Analysis of crack growth in a bar with complete circumferential surface crack : In Light Water Reactor Environment

Sammanfattning

This thesis is about analyzing a cylinder with a complete circumferential crack. The background of this thesis was an incident at Oskarshamn 3 in 2008, where a control rod extension was found to have cracks. This led to many investigations using different methods to analyze these cracks. The investigations showed that the cracks occurred and propagated due to thermal load. The load happens due to a cyclic temperature change, which leads to the material having different temperatures at different points, leads to stresses that are in cycles and starts cracking due to fatigue.OKG has ways and rules to follow when analyzing fractures on components. OKG needsto simulate and test the cracks; the simulations could be made in ISAAC, a calculation program for fracture mechanics assessments and damage resistance analyses. Sometimes, this software becomes hard to use for some types of loads. In a situation where a thermal load exists, ISAAC could be hard to use because it cannot create a thermal load on this specific geometry. This means investigating how thermal load is simulated in ISAAC and how well it performs is of interest. The method of predicting crack growth could be calculated in two different ways with two crack growth laws. This means that it is also of interest to see the difference between these crack growth laws. The crack growth laws are given in two reports, one in MD-02 and the other in ASME XI. The laws are not named in the reports, but in this thesis, they are named after the reports they are given in. The thesis aims to show the conservatism of the ISAAC software compared with Abaqus. It is also to show which of the crack growth laws is the most conservative.The methods used in this thesis simulate a cylinder with a complete circumferential crack and a thermal load. The thermal load is based on actual measured data but modified because it is too hard to say exactly how it acts, as the result changes occasionally. The temperature cycles between 85 ˚C and 276 ˚C with a hold time of ten seconds. The simulations are made in Abaqus and ISAAC to compare the results.The results from these simulations are the stress intensity factor (SIF). The results show a difference in SIF over crack depth between the software. The comparison between the software is that the SIF values for ISAAC become higher for deeper cracks than for small cracks. Thereis a big difference between the results, which means something is wrong when implementing the load in ISAAC. One explanation of this is the effect of using deformation-based and powerbased loads.The difference between the laws is also significant. It shows that the MD-02 has a higher crack growth rate than the modern law ASME XI. This means that the MD-02 is more conservative than ASME, but it does not mean it is the best way to analyze the crack growth rate.