INFLUENCE OF MECHANICAL VIBRATION IN TRANSPORT ON LEAK-TIGHTNESS OF METAL GASKET IN TRANSPORT/STORAGE CASK FOR SPENT NUCLEAR FUEL

Mechanical vibration in sea transport of spent fuel shipping cask has been measured and analyzed to result in possible cyclic displacement of ±0.02 mm to the metal gasket. In order to obtain a relationship between amount of lateral sliding of the lid and the leak rate, a 1/10-scale model of a lid structure of metal cask with a metal gasket of double O-ring type was manufactured. The gasket had a diameter of 10 mm and was coated with aluminum sheet. The metal gasket was thermally aged simulating heat from spent fuel in the cask. Static one-directional loading experiments showed no leakage when the lateral displacement was less than 0.1 mm. When the displacement increased up to 3 mm, the leak rate increased up to 10-6 Pa·m3 /sec, but recovered to 10-8 Pa·m3 /sec in 72 hours. Cyclic loading experiments showed the leak rate did not increase permanently when the lateral displacement was within ±0.02 mm. The leak rate increased permanently, when the displacement increased to more than ±0.035 mm. Dynamic one-directional loading experiments showed that the leak rate after the maximum displacement coincided with that at the same displacement by the static one-directional loading experiments. Namely, the leak rate did not depend on the loading rate nor displacement rate. These results indicate that the mechanical vibration in transport would influence the leak-tightness of the metal gasket required for storage if the amplitude of the vibration exceeded a threshold value.