# Neutronic Activity for Development of the Promising Alternative Water-Cooled DEMO Concepts
## 4 Discussion  
Starting from the CATIA models of the WCLL and WLCB outboard blankets, the CAD models of the 11.25\({}^{\circ}\) DEMO toroidal segment suitable for the neutronic simulations were elaborated for both blanket concepts. The models are well-detailed, and they include all essential geometry cells to represent the original blanket design. The models are built on a modular basis that enables replacing the different parts without destroying the whole structure. The DEMO generic model serves as the basis for the integration of all separate CAD models representing the blanket design (FW, manifolds, BZ). These separate CAD models were converted into the MCNP geometry representation and included in the DEMO MCNP generic model using the modular hierarchy. The main nuclear responses were calculated using the MCNP-6.2 code and the JEFF-3.3 nuclear data. The new design of the WCLL blanket, WCLL-db, provides better use of the blanket space for tritium breeding and the achieved TBR = 1.16 is sufficient to enable the sustainable tritium fuel cycle. There is the potential to increase the TBR with optimized FW (water content). A steel content in the breeder zone is a crucial factor for the TBR. Further reinforcement of the blanket structure will result in significant losses of the TBR. The temperature effects in the blanket materials should be accounted for in the TBR assessment. The use of the PbBi eutectic in the WCLL-db blanket has potential environmental risk due to the accumulation of \({}^{210}\)Po. The activation analyses performed to assess the amount of the \({}^{210}\)Po production in the WCLL-db DEMO reveals the significant total mass of this isotope in the reactor that could reach \(\sim\)500 g during the first phase of the DEMO operation. Considering the very low occupational limits for the DEMO personnel, this could be a potential show-stopper for the use of PbBi and as well for the WCLL-db in the DEMO. As the solution to this safety problem, the replacement of the PiBi with other Pb-based compounds was proposed. The use of such compounds could provide the same tritium breeding performances of the WCLL-db blanket similar to the one obtained with the initial design. The geometry modeling and the particle transport calculations for the WLCB breeder blanket concept were performed utilizing the hybrid procedure leading to the very flexible blanket layout. The combination of the modular structure of the geometry model and the repeated structure feature of the MCNP allows the comprehensive optimization simulations to outline and establish the most efficient blanket layout. Several geometry configurations \begin{table}
\begin{tabular}{c c c c c c}  & **FW (Front)** & **FW (Side)** & **Cooling Plates** & **Cooling Tubes** & **Manifold** \\ \hline Tritium & & & & & \\ generation, & 7.40 \(\times\) 10\({}^{9}\) & 1.28 \(\times\) 10\({}^{9}\) & 1.00 \(\times\) 10\({}^{8}\) & 3.43 \(\times\) 10\({}^{9}\) & 9.68 \(\times\) 10\({}^{8}\) \\ Bq/kg FPY & & & & & \\ \end{tabular}
\end{table} 
Table 5: Tritium generation in the heavy water coolant of the WLCB DEMO. of the WLCB blanket breeder zone were studied and found to be potentially capable of providing reasonable tritium breeding performance with the arrangement of the breeder ceramic: (a) in the poloidal tubes, (b) in the radial breeder pins, and (c) in the vertical beds. In all investigated variants with standard 90% \({}^{6}\)Li enrichment, TBR does not exceed TBR\({}_{90}\leq 1.13\), and therefore, it is below the required value TBR\({}_{90}\) < TBR\({}_{\rm{req}}\). In the case of the 98% \({}^{6}\)Li enrichment in the breeder ceramic, the gain of the TBR is \(\Delta\)TBR = +0.01, which does not significantly improve tritium breeding capability. The modifications applied to the blanket layout provided for the WLCB blanket the TBR\({}_{98}\leq 1.14\). Another way to increase the TBR is the utilization of advanced materials in the breeder zone.