## 2 DEMO Neutronics Model  
For the simulations with the MCNP code, two DEMO blanket concepts were considered: HCPB [9] and WCLL [10]. Both concepts utilize the same DEMO Base line model 2017 [11] that serves as a common basis for the integration of the breeder blankets. This model consists of 16 sectors toroidally repeated to build the DEMO tokamak. For the geometry modelling, a 22.5\({}^{\circ}\) sector was retrieved from the DEMO Base line CAD design. For the MCNP calculation, half of this segment (11.25\({}^{\circ}\)) sector is used assuming its toroidal symmetry in the tokamak and reflecting boundary conditions.  
The breeder blankets in both concepts utilize the same single module segmentation (SMS) technology involving the arrangement of one inboard (IB) and one and a half outboard (OB) blanket modules in the 11.25\({}^{\circ}\) geometry segment. The current HCPB DEMO design implies the use of the mixed Li4 SiO4 plus 37 mol.% of Li2 TiO3 with 60% \({}^{6}\)Li enrichment breeder material [9]. The breeder ceramic is enclosed in radial breeder pins arranged in the hexagonal lattice. The space around the pins is filled with Be12 Ti hexagonal prismatic blocks serving as a neutron multiplier. The cooling of the HCPB blanket is provided by 80 bar He coolant. The MCNP HCPB DEMO geometry model was generated making use of the McCad conversion tool [12], applying several geometry universes' hierarchy and MCNP-repeated structure function resulting in the fully heterogeneous 3D geometry representation [13], as shown in Figure 1.  
The WCLL blanket design [13] based on the implementation of a single module segmentation (SMS) conforms the dimensions of the breeder blanket space in the generic DEMO CAD model. The blanket U-shaped housing is 25 mm thick and attached to a massive back wall of 100 mm thickness that functions as a BSS. The blanket inner volume is filled by poloidally repeated so-called breeding elements, as shown in Figure 2. Water manifolds are located in front of the BSS and they feed with the FW and BZ with coolant.  
Figure 1: Envelop blanket design of DEMO (**left**), the MCNP model of HCPB (**top right**) and WCLL fully heterogeneous breeder blanket design (**bottom right**).  
Arranged between BZ and water manifolds are the PbLi (90% \({}^{6}\)Li) manifolds, feeding the BZ. Inlet and outlet PbLi channels are separated by square-formed steel pipes. The BZ is strengthened by vertical and horizontal stiffening plates that also serve to confine the PbLi flow. The cooling of the BZ is ensured by high pressure water (155 bar) with 295 \({}^{\circ}\)C inlet and 328 \({}^{\circ}\)C outlet temperatures enclosed in double-wall U-shaped tubes of 13.5/8 mm (outer/inner) size, as shown in Figure 1. The FW cooling is provided by water at the same thermo-hydraulic conditions routed in 7 \(\times\) 7 mm\({}^{2}\) square channels with 13.5 mm vertical pitch. All structural elements of the WCLL blanket are made of Eurofer steel.  
The MCNP geometry models for DEMO concepts do not assume any cut-outs for the arrangement of the in-vessel components in the tokamak design such as limiters, diagnostic systems and plasma heating systems (Neutral Beam Injector and Electron Cyclotron antennas). It means that the current blanket design is not fully realistic and it permits the application of various cut-outs, negatively decreasing the TBR by the \(\Delta\)TBR, assuming that the tritium breeding capability of the blanket is capable of compensating these effects. The assessed required TBRreq = TBRtarget + \(\Delta\)TBR should be at least 1.16 [14].