# Neutronic Activity for Development of the Promising Alternative Water-Cooled DEMO Concepts
## 2 WCLL-db Blanket Study
### Results of the Neutronic Simulations for the WCLL-db DEMO  
#### 2.3.1 The Main Nuclear Responses  
The particle transport simulations were carried out using the geometry model discussed above and MCNP6.2 [10] code with nuclear data from the JEFF-3.3 library [11]. The plasma neutron source was simulated within a subroutine [12] linked to the MCNP during its compilation. Statistical uncertainty of the MCNP nuclear responses does not exceed 0.01% for the TBR and less than 0.1% for other nuclear responses. The most essential nuclear response, TBR, was assessed for the new WCLL-db DEMO concept with a view to examining the tritium breeding performance of the promising layout of the PbLi-based blanket to compare the results with those from the reference WCLL design. The TBR was found to be TBR = 1.16, which is higher compared to the reference WCLL TBR = 1.14 [3], and it fits the DEMO TBR requirement. Optionally, the water manifold in the WCLL-db blanket could be further squeezed by 20 mm more, increasing the BZ and the TBR, respectively. This possibility was not studied because it should be first checked in the thermal-hydraulic calculations. The PbBi separating layer used in the blanket design in the cooling bundles and the first wall could be alternatively filled with noble gases such as helium, neon, etc. The choice of helium is technologically preferable due to its well-developed technology. In this case, no noticeable TBR reduction was found. As it was discussed above, the level of the simplifications applied to the CAD model of the WLCB-db blanket conforms to those adopted in the WCLL design project. The square edges of the FW used in the model instead of the round shape could reduce TBR not more than \(\Delta\)TBR \(<-0.005\); this result was obtained in a specially addressed study. This estimation is valid for both WCLL and WCLL-db blankets. All particle transport calculations presented in this work were performed by default with the nuclear data from JEFF-3.3 prepared for 0 \({}^{\circ}\)C. The WCLL blanket will operate at higher temperatures, for example, \(\sim\)310 \({}^{\circ}\)C for PbLi or water coolant. The steel structure in the blanket (especially the FW) has even higher temperatures. To simulate rough temperature distributions in the blanket and assess the temperature effect on the nuclear responses, two sets of the JEFF-3.3 nuclear data were used for such calculations:  
Figure 4: Horizontal cuts of the WCLL-db MCNP geometry model: inboard (**left**) and outboard (**right**) blankets. 900 K for the FW and 600 K for the rest of the blanket. The material densities in different geometry cells were adjusted respectively. A slight reduction of the TBR was found by accounting for the temperature effects in the nuclear data, \(\delta\)TBR \(<-0.5\%\). Thermal-hydraulic simulations performed to get a first-guess layout of the WCLL-db blanket were based on the results of the neutronic simulations performed for the reference WCLL blanket [3,13]. For the new WCLL-db design, the updated results for integral power generation in the blankets and radial profiles of the nuclear power densities are necessary to confirm or update the blanket layout. To this end, this information was provided for the midplane of the OB blanket, where the energy deposition achieves its maximum. Using these data and an FW neutron load profile [14], the energy depositions can be restored in all poloidal positions in the IB and OB blankets. The results for the nuclear power generation in different materials of the OB blanket are shown in Figure 5. An increase in the power densities in the manifold region comes from neutron moderation and absorption in water resulting in an increase of gamma irradiation there. Presented in Table 1 are the results for the nuclear energy generated in the WCLL-db blankets compared to the data obtained for the reference WCLL DEMO [13]. Shown in Figure 6 is a (n,t) reaction rate calculated on the OB side in the mid-plane of the reactor.