Plant operations are effected by the minimum pressurization-temperature curves. Personnel need to understand the information that is associated with the curves to better operate the plant.

EO 1.8STATE the two bases used for developing a minimum pressurization-temperature curve.

EO 1.9EXPLAIN a typical minimum pressure-temperature curve including:

a. Location of safe operating region

b. The way the curve will shift due to irradiation

EO 1.10 LIST the normal actions taken, in sequence, if the minimum pressurization-temperature curve is exceeded during critical operations.

EO 1.11 STATE the precaution for hydrostatic testing.

MPT Definition and Basis

Minimum pressurization-temperature (MPT) curves specify the temperature and pressure limitations for reactor plant operation. They are based on reactor vessel and head stress limitations and the need to preclude reactor vessel and head brittle fracture. Figure 4 shows some pressure-temperature operating curves for a pressurized water reactor (PWR) Primary Coolant System (PCS).

Note that the safe operating region is to the right of the reactor vessel MPT curve. The reactor vessel MPT curve ensures adequate operating margin away from the crack arrest curve discussed above. The curves used by operations also incorporate instrument error to ensure adequate safety margin. Because of the embrittling effects of neutron irradiation, the MPT curve will shift to the right over core life to account for the increased brittleness or decreased ductility. Figure 4 also contains pressurizer and steam generator operating curves. Operating curves may also include surge line and primary coolant pump operating limitations. The MPT relief valve setting prevents exceeding the NDT limit for pressure when the PCS is cold and is set below the lowest limit of the reactor vessel MPT curve.

Figure 4 PCS Temperature vs. Pressure for Normal Operation

If the limit of the MPT curve is exceeded during critical operation, the usual action is to scram the reactor, cool down and depressurize the PCS, and conduct an engineering evaluation prior to further plant operation.

During hydrostatic testing, minimum pressurization temperature precautions include making sure that desired hydrostatic pressure is consistent with plant temperatures so that excessive stress does not occur. Figure 5 shows MPT curves for hydrostatic testing of a PWR PCS. The safe operating region is to the right of the MPT curves. Other special hydrostatic limits may also apply during testing.

Figure 5 PCS Temperature vs. Hydrotest Pressure

Summarv

The important information in this chapter is summarized below.

Minimum Pressurization-Temperature Curves Summary

MPT curves are based on reactor vessel and head stress limitations, and the need to prevent reactor vessel and head brittle fracture.

MPT curve safe operating region is to the right of the curve.

MPT curve will shift to the right due to irradiation.

Normal actions if MPT curves are exceeded during critical operation are:

Scram reactor

Cool down and depressurize

Conduct engineering evaluation prior to further plant operation

The precaution to be observed when performing a hydrostatic test is to make sure the pressure is consistent with plant temperatures.