Fracture tests of steel pipes by internal detonation of hydrogen-oxygen gas

Abstract

In the reactor of BWR power plants, a small amount of hydrogen and oxygen gas is produced by radiolysis. The radiolysis gas flows through main steam pipes and reaches to branch pipes. There are some possibilities that the mixed gas accumulates and concentrates at the end of branch pipes which is closed by valves. Such flammable mixed gas might explode and cause higher pressure than operating pressure. Especially, detonation consists of a shock wave, which has significant high pressure. For assessment of the integrity of piping against detonation, it is necessary to investigate the strength of steel pipes subjected to detonation pressure. The objective of this study is to reveal empirically the fracture state of steel pipes under detonation pressure. Pipe rupture tests were conducted using carbon steel pipe specimens. The pipe specimens were ruptured by detonation pressure of the mixed gas which consisted of stoichiometric hydrogen-oxygen gas and about 5% nitrogen gas. The strains of the outer surface of the pipe specimens were measured by strain gauges. A high-speed camera photographed the behaviour of pipe rupture. The strain gauges showed that the circumferential strain of the ruptured specimen was over 8 %, and the high-speed camera indicated that the extension of the diameter became about 14 % before the rupture. A dimple pattern was observed on the fracture surface. Therefore, it was indicated that the fracture mode of the carbon steel pipes subjected to the detonation pressure is ductile fracture. Furthermore, we investigated the rupture of the pipe that had closed valve at one end. When detonation wave reaches at closed end, reflected wave occurs. The peak pressure of the reflected wave is twice or more as high as that of detonation wave. The pipe specimen which had the simulated closed valve at one end was ruptured by the reflected wave. The peak pressure of reflected wave was the highest at the end of the simulated closed valve, however, the simulated valve was not fractured and the pipe adjacent to the valve was ruptured by the reflected wave. This is because the thickness of simulated valve was much thicker than the adjacent pipe. It was concluded that the strength of the pipe adjacent to the closed valve is the most important part for the integrity of piping subjected to detonation pressure.