Study on flow measurement of transient thermal hydraulic phenomena using a thermal neutron beam as a probe

本项目为使中子射线照相技术与x射线或伽玛射线照相技术互补，成为一种先进的测量技术，进行了以下研究：(1)提高时间分辨率利用高通量稳定中子束、高性能变换器、高灵敏度相机等最新技术，构建了最高记录速度可达1000帧/秒的高帧率中子射线照相系统。还有一个带双微通道板的图像增强器。以1000帧/秒的记录速度成功地实现了金属矩形管道内两相流的可视化。通过统计分析低中子通量场下中子数测量结果和光衰减特性，估计了高帧率中子射线照相的时间分辨率极限。(2)热中子射线成像定量方法的发展：发展了考虑散射中子的免分数测量方法（sigma标度法）。通过实验、蒙特卡罗模拟和简化的中子模型验证了sigma标度方法。结果表明，该图像处理方法可以实现流态、气泡上升速度、气泡尺寸分布、界面面积浓度、液体速度场等流量测量。(3)应用于瞬态热液现象。本研究开发的高帧率中子成像系统和量化方法应用于各种热液现象：窄矩形管内垂直向上空气-水流动的可视化、小直径圆管内垂直向上空气-水流动的空隙率测量、蒸汽爆炸预混现象的可视化与测量、大密度比金属-氮气两相流动的流量测量与快中子增殖反应堆重大事故研究相关。

In this project, the following studies were performed to establish neutron radiography with the complementary characteristics to X-ray or gamma-ray radiography as an advanced measuring technique :(1)Improvement of temporal resolutionHigh-frame-rate neutron radiography system with the maximum recording speed of 1000 frame/s was constructed by gathering up-to-date technologies such as a high flux steady neutron beam, a high-perfomance converter, a high-sensitivity camera, and an image intensifier with double micro channel plates. Two-phase flow in a metallic rectangular duct was visualized successfully at the recording speed of 1000 frame/s. Temporal resolution limit of high-frame-rate neutron radiography was estimated by statistics analysis of neutron number measurement in a low neutron flux field and light-decay characteristics of a converter.(2)Development of quantification method of thermal neutron radiographic imageVoid fraction measurement method was developed by taking account of scattered neutrons (SIGMA-scaling method). The SIGMA-scaling method was verified by experiments, a Monte Carlo simulation, and a simplified neutronic medel. It was demonstrated that the image processing method enabled flow measurements such as flow regime, bubble rise velocity, bubble size distribution, interfacial area concentration, liquid velocity field, and so on.(3)Application to transient thermal hydraulic phenomenaThe high-frame-rate neutron radiography system and the quantification method devepoled in this study were applied to various thermal hydraulic phenomena : visualization of vertical upward air-water flow in a narrow rectangular duct, void fraction measurement of vertical upward air-water flow in small diameter round tubes, visualization and measurement of premixing phenomena in a steam explosion, and flow measurement of molten metal-nitrogen gas two-phase flow with a large density ratio in relation to a severe accident study in a fast breeder reactor.