# CS Measures for Nuclear Power Plant Protection: A Systematic Literature Review
## 4 Literature Review
### Risk Assessment and Threat Analysis for NPPs
The authors of [41] proposed an importance analysis method for cyber attacks against an NPP using the Probabilistic Safety Assessment (PSA) method. The authors started by identifying possible cyber attacks with failure modes. The authors demonstrated the proposed PSA method with two case studies. In the case studies, the risks of two cyber attack scenarios were quantitatively evaluated with two risk metrics: Core Damage Frequency (CDF) and Conditional Core Damage Probability (CCDP). The authors concluded that by identifying significant CDAs and classifying cyber attacks using quantifiable measures, it should be possible to develop a defense strategy against cyber attacks on NPPs that is both reliable and efficient. The authors of [51] presented a qualitative methodology for CS assessment that is appropriate for nuclear I&C systems. The authors conducted an assessment based on a questionnaire comprising 162 questions divided into five categories. An evaluation of the questionnaires was then conducted by weighting each response depending on whether the results showed the overall CS status for each category, and they conducted an evaluation of the system as a whole. The authors suggested that this methodology could serve as a CS index at an initial phase of the system development for the CS assessment of nuclear I&C systems. Nonetheless, further research needs to be conducted to assign detailed CS countermeasures in each category. Possible attacks may also be categorized into taxonomies to facilitate both their identification and preventive action planning. The authors of [7] highlighted that it is difficult to study cyber attack taxonomies for NPPs considering the characteristics of ICSs and the inadequacy of research compared to such issues in information technology. The authors suggested a template for a cyber attack taxonomy based on the characteristics of NPPs, exemplified a specific cyber attack case in the template, and proposed a systematic countermeasure selection strategy by matching the countermeasures with CDAs and security control in RG 5.71. The taxonomy included the attack procedure, attack vector, attack consequence, vulnerability, and countermeasure selection. The authors of [34] noted the importance of correctly classifying cyber attacks and distinguishing them from fault-induced safety events. For this purpose, the authors developed an event classifier to classify abnormal events in NPPs as either fault-induced safety events or cyber attacks. While the classifier was proven to be successful in distinguishing different types of abnormal events, more work is needed to enable the classification of combinations of safety events and cyber attacks. The authors of [20] proposed a Monte-Carlo-based exploration framework for generating cyber attack scenarios in Cyber-Physical Systems (CPSs). The method takes into account various failure modes of attacked components of the CPSs and outputs the possible effects of the cyber threats on the system. According to their analysis, actuators are the most vulnerable CPS components, as their failures may lead to the loss of system functionality and integrity. The authors of [22] proposed a method for calculating the degree of risk to a nuclear reactor's systems based on multiple factors for the purpose of safety. The authors presented a security threat mapping table, which was used to define and specify security problems. An important observation noted by the authors is that to create security profiles, environmental elements, security assumptions, and the organization's security policy must be analyzed. The model developed by the authors has already been implemented in reactor protection systems that are operating in the Republic of Korea. Nonetheless, additional research on I&CS protection profiles and security functions is suggested. A possible way to study attack vectors is by simulating threat scenarios both theoretically and with the aid of simulation platforms. The authors of [32] presented three threat scenarios related to the Electric Power Systems (EPSs) of NPPs.