Charpy impact tests discussed in the preceding chapter are most useful as a screening test to evaluate the relative effect of irradiation on producing embrittlement in different steels. However, Charpy data cannot be used directly for design. Design will probably be based on a defect-tolerant approach, where a plane-strain fracture toughness (thick sections) or a J-integral (thin-walled structure) approach are used [1]. Because of the simpler test procedure for a Charpy test, fewer fracture toughness data, especially of irradiated specimens, are available than are Charpy impact data. There is a qualitative correspondence between changes in Charpy impact properties and fracture toughness of irradiated steels, and the change in USE is often taken as an indication of the change in ductile fracture toughness [1].