A vacuum vessel wall with a postulated crack is modeled by using the two-dimensional solid finite elements. The J-integral value, a criterion of fracture mechanics, is calculated to estimate the integrity of the vacuum vessel wall with a postulated crack.

The analytical results show that the J-integral value of a crack having a quarter depth of the wall width is considerably less than the fracture toughness JIC when the membrane stress is less than the 0.2% proof stress of the unirradiated material, even if the vacuum vessel is irradiated to 3 dpa. This means that a shallow crack in the vacuum vessel wall does not become critical in the design load.