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[en] High Efficiency Particulate Air filters (HEPA Filters) are the main components in ventilation or confinement system for the retention of radioactive particles in nuclear installations. During abnormal conditions or accidents (e.g. fire accident, criticality in a nuclear fuel cycle facility and LOCA in power reactors) the resulting heat, smoke and humidity affect to a large extent the performance of HEPA filters. As a part of a research programme aims at the evaluation and improvement of the performance of HEPA filter media during abnormal conditions, the effect of elevated temperatures up to 400 degree C on the resistance of medium to penetration of water under pressure has been investigated. The test results showed that the resistance of the medium to penetration of water decreases with increase in temperature and thermal exposure time. This could be attributed to burnout of the organic binder used to improve the resistance of the medium to the penetration of water. The results also showed that at 400 degree C the resistance of the medium to the penetration of water disappeared. This was confirmed by inspection of the filter medium samples after exposure to high temperature using a scanning electron microscope. The inspection of the medium samples showed that the organic binder in the medium was deformed and finally collapsed at 400 degree C. Also, a best estimate model for the relation of filter medium resistance to water penetration under elevated temperature has been implemented. The results of this study can help in establishing a regulatory operating limit conditions (OLCs) for HEPA filter operation at high temperatures conditions in nuclear installations.