Fracture Behavior of Thermally Aged Primary Coolant Piping Made of Cast Stainless Steel

The fracture toughness of primary coolant piping made of duplex cast stainless steel can be reduced by thermal aging, at the operating temperature of a light water reactor, to that of so-called low upper shelf reactor pressure vessel steel. Compared to reactor pressure vessels, the crack detection ability of ultrasonic testing of primary coolant piping is inferior. It is therefore important to establish a standard procedure to assess the structural integrity of thermally aged primary coolant piping. Crack extensions on the inner surface of scale model primary coolant piping were measured in four point bending tests. The piping had been aged to simulate 60 years of operation at 325°C. The crack extension forces at the fronts of cracks were evaluated based on the measured tensile properties of the pipe material. They coincided well with the material’s J_mat-Δ a measured using a ITCT specimen. The J controlled crack extension was verified. Because the primary coolant piping has small mean radius to thickness, it will not be flattened so much as usual thin piping. It means that the crack extension force is less than that of thin walled pipe if the hypothetical elastic stress at the crack is same. Another point in estimating crack extension force is the tensile properties of thermally aged cast stainless steel. Methods of predicting the tensile properties and fracture toughness of thermally aged cast stainless steel have not been proposed until recently. By applying the proposed properties, which were confirmed to be appropriate by the test results of this study, and a recent study on crack detection ability, a simple procedure for assessing the crack extension force of cracks in thermally aged primary coolant pipe was prepared. It will be applied to the evaluation of structural integrity analyses for aged Japanese nuclear power plants.