Structural Integrity Assessment of RPV With Brittle Fracture Criterion: Issues and Advanced Approaches

A key input to calculation of the structural integrity of the RPV by the brittle fracture criterion is known to be the fracture toughness of a material. When calculating resistance of RPV to brittle fracture it was taken earlier that fracture toughness, K_JC, depends only on temperature, and does not depend on thickness of specimens if this thickness is sufficiently large so that the plane strain condition is satisfied. To date it has become clear by many researches that K_JC depends on a crack front length. This follows from the statistical nature of brittle fracture in steels. It means that for adequate assessment of RPV structural integrity, the dependence of K_JC on the postulated flaw size has to be taken into account. Moreover, it has been shown that the K_JC values for specimens with shallow cracks are larger than for specimens with deep cracks, and the K_JC values obtained from small-size fracture toughness specimens (for example, pre-cracked Charpy specimens) are larger than the values for CT specimens with thickness of 25 mm, and K_JC values may be affected load biaxiality. These findings are explained by the loss of constraint. Thus, for adequate assessment of RPV structural integrity by the brittle fracture criterion, the scale factor has to be taken into account, with two causes being responsible: the physical cause connected with the statistical nature of brittle fracture and the mechanical one related to the difference in constraint for specimens of various geometries. This analysis is important for assessment of RPV structural integrity because i) for most cases the postulated flaws in RPV, due to their size and location, may be considered as shallow cracks; ii) the K_JC values for irradiated RPV steels are usually obtained from small-size surveillance specimens that are as a rule, pre-cracked Charpy specimens; iii) for a postulated flaw in RPV, a load acts both perpendicular and along a crack front. i.e. loading is biaxial. For adequate assessment of RPV structural integrity by the brittle fracture criterion it is also necessary to take into account that distribution of the stress intensity factor along a crack front in RPV is heterogeneous, and loading is non-monotonic and non-isothermal. In the present report, advanced approaches are considered for assessment of RPV structural integrity that allow solving the above problems. The considered approaches have been included in Russian Standards for assessment of RPV structural integrity.