## 1 Introduction
\begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline  & **Current Status** & **Improvements** \\ \hline \multirow{3}{*}{Electric Power System} & * 2 EDGs/unit & * Movable vehicle \\  & - Loss of offsite power & for generator and batteries \\  & * 1 AAC DG/2 or 4 units & - \(\sim\)2014, All NPPs \\  & - Loss of cooling function (SBO) & \\ \hline \multirow{3}{*}{Cooling System} & * Redundancy (2 trains) & * Prepare supplementary methods \\  & * SFP has multiple sources & - fire truck, etc. \\  & - \(\sim\)2013, All NPPs \\ \hline \multirow{3}{*}{Fire Protection System} & * Fire hazards analysis/10 years & * Improving the firefighting plan \\  & * Fire protection plans & * Improving fire protection facility \\  & - \(\sim\)2015, All NPPs \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Safety improvements after the Fukushima accident (recreation of the figure in Ref. [1]). For redundancy, independent EDGs supply power to each bus in LOOP and SBO accidents. Another 800 kW diesel generator in the turbine building is installed for supplying power to non-Class 1E as well if the EDG system becomes unavailable, AAC DG is electrically connected to operate safety systems instead of failed EDG. Table 3 contains the actual diesel generator specifications and vendors of APR 1400 which is constructed in Korea. In this study, AAC diesel generators of Shin Hanul is selected as the reference system. Figure 2 and Table 4 represent the schematic diagram and specification of diesel generator, respectively. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Class 1E Components** & **Loading Sequence at LOOP** & **Power (kW)** \\ \hline Load Sequence Group \(\times\)2 & 0.0 s & 694.9 \(\times\) 2 \\ Safety injection pump 1 & 5.0 s & 715 \\ Safety injection pump 3 & 10.0 s & 715 \\ Motor driven AFW pump & 15.0 s (if required) & 930.9 \\ Containment spray pump & 20.0 s & 738.1 \\ (That interlocks with shutdown cooling pump) & 25.0 s & 1225 \\ Component cooling water pump & 30.0 s & 636.8 \\ Essential central chiller & 35.0 s & 806.0 \\ Essential ESW intake structure/ & 40.0 s & 304.9 \\ CCW heat exchanger building chiller & & 7461.7 \\ Total Diesel Load on DBA/LOOP excluding & & \\ Manual Load & & \\ \hline \hline \end{tabular} In case of failure of EDG, AAC DG supplies power to the Class 1E load instead of EDG.  
\end{table} 
Table 2: Class 1E load and sequence at LOOP condition. Figure 1: Schematic diagram of nuclear power plant and emergency power supply systems. From Table 4, it can be observed that the thermal potential of waste heat is quite high because a turbine outlet temperature is 306 \({}^{\circ}\)C, and the mass flowrate is 15.1 kg/h which is equivalent to about 3 MWth heat so that it is quite valuable to use this high potential waste heat as an additional power for operating a safety system. For example, additional electric power from the waste heat recovery system can be used for extended operation of AAC DG while using the same amount of fuel or installing more safety components for improving redundancy or diversity to utilize additional power. Consequently, the additional power from the AAC DG waste heat recovery system can be utilized in the nuclear safety system without revising the existing nuclear power plant's layout. In this paper, the supercritical carbon dioxide (S-CO\({}_{2}\)) bottoming cycles are studied in respect of best applicable