MPT Definition and Basis:

MPT (Minimum pressurization-temperature) curves specify the temperature and pressure limitations for reactor plant operation. That are based on reactor vessel and head stress limitations and the required to preclude reactor vessel and head brittle fracture. Figure displays a few pressure-temperature operating curves for a PWR (pressurized water reactor) Primary Coolant System (PCS).

Remember that the safe operating region is to the right of the reactor vessel MPT curve. A reactor vessel MPT curve ensures adequate operating margin away from the crack arrest curve elaborates above. The curves used through operations also incorporate instrument error to ensure adequate safety margin. The MPT curve will shift to the right over core life to account for the increased brittleness or decreased ductility Because of the embrittling effects of neutron irradiation.  Below figure also holds pressurizer and steam generator operating curves. Operating curves might also involve surge line and primary coolant pump operating limitations. A MPT relief valve setting avoids exceeding the NDT limit for pressure while the PCS is cold and is set below the lowest limit of the reactor vessel MPT curve.