## 6 Conclusions  
In this work, we developed and validated a testbed for conducting cybersecurity assessment in nuclear power plants. The main advantages of this setup are its realism, flexibility, and low cost. It allows the simulation of several cyber-attack scenarios against a simulated NPP communicating with its supervisory system (SCADA/HMI), through the Modbus TCP protocol. It is worth mentioning that this study was carried out based on a simulator of a hypothetical power plant (Asherah NPP) and that vulnerabilities that could lead to a similar attack on existing plants were not exploited. We also showed how it is possible to use this environment to generate the datasets needed for intrusion detection studies, and stated that it allows for implementation of defensive computer security architecture. For the proposed cyber-attack scenario, the performance of several simulations allowed to demonstrated how to force condenser cooling pump parameters against their nominal operating values is detrimental to the continuous operation of PWR-type NPPs. In particular, the impact assessment points to the risk that cyber-attacks against the condenser cooling pump speed control could result in material and financial damage to the NPP. The situation described also highlights the danger posed by Insiders, endowed with specific knowledge about the inner workings of the NPP ICS and acting within the Air Gap protected area. We realized that an important limitation of this testbed is the substantial memory load imposed by the employment of several virtual machines in the GNS3 topology, especially in terms of RAM. Thus, more modest computing environments could experience problems when trying to reproduce or expand the conditions described. However, it was possible to perform all the above procedures with a personal computer equipped with an AMD Ryzen 7 3700X 8-Core Processor 3.60 GHz and 32.0 GB of RAM installed. The RAM memories defined in the topology were: 6 GB for the Windows/ANS VM; and 2 GB for each of the Linux VMs. Several possibilities for future studies are envisioned. Such as:  
* Modification of the proposed topology, as by the inclusion of new independent PLCs or network equipment like firewalls, including for the testing of decoupling mechanisms between security zones;
* Choice of different ANS subsystems, for reproducing similar cyber-attack to the one performed in this work;
* Demonstration of different cyber-attacks like DoS and Replay;
* Production of datasets for ML algorithm training, with the goal of developing automated IDS, among others. Figure 34: CC_PumpSpeed and CD_Press Data—scatter plot. **Author Contributions:** Conceptualization, I.B.d.B. and R.T.d.S.J. ; Data curation, I.B.d.B. ; Formal analysis, I.B.d.B. and R.T.d.S.J. ; Funding acquisition, I.B.d.B. and R.T.d.S.J. ; Investigation, I.B.d.B. ; Methodology, I.B.d.B. and R.T.d.S.J. ; Project administration, I.B.d.B. and R.T.d.S.J. ; Resources, I.B.d.B. and R.T.d.S.J. ; Software, I.B.d.B. ; Supervision, R.T.d.S.J. ; Validation, I.B.d.B. ; Visualization, I.B.d.B. ; Writing--original draft, I.B.d.B. ; Writing--review and editing, I.B.d.B. and R.T.d.S.J. All authors have read and agreed to the published version of the manuscript. **Funding:** This research was funded by Agencia Brasileira de Inteligencia--ABIN--grant number 08/2019. **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** Not applicable. **Acknowledgments:** R.T.d.S.J. gratefully acknowledges the support of CNPq grants 465741/2014-2 and 312180/2019-5, the Administrative Council for Economic Defense--CADE grant 08700.000047/2019-14, the National Auditing Department of the Brazilian Health System DENASUS grant 23106.118410/2020-85, the General Attorney of the Union--AGU grant 697.935/2019, the General Attorney's Office for the National Treasure--PGFN grant 23106.148934/2019-67 and the University of Brasilia--UnB COPEI grant 7129.