Fault diagnostics is important for the safe operation of Nuclear Power Plants (NPPs). In recent years, data-driven approaches like neural networks, fuzzy and neuro-fuzzy approaches, support vector machine, K-nearest neighbors classifiers and inference methodologies, have been proposed and implemented to tackle the problem. Among these methodologies, Dynamic Uncertain Causality Graph (DUCG) has been proved effective in many practical cases. However, the causal graph construction behind the DUCG is complicated and, in many cases, redundant on the symptoms needed to correctly classify the fault. In this paper, we propose a method to simplify causal graph construction in an automatic way. The method consists in transforming the expert knowledge-based DCUG into a Fuzzy Decision Tree (FDT) by extracting from the DUCG a Fuzzy Rule Base (FRB). Genetic algorithm (GA) is, then, used for the optimization of the FDT, by performing a wrapper search around the FDT: the set of symptoms selected during the iterative search are taken as the best set of symptoms for the diagnosis of the faults that can occur in the system. The effectiveness of the approach is shown with respect to a DUCG model initially built to diagnose 23 faults originally using 262 symptoms of Unit-1 in the Ningde NPP of the China Guangdong Nuclear Power Corporation (CGNPC). Final results show that the FDT whose symptoms and diagnosis strategy has been optimized by GA, can drive the construction of DUCG and lower the computational burden without loss of accuracy in diagnosis.

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