not be frozen due to the low temperature. The decay heat produced in the fuel-salt is transferred to the heat sink through several natural ciculation links, so the studies about the natural circulation flow and heat transfer will be performed, and thermal-hydraulic coupled problems in the system could be analyzed. On the basis of the calculation and analysis results, experiment loop will be built and experimental research will be conducted to determine the heat transfer coefficients of the heat-exchanging element,the temperature variation characteristics of the molten and the thermal-hydraulic coupled problems in the natural-circulation system. The overall performance of the passive system is determined by mutual confirmation between experimental results and calculating results. The research will provide a fundamental basis for engineering application of the passive decay heat removal system in molten salt reactor.

由于熔盐堆没有固体燃料元件，停堆后液体熔盐燃料在排盐罐内贮存，因此不能采用现有水冷堆的方法导出燃料的衰变热，需要研究从排盐罐内液体燃料中导出衰变热的新方法及所涉及的科学问题。本项目拟在排盐罐内加特殊形式的换热元件导出燃料的衰变热，换热元件具有冷却介质与熔盐隔离性能好、传热温差适当、靠工质自然循环输出热量等特点。对换热元件热量输出和熔盐的热分层等问题进行研究，使热量输出与衰变热产生相适应，保证排盐罐内熔盐温度不过高，也不出现熔盐低温冻结。熔盐内产生的衰变热要通过多个自然循环过程排往最终热阱，需要对多重自然循环耦合系统的传热滞后、流动不稳定性等问题进行研究。在计算分析的基础上，搭建实验装置，验证换热元件的传热特性、熔盐的温度变化特性以及多个自然循环系统耦合特性等。通过实验结果与计算结果的相互验证，确定非能动余热排出系统的总体性能，为熔盐堆非能动余热排出系统的工程应用提供基础依据。

针对液体燃料熔盐堆停堆后燃料盐在排盐罐内贮存的特点，对高温熔盐的衰变热导出和非能动热量排出过程所涉及的科学问题进行深入的基础研究，从理论和实验两个方面证明液体燃料反应堆非能动余热排出的可行性。.在液体燃料热量导出问题研究中，研究了不同边界温度条件下熔盐与套管式换热元件之间的传热特性，得到换热元件的换热量。正常工作温度范围内，热量从最外层套管传递到中间层套管的过程中辐射传热占总传热量的58%-67%，起主导作用。在套管式的结构中，辐射传热量与套管壁面温度四次方之差成正比，因此套管式结构有利于换热元件排热功率与衰变热相匹配。对系统传热路径上的热阻组成情况进行了分析，其中气隙层热阻起主导作用，是限制系统排热功率的主要因素，也是系统能够在温度和压力较低的条件下运行的主要因素。.基于CFD计算对排盐罐内熔盐热分层效应和自然对流现象进行模拟，结合实验得到的换热元件传热量随熔盐温度变化的关系，得到稳态和瞬态条件下熔盐温度分布。对排盐罐内局部特殊现象和热分层形成的过程进行认识和分析。采用无量纲数对比了内热源和外热源条件下排盐罐内热分层发展规律的区别。.通过非能动余热排出系统内自然循环瞬态流动特性的实验研究，对换热元件内出现的流动不稳定现象进行分析，研究系统加热功率和入口温度变化对自然循环流动不稳定性的影响，确定了自然循环不稳定边界，给出了套管式自然循环回路中密度波不稳定发生的判据。利用程序对多重自然循环系统进行耦合计算，在考虑衰变热随时间变化的情况下，给出耦合后的非能动余热排出速率与衰变热变化的计算结果，确定系统排热能力与衰变热产生的平衡关系。.对自然循环系统性能进行实验研究，分析系统启动过程和衰变热导出过程中系统的输热能力及耦合特性，确定系统内各热工参数的变化特性。给出了熔盐与水大温差传热情况下排盐罐内熔盐的温度变化特性和热分层发展规律。结合非能动余热排出系统综合测试的实验结果，最终确定系统的总体性能，为熔盐堆非能动余热排出系统的工程应用提供基础依据。