Sleeve and socket welds are often used in small bore nuclear power plant pipework where access is too limited to allow a conventional butt-weld. These welds are also used in process plants and pipelines as a permanent repair to reinforce areas such as cracks and corrosion that might threaten the structural integrity of the component. The fillet weld associated with this type of joint is particularly susceptible to lack of fusion defects which can be problematic to detect using conventional volumetric inspection techniques. The stress concentration associated with this type of defect will impact the fatigue life and pressure retaining ability of the joint. This paper provides examples of Copper/Nickel (CuNi) sleeve weld defects and presents an approach for determining the fatigue life of socket welds due to pressure cycling within a Pressurised Water Reactor (PWR) environment. This approach is based on modelling lack of fusion features using a database of sleeve and socket weld Non-Destructive Testing (NDT) records and calculating the stress range in the remaining ligament using textbook calculations. Sensitivity studies presented herein show the impact of lack of fusion and pipe size/thickness on fatigue life.