STRUCTURAL RESPONSE OF THIN-WALLED AUSTENITIC PIPES TO INTERNAL GASEOUS DETONATION LOADING

One of the main risks with the occurrence or by the use of hydrogen in oxygen-containing environment is the easy ignitability of the gas-mixture. An example of unwanted aggregations of such a gas-mixture is the generation of gaseous hydrogen and oxygen in safety-relevant piping of nuclear power plants due to radiolysis. Radiolysis is the dissociation of water under the influence of gamma and neutron radiation. Within the scope of a research project funded by the German Federal Ministry of Economics and Technology (BMWi), the assessment of incident situations with appearance and ignition of different amounts of radiolysis gas should be provided. Of central importance for the pipe behavior under detonative loading is the knowledge about the pressure-profile of the deflagration (subsonic combustion before entry of detonation) as well as the mechanisms of an acceleration of the deflagration up to the detonation, which starts as a so-called overdriven detonation in a volume of pre-compressed gas and then decays to the stable detonation. Furthermore pressureprofile and speed of the detonation wave are elementary pipe load parameters. Detonation tests and numerical evaluations were perfbrmed to simulate detonations of radiolysis gas in pipes with O.D. x t (outer diameter x wall thickness) = (114 x 6) mm made of austenitic steel under operating pressure of p 0 = 7 MP a. For the consideration of mixtures of radiolysis gas and steam inside a piping the volume proportion in percent of oxyhydrogen (H-2+1/2O(2)) to nitrogen (N-2) was varied. The influence of the overdriven detonation is essential for the deformation and rupture behavior of the pipe and depends strongly on the ratio of oxyhydrogen to nitrogen. Due to the high-rate response of the pipes to the detonation, multiple longitudinal cracks and fragmentation occurred in certain cases. For the evaluation of the fracture mechanisms accompanying metallographic investigations were performed. For the numerical investigations an appropriate material constitutive law was chosen in order to describe the material behavior including strain rate sensitivity and thermal softening at high deformations. Material parameters are obtained from small-size specimen tests. The material model includes necking and was validated by numerical simulations of tensile and compression tests. For the numerical simulation of gas detonations in pipes the full pressure profile including deflagration and overdriven detonation is of a central importance. In this publication pressure profiles were obtained by tests and calculations. The numerical simulations of detonations inside the pipes were verified by the detonation tests using results of optical deformation measurement.