## 4 Results of the Neutronic Simulations  
The particle transport simulations were carried out, making use of the geometry models for different DEMO tokamak toroidal segments presented above and MCNP6.2 [11] code with nuclear data from the JEFF-3.3 library [12]. The explicit plasma neutron source description was introduced in the MCNP input deck, making use of the model developed in [13]. The statistical uncertainty of the MCNP nuclear responses for the TBR did not exceed 0.01%. For the geometry models with the breeder blanket space filled only with the blankets, the results of the TBR calculations were 1.173, 1.150 and 1.140 for the HCPB, WCLL and WLCB BB concepts, respectively.  
The approach adopted in this study for the DEMO tritium breeding performance assessment in respect to the inclusion of the IVCs was to estimate a relative variation of the TBR given as a percentage. For each DEMO blanket concept, a starting point for the assessments was the tokamak configuration without any IVCs included. The calculations with the set of various DEMO toroidal segment configurations allowed us to assess the TBR reduction caused by separate IVCs arranged in the tokamak. Assuming that the effect from every IVC is independent, the global TBR reduction for the layout presented in Figure 1 was calculated as a superposition of the individual results. As was mentioned above, this approach was proven to be valid for such kinds of assessments.  
The results of the simulations with different optional tokamak configurations are presented in Table 1. The results obtained in the previous study [2] with the inclusion of the breeder blankets filled with the representative homogeneous material mixture are given in Table 1 for a comparison. The effect of the detailed heterogeneous representation of the breeder blankets compared to the homogeneous one did not exceed 0.5%, and therefore the former approach can also be used to perform quick assessments of the relative TBR deviations in the DEMO tokamak. The inclusion of the various limiters in the DEMO tokamak reduced the TBR by up to ~8%. The most significant effect on the TBR reduction ~4% came from the UL. The TBR losses due to the inclusion of the IVCs in the two main breeder blanket concepts (HCPB and WCLL) considered as the driver blankets in the DEMO were similar and were assessed to be ~10.5%. The results for the alternative blanket concept (WLCB) were slightly less, at ~9.3%. The study performed in [14] assuming only equatorial port cut-outs in the blanket assessed the TBR losses in the DEMO tokamak and found that they were no greater than 8.7%.  
Figure 6: Cuts of the 22.5\({}^{\circ}\) MCNP geometry models for the HCPB DEMO: UL (**a**) and integration of IML, UL and OML without OLL (**b**).  
Similarly, the reduction in the nuclear power generation in the breeder blankets due to the arrangement of the IVCs was assessed for all concepts (Table 2). The full 360\({}^{\circ}\) tokamak model was not developed to obtain the data presented in Table 2. Instead, the results obtained for the one toroidal segment were extrapolated to the full tokamak. Assuming that the breeder blanket volumes occupied by the cut-outs do not participate in the nuclear power generation, reductions of 6.4%, 6.16% and 5.86% were obtained for the full power production in the HCPB, WCLL and WLCB BB concepts, respectively. This could result in a 105 to 124 MW loss of power production in the DEMO with different blanket concepts [15].