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

本项目主要开展了以下几方面的研究工作，以建立一种与X射线或γ射线照相互补的先进的中子照相测量技术：（1）提高时间分辨率通过集成高通量稳定中子束、高性能转换器、高灵敏度相机和双微通道板像增强器等最新技术，构建了最高记录速度为1000帧/秒的高帧频中子照相系统。在1000帧/秒的记录速度下，成功地显示了金属矩形管道中的两相流。通过对低中子注量率场中子数测量结果和转换器光衰减特性的统计分析，估算了高帧频中子照相的时间分辨率极限。（2）热中子照相图像定量化方法的发展提出了考虑散射中子的空隙率测量方法（SIGMA定标法）。通过实验、Monte Carlo模拟和简化的中子学模型验证了SIGMA标度方法。结果表明，图像处理方法使流动测量，如流态，气泡上升速度，气泡尺寸分布，界面面积浓度，液体速度场，等等。（3）瞬态热工水力现象的应用本研究开发的高帧频中子照相系统和定量方法应用于各种热工水力现象：窄矩形管道中垂直向上的空气-水流的可视化，小直径圆管中垂直向上的空气-水流的空隙率测量，蒸汽爆炸中预混合现象的可视化和测量，与快堆严重事故研究有关的大密度比熔融金属-氮气两相流的流量测量。

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.