熔盐堆余热排出系统闪蒸流动的不稳定性

Flashing flow instability in molten salt reactors residual heat removal system

套管式换热元件是熔盐堆非能动余热排出系统的重要组成部分之一。设计并搭建了单根套管式换热元件自然循环实验台,模拟熔盐堆停堆后排盐罐中熔盐的显热与衰变热的导出过程。在这种以套管式换热元件为基础的自然循环回路中,发现了闪蒸流动不稳定现象的存在。分析了在闪蒸过程中压力与温度的变化情况,以及这种闪蒸流动不稳定性的特点,并研究了在闪蒸过程中的换热系数的变化情况。结果表明,当外层套管外壁温度从440oC上升到700oC时,单根套管式换热元件的换热量会从550 W上升到1 900 W。此外,在闪蒸过程中,换热元件中的沸腾换热系数在800-1 700 W·(m^2·K)^(-1)之间变化,换热元件壁温与自然循环流量的周期性变化引起管壁周期性热应力与系统的周期性机械振动。因此,在熔盐堆余热排出系统的设计过程中,为保证热量导出的安全性与稳定性,需要考虑到闪蒸现象。

Background： In contrast with pressure water reactor, molten salt reactors （MSR） have low pressure, small size, high multiplication efficiency, and little residual radioactivity. The residual heat removal system is an important part of nuclear power reactor to ensure the reactor safety. However, little attention has been paid to the passive residual heat removal system （PRHRS） of the molten salt reactor in the literature so far. Purpose： This study aims to make some preliminary work for the design of the passive residual heat removal system and to make sure the safety and stability of residual heat removal of MSRs. Methods： The heat transfers and flow characteristics are analyzed for the cooling thimble of MSRs. An experimental loop is designed and built with a single heat transfer element to simulate the sensible heat and decay heat removal process in drain salt tank when the reactor is shut down. The changes of pressure and temperature are analyzed through which the characteristics of flashing flow instability are studied. Moreover, the changes of heat transfer coefficients are analyzed. Results： The heat transfer rate increases from 550 W to 1 900 W when the thimble outer wall temperature increases from 440 ℃ to 700 ℃. And the heat transfer coefficient changes from 800 W·（m^2·K）^-1 to 1 700 W.（m^2·K）^-1 during the flashing process. Conclusion： The flashing phenomenon will cause the periodical change of natural circulation flow rate and temperature of pipe wall, resulting in the periodic fluctuation of wall temperature and periodic mechanical vibration of the system. In the process of designing the molten salt reactor passive heat removal system, the flashing flow instability phenomenon should be taken into consideration to ensure the safety and stability of heat removal.