A reactor pressure vessel (RPV) is assumed to be subjected to pressurized thermal shocks (PTSs) as a result of the emergency cooling water injected during a loss-of-coolant accident (LOCA). The cooling flow is not homogeneous but typically in a plume shape (stripe cooling) flowing from the cold leg through the inlet nozzles. This paper aims to analyze the non-uniform cooling effect on the RPV integrity.
In this paper, both deterministic and probabilistic methods are used to analyze the integrity of a model RPV subjected to PTS. RELAP5, GRS-MIX, CFD and other semi-analytical methods are used to analyze the transient with and without considering plume cooling effect. Finite element method (FEM), extended finite element method (XFEM) and weight function method are used to calculate KI of the postulated cracks. The FAVOR code is used to calculate the conditional probabilities for crack initiation and failure of the RPV considering different crack distributions.
KI based on CFD input is the highest, followed by that based on reference transient, GRS-MIX and RELAP5. Peak KI of the cracks inside the plume increases about 33% compared with that outside. According to the maximum criteria, the maximum allowed RTNDT are 56.9 °C, 90.2 °C, 98 °C, 115.7 °C and 136.2 °C for the crack in the nozzle region based on CFD transient, the cracks in the ring region based on the CFD, reference data, GRS-MIX and RELAP5 calculated transient, respectively. These values are 36 °C, 68.5 °C, 73 °C, 81 °C and 104 °C according to the tangent criteria. The conditional probability inside the plume is more than nine orders of magnitude higher than outside the plume. Considering plume cooling effects increases the total failure frequency by 1–2 orders of magnitude. In order to be conservative, it is necessary to consider the plume effect in the integrity assessment.