The cyber security problem is posing new challenges to the current safety analysis of nuclear power plants. Historically, analogue control systems in the absence of interactive communications are immune to cyber-attacks; however, digital control systems with extensive interconnection of reprogrammable components are intensely vulnerable to cyber-attacks which shed light on the significance and urgency of the cyber security. The current cyber security approaches, which merely focus on information networks, have not given multi-faceted considerations to instrumentation and control (I&C) systems. The cyber-attack on I&C systems may lead to more severe consequences, including the abnormal change of parameters, the malfunction of equipment, and even the accident condition. The existing cyber security approaches for information networks, such as firewalls, encryption, can enhance the cyber security of I&C systems, but are often insufficient in addressing challenges associate with the I&C systems which link cyber space and physical systems. The defense approach based on physical information should be developed to meet the emerging challenges. In this paper, we propose the cyber-physical security (CPS) approach based on the physical process data for the cyber defense. This approach does not intend to replace current cyber defense mechanisms. It could be served as the last barrier for security defense. The goal of the CPS defense approach is to detect attacks at the beginning of the occurrence of physical process anomalies cause by cyber-attacks. A practical implementation of the CPS approach is proposed and its influence on the existing infrastructure is discussed. The statistical analysis techniques are utilized on physical process data for attack detection. The method of dynamic principal component analysis (dynamic PCA) is employed to characterize the correlation of multiple variables in the normal operational condition. In the abnormal operational occurrence, the chi-square detector is able to distinguish adversarial cyber-attacks from ordinary random failures.

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