Analysis of the gas diffusion process during a hypothetical air ingress accident in a modular high temperature gas cooled reactor

In order to simulate the diffusion process during a hypothetical air ingress accident in a modular high temperature gas cooled reactor, a one-dimensional coupled diffusion-convection model has been established. In this analysis it is shown first, that experiments performed at the Japan Atomic Energy Research Institute (JAERI) have been recalculated successfully, thus validating the new model. Applying this model to the NACOK facility, now under construction at the Institute for Safety Research and Reactor Technology (ISR) of the KFA Research Center Jiilich, a delay time until the onset of natural circulation was found to be 14 hours. For the 200MW HTR-MODULE designed by the SIEMENS company about 39 hours are predicted. It was found that pressure disturbances, e.g. caused by environmental noise could significantly shorten the time delay until the onset of natural circulation. The onset time delay remains constant below 40 dB noise, reaches half of its value at 80 dB and tends to zero at 120 dB. Thus, the relevance of the delay effect with respect to safety related questions is reduced