熔盐堆动态燃耗行为研究

The Molten Salt Reactor (MSR), as the only one reactor adopting liquid fuel among Generation IV reactor concepts, has unique advantages in Th-U breeding and transuranic transmutation. However, because of its significant differences from the solid fuel reactors, the physical analysis of MSR presents much more challenges since almost all current reactor physical analysis tools are developed for solid fuel reactors. During the operation of MSR, the fuel salt entrains fissile materials and fission products (FP) experiences neutron flux exposure as it flows through the core, while undergoes only nuclide decay when the fuel salt flows out to the outer-loop of core. The flow cycle time is quite short, which is less than 50 s. This phenomenon is bound to result in the much different nuclide evolution law from that of solid fuel reactors. At the same time, the flowing fuel salt and fission gas continuously permeates to the moderator of graphite, and the fission products are online removed. These unique processes make the nuclide evolution law of molten salt reactors be more complicated. From this point, developing a new burnup calculation method specific for MSR is extremely necessary. In this study, we firstly modify the traditional burnup differential equations to precisely describe the unique mechanisms of MSR such as the circulation of molten salt, permeation of undissolved fuel salt and fission gas to the graphite, et al. Then, the advanced numerical methods are employed to solve the modified burnup differential equations for calculating the three-dimensional dynamic burnup of MSR. Based on the developed burnup calculation method, we attempt to reveal the influence of the flowing of molten salt, graphite permeation, online reprocessing and refueling on the core reactivity, breeding performance and source term analysis. This project belongs to the basic reactor physics research, its obtained results would greatly enhance the development of MSR.

作为唯一一种采用液态燃料形式的先进反应堆堆型，熔盐堆在钍铀增殖以及锕系核素嬗变方面具有独特的优势。然而由于与固态燃料反应堆存在显著差异，导致其反应堆物理分析极具挑战性。熔盐堆系统运行期间，燃料熔盐以极短的循环周期不断流经堆芯中子辐照区与外回路衰变区，其核素演化规律完全不同于固态燃料反应堆。与此同时，燃料熔盐及不溶裂变气体不断向石墨渗透、裂变产物不断在线去除，这些导致熔盐堆核素演化规律更加复杂。基于此，本项目拟通过修改传统燃耗微分方程以及采用先进数值方法精确求解的方式模拟熔盐堆三维燃耗动态变化，揭示熔盐流动、石墨渗透、在线加料以及后处理等多物理机制对堆芯反应性、增殖性能以及源项分析的影响规律。本项目属于基础物理研究，为熔盐堆发展所亟需解决的关键科学问题，对提升熔盐堆物理分析水平具有非常重要的实用性。

作为唯一一种采用液态燃料形式的先进反应堆堆型，熔盐堆在钍铀增殖以及锕系核素嬗变方面具有独特的优势。但熔盐堆系统复杂的多物理耦合机制，给熔盐堆物理分析与仿真模拟带来了极大挑战。熔盐堆运行过程中，为了快速导出堆芯所产生的热量，要求熔盐流动循环周期尽可能短（秒量级），这种快速流动效应造成燃料核素在堆芯中子辐照环境与外回路衰变环境之间不断快速转换，造成核素演化规律完全不同于固态燃料反应堆。在线加料与燃料后处理是熔盐堆物理模拟分析的另一关键问题，将会导致核素演化偏离本征轨迹。由于采用石墨慢化，堆芯运行期间，同时会出现熔盐以及裂变气体向石墨渗透现象，而熔盐的流动特性以及氦泡的鼓入（去除裂变气体）使这种渗透更加复杂。. 本项目围绕熔盐堆复杂多物理机制耦合下核素动态演化难于模拟的关键科学问题，基于熔盐堆运行的具体流程，开展了极短流动循环周期下的核素演化规律研究、燃料熔盐及裂变气体向石墨渗透情况下的燃耗行为研究、以及在线加料以及后处理模式下的燃耗行为研究。实现了极短熔盐流动周期（秒量级）内的核素演化行为模拟，系统揭示了燃料熔盐流动对不同类型、不同寿期关键核素演化规律的影响，结果显示燃料熔盐流动对短半衰期核素影响显著。基于特征线方法，发展了熔盐堆高保真模拟分析模型，成功解决了燃料熔盐渗透至石墨后形成不规则几何难于模拟的难题。考虑堆芯运行过程中温度及压强变化，基于裂变气体在燃料熔盐、氦泡及石墨间相互迁移机制，采用固-液-气传质模型，建立了熔盐堆一回路裂变气体迁移扩散模型，揭示了关键参数对氙毒的影响规律。结合燃料熔盐流动、氦泡鼓入去除裂变气体、在线加料以及在线后处理机制，发展了熔盐堆动态燃耗模拟分析工具，系统揭示了在线后处理效率对钍铀循环性能的影响规律。. 本项目是熔盐堆研究领域的基础科学研究，研究成果有助于深化熔盐堆物理分析，为熔盐堆堆芯设计提供重要保障，对熔盐堆的发展具有重要的实用价值与科学指导意义。