Three dimensional Fracture Analysis of the Reactor Pressure Vessel inlet nozzle under Pressurized Thermal Shock

摘要

This paper presents a 3D FE mechanical analysis of the brittle fracture risk of a Reactor Pressure Vessel (RPV) inlet nozzle corner submitted to a Pressurized Thermal Shock (PTS). The input transient loading conies from a 3D thermo-hydraulic analysis of a Loss of Coolant Accident resulting from a 2" small break LOCA in one of the hot legs of the primary system. The present study investigates the margins against brittle fracture along the crack front. The aim is to show that a full 3D study of the PTS captures all the physical phenomena, namely the cold tongue and its effect on a defect postulated in the most severe location.The transient has been computed using a 3D thermal-hydraulic model with STAR-CD software. The selected transient is one of the most severe in 3rd category and a 20 mm deep crack has been postulated in the inlet nozzle corner area where the cold tongue effect is maximum. The mechanical analysis has been performed with SYSTUS Finite Element software which contains fracture mechanics modules for refined meshing of 3D flaws and fracture analyses.The margin against brittle fracture is evaluated at the tip of an open defect in the inner RPV inlet nozzle corner, according to the codified approach. The approach is based on Linear Elastic Fracture Mechanics and compares the Stress Intensity Factor along the crack front during the transient with the End of Life fracture toughness.Under such complex loading, the 3D analysis gives a fully representative picture of the defect behavior under real conditions. The using and the mastering of complex numerical tools allow quantifying accurately the margin to brittle fracture, in order to improve the demonstration of the AREVA reactors safety.