Seals of small and medium modular reactors are considered as not important components compared with seals for the large modular reactors which are current-generation plants. Leakage of seals has been a safety concern, however seal maintenance and replacement are costly and time consuming for the current-generation plants. Therefore, integral designs of small and medium modular reactors eliminate the need for seals. However it is impossible to eliminate seals completely from the detail design and equipments. Seals consisting of organic materials are estimated as serious damaged components at design basis accident (DBA). Especially it is no exaggeration to say that rubber gaskets are weakest gasket at DBA. In the point of view we must know about behavior of rubber deterioration under condition at DBA. It is very important to select rubber compound from a viewpoint of not only adequate rubber that has characteristic required for each specification but also consideration about rubber deterioration under condition at normal plant operation and DBA. However rubber for gasket has not been selected based on precise studies with enough data concerned about the rubber deterioration because there have been less studies in the world. From compression set test, swelling test, and Fourier transform infrared spectroscopy (FT-IR) analysis, it has been indicated that chemical structure of both Ethylene propylene diene monomer rubber (EPDM) and Silicone rubber might be broken by radiation, and crosslinking of elastomer by radiation might progress at the same time. In fact, EPDM compounds have good results of compression set after radiation exposure and Silicone rubber does not have good results. It had been predicted that breakage and crosslinking of elastomer chemical structure caused by radiation would have been more serious on Silicone rubber. On the other hand, it has been estimated that breakage of chemical structure of Fluoro-rubber is more influenced by crosslinking of elastomer by radiation, than influence by low molecular elastomer caused by deterioration have been generated. From these results it is considered that Fluoro-rubber is weak against radiation compared to another elastomer. If Fluoro-rubber must be chosen because of its high thermal resistance for example, a radiation condition of environment should be considered seriously. Unfortunately we could not find significant correlation between compression set and general properties. However results of elongation at breakage indicates possibility of relationship to compression set.