# System Studies on the Fusion-Fission Hybrid Systems and Its Fuel Cycle
## 2 Materials and Methods  
All three hybrid reactors, assumed by the road map of the NRC \"Kurchatov Institute\" project, will be based on a tokamak with a blanket containing fissile materials and lithium. The demo hybrid reactor DEMO-FNS is the first of the reactors mentioned above, and its contemporary design is the most detailed and defined. The other two reactors will have a similar construction with slightly different parameters. All of the tokamaks of the hybrid systems considered have the following identical parameters: major plasma radius \(R_{0}\) = 320 cm, minor plasma radius \(a\) = 100 cm, plasma current \(I_{\mathrm{p}}\) = 5 MA, toroidal magnetic field \(B_{\mathrm{t0}}\) = 5 T, fusion power \(P_{\mathrm{fus}}\) = 40 MW (that corresponds to \(\sim\)\(1.4\times 10^{19}\) n/s for D-T reaction) and effective fuel irradiation time is 5 years. Each reactor is due to commence operations in 2033, 2045 and 2055 respectively. The differences that are important for this study are listed below in Table 3:  
The 3D geometrical model of DEMO-FNS for neutron transport calculation via the Monte-Carlo method is shown in Figures 4 and 5. The blanket contains 18 assemblies with minor actinides (transmutation area), with lithium salt in the remaining space (tritium breeding area). Figure 4: Horizontal equatorial cut of DEMO-FNS (fusion neutron source) model. TFC—Toroidal field coil. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **DEMO-FNS** & **PIHR** & **IHR** \\ \hline Capacity factor & 0.3 & 0.8 & 0.95 \\ Fuel loading (MA + Zr), t (H\({}_{2}\)O—coolant) & 26.24 & 26.24 & 41.68 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Capacity factor and fuel loading with MA (minor actinides). Two coolants for the transmutation area are examined: CO\({}_{2}\) and H\({}_{2}\)O. The coolant flows vertically, along the fuel rods, inside the assemblies' casings. H\({}_{2}\)O is chosen as a basic option for the blanket. The average coolant density inside the assemblies for H\({}_{2}\)O is 0.37 g/cm\({}^{3}\) and for CO\({}_{2}\) is 0.14 g/cm\({}^{3}\). Utilization of CO\({}_{2}\), instead of H\({}_{2}\)O, causes \(k_{\text{eff}}\) growth (up to 1.04). It is thus necessary to decrease total fuel loading in the case of CO\({}_{2}\) as a coolant to 19.7 t (for DEMO-FNS). A metal alloy of MA and Zr was chosen as a fuel. This alloy has a theoretical density of 15 g/cm\({}^{3}\). Many researchers have considered this type of fuel [10; 14; 15], and it has even been utilized in a fast reactor [16]. A detailed fuel inventory is shown in Table 4.  
Calculations on nuclide kinetics were carried out by the program FISPACT-II [17] using a constant neutron spectrum. The neutron spectrum for the transmutation area was obtained as a result of neutron transport calculations using the Monte-Carlo method. The spectrum is volume-averaged for the whole transmutation area. The utilization of a constant neutron spectrum for calculations on nuclide kinetics gives some error in the results, because of the impact of fuel inventory evolution on the spectrum and vice versa. However, in the case of a subcritical system with an external neutron source, this error should not be very high. Different researchers address contradictory assessments on this error [11; 12]. The neutron data library ENDF/B-VI is used for neutron transport calculation. The spectrum was obtained for 709 energy groups that correspond to the CCFE-709 group structure. The nuclear data library TENDL_2014 is used for nuclide kinetics calculations. This explains the reason for utilizing the CCFE-709 group structure for the neutron spectrum. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Nuclide** & **Fraction, \%** & **Nuclide** & **Fraction, \%** \\ \hline Np-237 & 28.67 & Zr-90 & 2.05 \\ Am-241 & 62.10 & Zr-91 & 0.45 \\ Am-242m & 0.06 & Zr-92 & 0.70 \\ Am-243 & 4.63 & Zr-94 & 0.72 \\ Cm-244 & 0.49 & Zr-96 & 0.12 \\ \hline
**MA** & 95.96 & **Zr** & 4.04 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Fuel inventory (15 g/cm\({}^{3}\)), % (mass).