A Probabilistic Approach to Heatup and Cooldown Operating Limits for a Nuclear Pressure Vessel

During plant heatup and cooldown, the stresses in a nuclear pressure vessel are the membrane stress associated with system pressure and the stress caused by the thermal gradient across the vessel wall. The system pressure and the rate of temperature change (heatup or cooldown) must be controlled in a manner that protects against failure of the vessel. The current basis for setting these pressure-temperature operating limits is a deterministic fracture mechanics analysis that conservatively applies a safety of factor of two to the system pressure and assumes a large flaw corresponding to 1/4 of the thickness of the vessel. This calculation produces an upper bound estimate of the stress intensity at the flaw tip, which is compared to a lower bound estimate of the material fracture toughness to set the pressure-temperature operating limits. This methodology results in restrictive operating limits that correspond to an extremely low probability of vessel failure. The development of state-of-the art probabilistic fracture mechanics for the FAVOR Code has made it possible to determine the risk of vessel failure associated with these restrictive limits. Relaxation of these limits may be possible with negligible impact on the risk of vessel failure. Overall plant risk may be reduced by eliminating other potential challenges to plant integrity that are associated with the current restrictive operating limits.