核石墨的孔结构与熔盐浸渗特性研究

Porosity properties and molten salt impregnation characteristics of nuclear graphites

对于液态燃料熔盐堆而言,核石墨的浸渗问题非常重要,关系反应堆运行安全性。因此,对核石墨的熔盐浸渗的研究必不可少。核石墨是多孔材料,其孔结构决定了其浸渗特性。本研究主要针对中国科学院上海应用物理研究所的液态燃料熔盐堆项目——钍基熔盐堆核能系统(Thorium-based Molten Salt Reactor,TMSR)而开展。利用光学显微镜、压汞仪以及真密度仪研究分析了4种具有代表性的核石墨的孔结构,并利用高压反应釜研究了它们在不同压强下的熔盐(氟化盐,650°C)浸渗特性。结果表明,不同核石墨的孔结构具有明显差异;核石墨的熔盐浸渗与压汞浸渗相似;石墨的孔结构(如入孔孔径、开孔率等)决定了一定压强下石墨是否会发生熔盐浸渗以及浸渗量的多少。

Background： Nuclear graphite is a porous material and its pores can be easily impregnated with the molten fuel salt in a high pressure environment in the molten salt reactor （MSR）. A seepage of the fuel salt into the graphite leads to the formation of local hot spots, which significantly damage the graphite, thereby reducing the service life of the graphite components. Purpose： This study aims to investigate the relationship between porosity property and molten salt impregnation of nuclear graphite. Methods： The porosity properties were examined by optical microscopy （OM）, mercury porosimetry, and helium gas pycnometry. The impregnation tests were performed by impregnating graphites with molten fluoride salt at a temperature of 650~ centigrade and a pressure of 0.1 MPa and 0.5 MPa respectively. Results： The porosity results indicated that the IG-110/IG-430 showed uniformly-distributed gas-evolved pores, small entrance pore diameter 2-4 μm, and a high open porosity; NBG-17/NBG-18 showed a big gas-evolved pores, wide range entrance pore diameter, and a high closed porosity （numerous calcination cracks in filler）. Conclusion： The impregnation results showed that the impregnation mechanism of molten salt was similar to mercury. Only the applied pressure is greater than threshold pressure, the impregnation starts. With the increase of pressure, the small pores will also be filled with salt. The distribution of molten salt indicated that most of the cracks （calcination cracks and Mrozowski cracks） were closed.