## 6 Conclusions  
In this paper, results of the thermodynamic cycle analyses and sensitivity analyses of various S-CO\({}_{2}\) cycles for the waste heat recovery of a 7.2 MW diesel engine used in a nuclear power plant are presented. The diesel engine can provide two heat sources for the waste heat recovery system: one from exhaust gas and the other from scavenge air. This is unique compared to other heat sources such as fossil fuel power plants, concentrated solar power systems, nuclear power plants, etc. Thus, it was necessary to review the S-CO\({}_{2}\) cycle layouts for dual heat sources application.  
As a result of cycle comparisons, it was found that the partial heating cycle produces the highest net work while having a simple layout and small number of major components. Particularly, the partial heating cycle has additional advantage in respect of easier cycle operation scheme since it requires single turbine and compressor only.  
Furthermore, the partial heating cycle was modified by changing the flow split merging point from cold side inlet to cold side outlet of exhaust gas heat exchanger, to additionally use the scavenge  
Figure 31: S-CO\({}_{2}\) cycle performance in part load conditions.  
Figure 30: Quasi-steady state analysis result (63% load).  
air heat from diesel engine turbocharger. The waste heat recovery is sharply increased by applying dual heat sources from exhaust gas and scavenge air. This study identified that waste heat recovery can be increased more by changing the flow split merging point of a partial heating cycle for the diesel engine application.  
The main components, including turbomachinery and heat exchangers, are designed with in-house codes which have been validated with experiment data. Based on the designed cycle and components, the S-CO\({}_{2}\) cycle performance under partial load conditions is analyzed with an in-house quasi-steady state cycle analysis code to find the optimal operating condition and to establish the control logic for the part-load operation. In this paper, part-load operation and optimal operating conditions were calculated varying from 63-100%. For the off-design analysis it is performed while assuming only mass inventory control is applied. RPM was kept at design value for the grid stability. Part load analysis shows an output difference of as much as 100 kW or more between the optimal and non-optimal points. Therefore, it is necessary to further study the strategy of controlling the operation point for each off-design operation mode. Moreover, the performance change of off-design optimum point for varying thermal load is important for economic analysis of the system.  
The additional power from the AAC DG waste heat recovery system can operate a nuclear safety system without revising the existing nuclear power plant's layout too much. By producing additional 645 kWe from the S-CO\({}_{2}\) bottoming cycle from 3 MWth waste heat, nuclear safety can be improved by potentially extending AAC DG operation time or adopting more safety components to improve redundancy or diversity. Furthermore, more instrument and control systems can be utilized during accident from the additional power of the waste heat recovery system.  
Conceptualization, J.K.H.; Methodology, J.K.H. and M.S.K.; Software, J.K.H., M.S.K. and S.S.; Validation, J.K.H., M.S.K. and J.L.; Formal Analysis, J.K.H. and M.S.K.; Investigation, B.S.O.; Resources, J.L.; Data Curation, S.S.; Writing-Original Draft Preparation, J.K.H.; Writing-Review & Editing, J.I.L.; Visualization, M.S.K.; Supervision, J.I.L.; Project Administration, J.I.L.; Funding Acquisition, J.I.L.  
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (2017M2B2B1071971).  
The authors declare no conflict of interest.