A gasket design suitable for helium circuits of generation IV Gas-Cooled reactors

The article focuses on the problem of helium leakage from high-pressure reactor circuits through flange joints. The use of a laboratory apparatus developed to identify the most suitable gasket is demonstrated on two sample gaskets selected on the basis of preliminary measurements. The apparatus works on the principle of the sample placement in a standardized flange located in a space isolated from the environment. At laboratory temperature, the pressure inside the simulated pipe can be maintained at a maximum of 35 MPa; at the maximum operating temperature of 450 °C, the maximum pressure possible is 7 MPa. Gas analysis outside and inside the flange joint is provided by two gas chromatographs, one of which is equipped with a thermal-conductivity detector (TCD) and the other with a pulsed discharge helium ionization detector (PDHID). In particular, the study discusses the effect of elevated temperature on the sealing properties of the samples, the influence of graphite-layer thickness on gas diffusion and effusion, and finding the optimal leak-measurement time based on the standard deviation. The best sample tested was a kammprofile gasket with a steel insertion and two graphite layers of the thickness of 1.60 mm. At the temperature of 300 °C and the pressure difference of 5.7 MPa, this sample achieved a specific helium-leak rate of 1.5 10−5 mg s−1 m−1. In this case, the observed rate of nitrogen diffusion against the direction of the pressure gradient was almost three orders of magnitude lower, namely 8.9 10−8 mg s−1 m−1. Considering the detection limits of the analyzers used, the best experimental time for gaskets with such low gas-transport values has been determined as 200 h.