4.206. Soft sealing materials are commonly used in multiple containment applications, such as in the sealing of ventilation valves or the inflatable sealing of air locks. Although these materials contribute to a very high leaktightness of the containment under normal conditions, their behaviour in design basis accidents should be properly demonstrated. Potentially damaging effects for soft sealing materials include embrittlement and cracking due to high tempera­tures and irradiation, dissolution due to moisture and steam, and swelling or shrinkage due to temperature fluctuations. Specific consideration should be given to the protection of these materials from the direct effects of hydrogen burning and/or the accumulation of radioactive aerosols. In extreme conditions such materials may degrade to the extent that their mechanical properties are altered.

4.207. The anticipated lifetimes of soft sealing materials and the ageing mechanisms that affect their performance should be assessed, and appropriate replacement intervals should be established (para. 4.39). Sealing components should be designed to be easily inspectable and replaceable.