## 4 Results and Discussion  
The SCALE computer code system is widely used for depletion and decay analysis of RBMK spent fuel [21; 22; 23; 24; 25]. The modeling approach of specific RBMK-1500 SF characteristics (Stage No. 1 modeling results) was comprehensively presented in [17]. In this paper, it was concluded that the gamma source of spent fuel rapidly decreases during a 50 year storage period. For the neutron source, this period is 100 years. After 300 years of storage time, low-energy gamma photons are dominant in the emissions. As for the neutron source, neutrons with higher energies are dominant. Stage No. 2 modeling results are presented in this study. Firstly, we provide a discussion on the energy groups of gamma \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Group No.** & **Neutron Energies, MeV** & **Group No.** & **Gamma Energies, MeV** \\ \hline
1 & 1.00 \(\times\) 10\({}^{-11}\)-1.00 \(\times\) 10\({}^{-8}\) & 1 & 1.00 \(\times\) 10\({}^{-2}\)–4.50 \(\times\) 10\({}^{-2}\) \\
2 & 1.00 \(\times\) 10\({}^{-8}\)-3.00 \(\times\) 10\({}^{-8}\) & 2 & 4.50 \(\times\) 10\({}^{-2}\)–0.1 \\  
\hline \hline \end{tabular}
\end{table} 
Table 1: Neutron and gamma energy groups and energy boundaries. and neutron emissions from the spent fuel that determine gamma and neutron dose rates on the outer surface of the cast iron and metal-concrete containers for different storage periods; secondly, we present the contributions in percentage of neutrons and gammas to the total dose rate during extended storage times; lastly, we present the total dose rates and their variation with storage time on the side, top, and bottom walls of the cast iron and metal-concrete containers. The side wall of the cast iron and metal-concrete containers was selected in order to analyze the energy groups of gamma and neutron emissions from the spent fuel, which mainly determine gamma and neutron dose rates on the outer surface of the containers. Figures 3 and 4 shows the gamma and neutron energy groups, respectively, as well as their contributions to gamma or neutron dose rates for 5, 50, and 300 years of storage time. As it can be seen from Figure 3, 97-99% of the gamma dose rate is caused by photons that are emitted from the spent fuel with energies within 7-12 gamma energy groups (energy boundaries are presented in Table 1). The influence of the remaining 13 gamma energy groups on the gamma dose rate is insignificant because low-energy gamma photons are efficiently shielded, and the number of high-energy gamma photons is rather small. The input gamma dose rate from various energy groups (see Figure 3) is different at different storage periods. For longer storage periods (50, 300 years) gamma photons of Group No. 7 (0.6-0.8 MeV) provide a major contribution on the side wall of both container types. In the case of neutrons, as can be seen from Figure 4, about 99% of the neutron dose is caused by neutrons of 21-27 neutron energy groups (see Table 1 for group energy boundaries). The contributions of the remaining 20 neutron energy groups on the neutron dose rate are negligible. Unlike the contributions of gamma energy groups, the neutron dose rate for all storage periods is determined by the same dominant neutron energy groups. The total dose rate is the sum of neutron and gamma dose rates. Contributions (in percentage) of each component to the total dose rate at various container surface points during the storage time are presented in Figures 5-7. Steel lids at the top of the containers (see Figure 5) provide effective shielding of gamma photons; therefore, the total dose rates are mostly caused by neutrons for both containers. In order to reduce the neutron dose rates, additional concrete covers (not taken into account during the modeling) are placed on top of the containers at the storage facility. The CASTOR(r) RBMK-1500 container's side and  
Figure 3: Energy groups that determine the gamma dose rate (low-energy gamma photons are efficiently shielded; the number of high-energy gamma photons is rather small).