# CS Measures for Nuclear Power Plant Protection: A Systematic Literature Review
## 4 Literature Review
### Measures for NPP Protection  
After obtaining an understanding of the threats and vulnerabilities that afflict the digital systems of NPPs, it is necessary to establish measures to protect them from computer threats and techniques for the mitigation of cyber attacks. As indicated by [30], the cyber/physical security of NPPs may require the management of different levels/layers of protection in accordance with the standard set by RG 5.71. According to [19], standards of minimum acceptable risks are challenging to develop due to the constant progress and modernization of information technology and digital systems, in addition to the new threats and tools that are constantly being developed. For the same reason, standardized measures for protection from or mitigation of cyber attacks are just as arduous to define. In the series of documents published by the International Atomic Energy Agency (IAEA) [50], it was suggested that the basic principle to follow to protect against computer threats is the use of the defense-in-depth model. This model is based on the idea of using multiple layers of often independent protection measures to guarantee multi-level protection without single points of failure. The authors of [19] described an action list of activities to conduct during the development, implementation, maintenance, and improvement of computer security in NPPs in accordance with the procedures defined in the international standards and regulatory guides. Approaches to CS threat prevention, detection, and response of (IT) systems that have proven to be successful have also been suggested to be used for I&C systems of NPPs. Naturally, differences in real-time operational requirements, distinct communication protocols, and requirements for continuous availability of the systems need to be considered when adapting these approaches for I&C systems in NPPs. The authors of [49] proposed specific approaches to implementing a framework for prevention, detection, and response for I&C systems. For prevention, the authors suggested monitoring and auditing I&C systems to meet the real-time requirements. For detection,an intrusion detection approach based on physical data was proposed in order to deal with the distinct communication protocols. Finally, for response, the intrusion-tolerant control was proposed to maintain continuous availability. According to the authors, this overall solution, when combined with a safety design principle, should provide an overall solution for CS of I&C systems. In addition to to adapted frameworks for CS, a number of approaches specifically developed for the CS of I&C systems of NPPs have appeared in the literature. The authors of [35] proposed a framework for CS assessment using an Early Fault Diagnostics System (EDS) during the operational stage. The EDS aims to prevent the evolution of the incident/accident by using a troubleshooting process in any plant operational mode. This would allow plant operators to identify any significant deviation of plant parameters from their normal value, well before reaching any undesired threshold that would potentially lead to a prohibited plant state, together with the cause that generated the deviation. The authors of [47] laid out a new systems-engineering-focused approach for addressing threats to NPPs, which they named System-Aware Security (SAS) for NPPs. The authors theoretically demonstrated a possible application of the methodology in order to address cyber attacks employed via embedded infections in NP systems. The authors highlighted that the approach still has several limitations, and further work in component integration and security analysis may be necessary to improve the model. To improve the robustness of key equipment under cyber attacks, the authors of [43] proposed a localized kit for key equipment in a process as a complementary detection method. The authors highlighted that the proposed model reduced the total number of variables used and improved the computational speed when compared to other models.