Master curve approach to integrity assessment of RPV steels

It has been the practice to assess the integrity and safety of nuclear reactor pressure vessels (PV) in operation by making use of the ASME Codes, ASTM Standard E185 together with USNRC Regulatory guide 1.99, Rev 2. Here, the fracture toughness of the PV is derived from a reference fracture toughness curve using an indexing parameter RT_NDT which in turn is obtained from the results of Charpy impact tests and the Drop Weight nil-ductility transition tests. Hence, such an evaluation based on the indexing parameter RT_NDT would be an indirect approach of assessing the PV. Further, these assessments have been found to be conservative with respect to the actual condition of the PV. Under such scenario there is a specific need to have a more direct and established test methodology that could be resorted to for ascertaining the fracture toughness of the PV reliably. The new approach is indeed the Master curve approach as furnished in ASTM Standard E 1921. This makes use of an indexing parameter T₀ termed as Reference Temperature, defined as the temperature at which the median fracture toughness for a 1 CT specimen would equal 100 MPa√m. This is not only a direct approach for the evaluation of the PV's toughness but it also takes into account of the statistical aspects while analyzing the data