<p>The integrity assessment of the reactor pressure vessel (RPV) is of high importance as it is the main component of the primary loop in a nuclear reactor and counts as an irreplaceable part. The core of the RPV can be considered as hollow circular cylinders disregarding the geometrical details due to nozzles. During an event such as a loss of coolant accident, the RPV is subjected to stresses originated by a combination of pressure and temperature gradients. Under the presence of initial cracks, such defects can concentrate stresses, which might provoke crack growth. Irradiation on material can significantly reduce the fracture toughness and in the possible scenario of crack growth, this can promote undesirable unstable crack propagation. Experiments on real size samples to examine the fracture behavior under these loading conditions are extremely expensive and one possible way to circumvent this problem is to perform experiments on mock-ups.</p><p>This contribution investigates the prediction capabilities not only of crack initiation but also of crack growth and arrest by means of a rather simple method based on the closed-weight function formula for the stress intensity factor (SIF) for axial crack in hollow cylinders subjected to thermal shock. The method is explained together with some illustrative examples for real low allow steel used in nuclear applications. In order to obtain the temperature and stress distribution in the cylinder during the thermal shock, a finite element (FE) model is defined to obtain the uncoupled solution of these two fields needed for the closed-weight function. Since the material exhibits a ductile-brittle transition fracture behavior, the temperature-dependent fracture toughness for initiation and for arrest are described using the ASME model. The solution for the SIF is based on the linear elastic fracture mechanics (LEFM) and therefore only elastic material is assumed and the crack can propagate in brittle manner. The crack initiates propagation if the SIF value at the crack tip reaches the fracture toughness for initiation and propagates unstably in mode I unless the fracture toughness for crack arrest is reached. The quality of the solution is checked by comparing the obtained solution for a “stationary” crack with the calculated extended finite element method (XFEM) solution for the same loading transient. The results show that for some geometries of the cylinder, the crack stops and in some other cases the crack propagates indefinitely and the cylinder fails. The method does not require expensive computational resources and gives fast reliable results. The closed-weight function method provides a powerful and economical way to predict the crack propagation and arrest of the initial crack. This is an advantage when an optimization of the structure is needed. Thus, the second part of the paper shows the results for the optimization of a RPV, downscaled to an experimental mock up subjected to a realistic thermal shock.</p>

Crack Propagation Calculation for Axial Cracks in Hollow Cylinders Subjected to Thermal Shock

Paper Proceedings: https://asmedigitalcollection.asme.org/PVP/PVP2021/volume/85314

ASME 2021 Pressure Vessels & Piping Conference (PVP2021)

Supporting the need among researchers worldwide for additional online and multimedia content, ASME now offers the ASME Conference Video Collection. This platform will collect and provide long-term access to conference presentation videos, encompassing all technology areas within ASME's technical conference and events program. About 30,000-40,000 engineers across all mechanical engineering and related disciplines participate in ASME conferences each year. This includes authors of approximately 6,000 to 8,000 technical papers published by ASME, the majority of whom also submit video presentations for our virtual conferences. The video collection will be available as an add-on to existing product packages or as a separate subscription.