Assessment of the EPDM Seal Life Time in Nuclear Power Plants

Polymeric materials used in nuclear power plants (NPP) are usually during their operational lifetime exposed to high number of various deteriorative effects resulting in their degradation. In the case o-rings and gaskets these factors consist predominantly of long-term irradiation (at rather low dose rates) and enhanced temperature in combination with mechanical stress. The related chemical and physical changes that appear as a consequence of ageing decrease the sealing force, witch results in the loss of the tightness and the seal failure. Within the NRI Rez testing laboratory, it has been tested the EPDM gasket which was designed as a sealing for hermetic doors and manholes in the NPP containment. To assess the service life time, the seal has been acceleratelly aged to simulate its long term operation in harsh conditions - temperature up to 60 degree, dose rate up to 1 Gy/h. 7 and 14 mm thick samples with the hardness of 50 and 60 ShA, respectively, have been during the ageing compressed between two massive metal plates in the interval between 0 and 35 % of the initial thickness. Mechanical properties, compression set (the amount of residual deformation in the seal after a distorting load is removed) and the leakage tests have been carried out. To achieve near equilibrium set values, simulating the seal relaxation during the reactor shut down and open doors (1 month per year), the compression set have been measured after 24 hour relaxation of the seal at 100 degree. The primary seal function is preventing escapes of unwanted fluids not only during normal operation, but also during accidents like LOCA (loss of coolant accident). Therefore LOCA has been simulated and the tightness at the overpressure up to 0.5 MPa. Have been tested. On the base of the results minimal necessary service compression to keep the tightness at all conditions including accident was found and the service life time has been assessed for both seal thickness