This application deals with the procedure to determine the heatup and cooldown curves (P/T curves) expressed in terms of allowable values for the pressure, P, and temperature, T, of the reactor coolant system. These curves, which are part of the plant technical specifications, are required by the U.S. legal document 10 CFR 50 Appendix G. The procedure follows the ASME Code Section III Appendix G, from Section III or Section XI. It is necessary to consider, among others, the requirements of the upper-shelf energy as well as the pressurized thermal shock to ensure the structural integrity of the reactor vessel. As the material toughness of this component varies with the neutron radiation the P/T curves must be updated periodically. The analysis is conducted using the procedure of the ASME Code, applied to a postulated semi-elliptical axial crack on the inside or outside surface of the reactor vessel shell, in the beltline region.