At nuclear power plants, containment vessel acts as pressure barrier in such an emergency as a loss-of-coolant accident. It is important as safety equipment to prevent radiological from leaking outside. Rubber gaskets, which are used for sealing faces of containment vessel, are needed to maintain certain sealability not only in stable condition but also in an emergent situation. Among important characteristics of the rubber gasket are not only physical property changes after general aging test (for example, tensile strength changes after heat resistance test) that indicate long-term stability of gasket itself, but also after radiation resistance test which gives potential to good substitute characteristic in terms of sealability in such cases. Physical property changes after general aging test do not always substitute sealability in an emergency, because they do not reflect effects of radiation. That is why nuclear power plant engineers must choose suitable rubber materials that have high performance in radiation resistance. In Japan, silicone rubber gaskets have been used for containment vessel for a long time since 1970th, but in the United States, ethylene propylene diene terpolymer (EPDM) gaskets have been widely used. NICHIAS has silicone rubber and EPDM materials for containment vessel and these gaskets have been used in nuclear power plant. But it is not obvious why different materials have been used in two countries because few relative comparisons of the two materials have been carried out. Especially silicone rubber and EPDM gaskets have many combinations of chemical compositions, so it is difficult to evaluate gasket suitability for containment vessel. There are many kinds of studies concerning long-term stability and life of gasket, but we all must know what characteristics relate sealability under radiation exposure condition and are suitable for guidepost of sealability under previous condition. This report compares silicone rubber gasket with EPDM gasket on physical property changes under irradiation and thermal treatment. We report compression set test results about one each type of silicone rubber and EPDM gasket under irradiation from Co60. And we also report relationships in physical property changes between irradiation and thermal treatment. Finally NICHIAS predicts long-term and emergency sealability of these gaskets from the results of evaluation. We hope it will be a part of design guideline of rubber gasket for containment vessel.

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