BRITTLE FRACTURE MECHANISM
Figure 3 is a graph of stress versus temperature, showing fracture initiation curves for various
It is clear from the above discussion that we must operate above the NDT temperature to be
Figure 3 Fracture Diagram
certain that no brittle fracture can occur. For greater safety, it is desirable that operation be
limited above the FTE temperature, or NDT + 60F. Under such conditions, no brittle fracture
can occur for purely elastic loads.
As previously discussed, irradiation of the pressure vessel can raise the NDT temperature over
the lifetime of the reactor pressure vessel, restricting the operating temperatures and stress on
the vessel. It should be clear that this increase in NDT can lead to significant operating
restrictions, especially after 25 years to 30 years of operation where the NDT can raise 200F
to 300F. Thus, if the FTE was 60F at the beginning of vessel life and a change in the NDT
of 300F occurred over a period of time, the reactor coolant would have to be raised to more
than 360F before full system pressure could be applied.