Nuclear power plants (NPPs) are complex dynamic systems with multiple sensors and actuators. The presence of faults in the actuators and sensors can deteriorate the system's performance and cause serious safety issues. Although concerns about faults in the sensors and actuators in NPPs is a similarly important topic, only a few papers have discussed it. In this study, fault detection and diagnosis (FDD) based on neural networks (NN) and K-nearest neighbour (KNN) is addressed for a pressurized water reactor (PWR). Fault detection is first determined based on the NN. Second, the KNN algorithm is used to classify the faults. The proposed approach is capable of classifying a variety of actuator faults, sensor faults, and multiple simultaneous actuator and sensor faults. A set of simulation results is provided to demonstrate the accuracy of the FDD method. The classifier performance is further compared with other machine learning techniques.INDEX TERMS Fault classification, fault detection, K-nearest neighbor (KNN), neural networks (NNs), nuclear power plants (NPPs), pressurized water reactor (PWR).