## 2 DEMO Tokamak Layout  
Similar to any nuclear facility, the operation of the DEMO fusion reactor is maintained and controlled by utilizing the dedicated auxiliary systems allocated in the tokamak. These systems do not contribute to the tritium production in the tokamak, and they consume space suitable for tritium breeding. These systems are designed to ensure the secure functioning of the complex fusion system by protecting the breeder blankets against plasma disruption through an arrangement of the limiters; feeding the plasma with atoms of deuterium and tritium; heating and maintaining the plasma through, for instance, neutral beam injectors and electron cyclotrons; and controlling the plasma stability through diagnostic systems arranged in cassettes. These auxiliary systems are supposed to be toroidally distributed over the tokamak to ensure the reliable operation of the facility.  
The DEMO concept 2017 of the tokamak [6] includes 16 toroidal segments; each 22.5\({}^{\circ}\) big segment serves as an envelope to allocate a breeder blanket volume, a vacuum vessel, a divertor, an equatorial port and upper ports, toroidal magnetic field coils, poloidal field coils, a cryostat and a central solenoid [6]. For the computations, we adopted the same optional tokamak layout adopted in [2] that was suggested in the DEMO project to fulfill all DEMO system requirements. This DEMO layout, shown in Figure 1, includes the following systems:  
* Four inboard mid-plane limiters (IMLs);
* A representation of diagnostic port plugs, the design of which is not yet available;
* Four outboard mid-plane limiters (OMLs);
* Eight upper-port limiters (ULs);
* Four outboard lower limiters (OLLs);
* Three neutral beam injector (NBI) systems;
* Nine electron cyclotron (EC) antennas.  
The tritium breeding performance of the DEMO primarily depends on the coverage of the space around the plasma with the breeder blankets. Any cut-out in the breeder blanket reduces the useful volume for tritium generation and, hence, complex layouts of the NBI and EC systems arranged outside the tokamak do not affect tritium breeding generation in the blankets. The DEMO tokamak schematic layout shown in Figure 1 supposes an unsymmetrical arrangement of the IVCs. Typically, an approach in the DEMO-related neutronic simulations implies modeling and utilization of one toroidal segment (for example, 22.5\({}^{\circ}\)), assuming a tokamak symmetry. A propagation of the results to the 360\({}^{\circ}\) tokamak is simulated by the introduction of reflecting boundaries in the geometry model. A logical distortion introduced with such simplification compared to the modeling based on the realistic DEMO tokamak configuration was studied in [2]. The possible effect of a neutron inter-streaming between IVCs in the realistic tokamak layout was found to be negligible. The approach adopted in this work that utilizes geometry models of 22.5\({}^{\circ}\) size toroidal segments with the reflecting boundaries to simulate the whole tokamak enables satisfactory global assessments of various effects. The computations were performed as follows: for each segment of 22.5\({}^{\circ}\) toroidal size, a separate geometry model was created according to the IVCs arrangement presented in Figure 1; the \(\text{TBR}_{\text{req}}\) was calculated based on symmetrical replications of this segment in the tokamak; the final reduction of the TBR due to the adaptation of all IVCs presented in Figure 1 was calculated as a superposition of the results obtained for separate IVCs.