## 4 Conclusions  
A newly developed modular dry storage system for PWR SNF was proposed, and air inlets and outlets were arranged at the top and bottom of the structure to enable non-powered natural convection cooling in a square-box-type concrete structure. The \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Condition** & **Load Combination** & **Ref.** & \
\begin{tabular}{c} **Maximum** \\ **Stress** \\ **(MPa)** \\ \end{tabular} &
\begin{tabular}{c} **Allowable** \\ **Stress** \\ **(MPa)** \\ \end{tabular} \\ \hline \multirow{3}{*}{Normal} & (1) 1.4 D \({}^{1}\) + 1.7 L \({}^{2}\) & \multirow{3}{*}{ACI-349} & 6.27 & \multirow{3}{*}{30} \\  & (2) 1.4 D + 1.7 L + 1.7 W \({}^{3}\) & & 6.32 & \\  & (3) 0.9 D + 1.7 W & & 4.01 & \\ \hline \multirow{3}{*}{Off-normal} & (4) 1.05 D + 1.3 L + 1.05 To \({}^{4}\) & & 17.51 & \multirow{3}{*}{30} \\  & (5) 1.05 D + 1.3 L + 1.3 W + 1.05 To \({}^{6}\) & & 4.01 & \\ \cline{1-1} \cline{3-5}  & (6) 1.05 D + 1.275 L + 1.275 To & & 17.51 & \multirow{3}{*}{Accident} \\  & (7) 1.05 D + 1.275 L + 1.275 To & & 21.81 & \\ \cline{1-1} \cline{3-5}  & (8) 0.9 D + 1.275 W + 1.275 To & & 22.4 & \\ \hline \multirow{3}{*}{Accident} & (9–11) 1.0 D + 1.0 L + 1.0 To + 1.0 Ess \({}^{6}\) & & 17.17 & \multirow{3}{*}{30} \\  & (12–14) 0.9 D + 1.0 To + 1.0 Ess & & 17.26 & \\ \cline{1-1} \cline{3-5}  & (15) 1.0 D + 1.0 L + 1.0 Ta \({}^{5}\) & & 26.2 & \\ \cline{1-1} \cline{3-5}  & (16-18) 1.0 D + 1.0 L + 1.0 Ta \({}^{5}\) & & 27.02 & \\ \cline{1-1} \cline{3-5}  & (19–21) 0.9 D + 1.0 Ta + 1.0 Ess & & 27.01 & \\ \hline \hline \end{tabular} \({}^{1}\) D: dead load, \({}^{2}\) L: live load, \({}^{3}\) W: wind load, \({}^{4}\) To: normal thermal load, \({}^{5}\) Ta: transient thermal load, \({}^{6}\) Ess: seismic load.  
\end{table} 
Table 4: Load combinations for structural evaluation of DSM. Figure 12: Maximum stress of load combination 8. Figure 13: Maximum stress of load combination 16. DSM cylinder was installed between the canister and the external air layer, so that the stainless steel material of the canister does not contact external air. This structure can secure soundness for long-term storage, as there is no corrosion effect due to chlorine ions from the outside air. In addition, the storage method using a canister is advantageous in linking to the next management step, because when delivering SNF from a storage facility in an NPP to an intermediate storage facility, canisters containing SNF can be directly transported without repackaging. Based on these advantages, the conceptual design of the DSM was developed for future application. To investigate the applicability of the conceptual design, source terms, criticality, radiation shielding, thermal management, and structural evaluation were performed in accordance with the design requirements. The reference fuel for evaluating the integrity of the dry storage module was PLUS7, and a fuel with a maximum initial concentration of 5.0 wt.%, a maximum average burnup of 45 GWD/MTU, and a minimum cooling period of 10 years was selected. As a result of performing the safety evaluation for each field of the reference fuel, it was confirmed that the developed DSM meets the design requirements in terms of the criticality aspect, and that the shielding performance of the module structure was sufficient. The thermal evaluation was performed below the temperature limit to ensure the integrity of the concrete and fuel, but it was thought that additional margins would need to be secured for the effective operation of the facility. In particular, the biggest load generation factor in the structural analysis was also thermal load. Therefore, in order to secure the temperature margin of the DSM, we concluded that the convective heat transfer performance can be improved by controlling the specifications and material of the insulation plate, the distance between the cylinders, the helium filling between the cylinder and the canister, and the shape of the outer pin of the cylinder.