# CS Measures for Nuclear Power Plant Protection: A Systematic Literature Review
## 4 Literature Review
### Risk Assessment and Threat Analysis for NPPs  
The CS guides and regulations formulated by international organizations and countries, such as RG 5.71, mostly provide general guidance for CS risk assessment. According to [19], international regulatory guides present a series of shortcomings:  
* Lack of assessment methods for computer security conditions and identification of security vulnerabilities;
* Lack of assessment methods for computer risks and threats;
* The development stage does not consider computer security issues;
* Lack of consideration for aspects of training and attestation of personnel concerning computer security. The authors of [50] suggested classifying attacks by their format/character:  
* Information-gathering attacks;
* Active attacks to disable or compromise the proper functioning of one or several computers or other devices critical to a facility's safety or security;
* Concurrent modes of attack. The authors of [23] argued that the vulnerability assessment methodologies proposed by the US Nuclear Regulatory Commission (NRC) require several key additions andchanges to increase their efficacy. Some of the changes suggested by the authors include conducting and storing a comprehensive inventory of digital systems and components, conducting more penetration testing, using vulnerability databases and analysis software, and improved use of vulnerability assessment methods. The authors of [19] suggested categorizing threats to NPPs into cyber threats at the stage of development of I&C systems and cyber threats at the stage of operation of I&C systems at NPPs. Additionally, these threats could also be sub-divided based on whether they are internal or external. To identify possible risks and overcome the gaps found in the current regulatory frameworks, many vulnerability assessment methodologies have been proposed in the literature. A potential security risk analysis of NPPs should include probability of attack by the adversary, attack purpose, and attack ability [42]. To evaluate CS risk in accordance with regulatory guides such as RG 5.71, the authors of [28] proposed a CS risk model using a Bayesian Network (BN) for a Reactor Protection System (RPS) of a nuclear reactor, as well as a methodology for applying analytical results from a BN model to an event tree model. The model was developed to overcome the limitations of previously used fault trees as a Probabilistic Safety Assessment (PSA) method. Another risk assessment framework was proposed by [24]. The framework proposed by these authors evaluates risk by defining the difficulty and consequences of a cyber attack, basing assessment methods on Bayesian belief networks and probabilistic safety assessment methods. The authors demonstrated the feasibility of the proposed framework by quantitatively evaluating several cyber attack scenarios based on the developed models for difficulty and consequences as a case study. Finally, the authors suggested that the framework may be used for risk-informed regulation of cyber attack scenarios and CDAs with quantitative goals, as well as risk-informed CS strategies and related evaluation efficiencies. According to RG 5.71, periodic checking of the I&C systems should be conducted to identify any possible CS vulnerabilities. For this reason, CS vulnerability checking systems, such as the one proposed by [31], should be used to reduce the impact of vulnerabilities, as well as to ensure compliance with the automatic check regulatory guidelines. A fundamental requirement of these scanning tools is that they should not generate excessively high network traffic or overhead on the objects to be scanned. The work by [45] determined the levels of CS protection for the subsystems and equipment of a digital I&C system of an NPP. These levels were determined using the CS defense-in-depth model, an example of which is shown in Figure 2. The authors tried to identify the potential CS risk factors, namely, assets, threats, and vulnerabilities. The proposed risk analysis was then carried out each of the identified levels.