# Neutronic Activity for Development of the Promising Alternative Water-Cooled DEMO Concepts
## 3 WLCB Blanket Study
### Enhancement of the Tritium Breeding Capability in the WLCB Blanket  
The data presented in Table 3 were obtained due to the comprehensive optimizations of the WLCB blanket geometry layout. The insufficient tritium breeding performance of the blanket, TBR\({}_{90}\) < TBR\({}_{\text{req}}\) in all cases, indicates a deficiency of the blanket physics: the not sufficient neutron multiplication and the excessive neutron absorption in the blanket materials. No geometry modifications made resulted in the TBR exceeding 1.13. The next level of the WLCB design optimization used to increase the TBR is the choice of alternative materials. The most evident modifications were adopted to the breeder ceramic. To this end, the \({}^{6}\)Li enrichment was increased up to 98%. The gain in the TBR was found in all cases \(\Delta\)TBR = 0.01, and therefore TBR\({}_{98}\) does not exceed 1.14 in all cases. Another theoretical opportunity to enhance the TBR is the increase of the packing factor of the breeder ceramic pebbles. The simulation performed to study this effect showed almost no effect on the TBR gain, while the packing factor increased from 64% to 80%. The same effect was reported for the HCPB DEMO blankets [17], where TBR achieved the maximum with the packing factor around \(\sim\)64%. The tritium generation increases in the small radial layer in the front region of the BZ with the increase of the packing factor of the breeder ceramic, and the rear part of the BZ contributes respectively significantly less to the total result. The next modification applied to the breeder ceramic was the choice of another, more efficient material providing higher tritium release under neutron irradiation compared to the standard lithium orthosilicate. An octa-lithium plumate Li\({}_{5}\)PbO\({}_{6}\) (melting point > 800 \({}^{\circ}\)C) and Li\({}_{5}\)ZrO\({}_{6}\) (melting point 1336 \({}^{\circ}\)C) [17; 22; 23] were chosen as the most advanced breeder ceramics to substitute the Li\({}_{4}\)SiO\({}_{4}\) in the whole breeder zone. With the \({}^{6}\)Li enrichment of 98% in lithium, the tritium breeding in the WLCB increased significantly up to TBR\({}_{\text{adv}}\) = 1.18 (see the results for this option in Table 3) and up to TBR\({}_{\text{adv}}\) = 1.17 (case #5, not presented in Table 3) with Li\({}_{5}\)PbO\({}_{6}\) and Li\({}_{5}\)ZrO\({}_{6}\) breeder ceramics respectively. Despite the very promising results, the technology of these ceramics must be well-established, validated, and certified for practical use in the DEMO project. The significant neutron absorption in the BZ of the WLCB blanket could be noticeably suppressed in the case of heavy water use for blanket cooling. The change of the coolant substance is a substantial alteration of the WLCB blanket layout that requires additional studies to prove its feasibility in the DEMO project. The appropriate modifications were implemented  
Figure 12: The 3D distribution of the RR\({}_{(n2n)}\)/RR\({}_{\text{abs}}\) ratio in the equatorial region of the central outboard blanket. to account for the heavy water cooling: the descriptions of the FW, cooling plates, and cooling tubes were modified in the MCNP geometry model accordingly. The gain of the tritium generation was found to be significant: \(\Delta\)TBR\({}_{\text{DZO}}\) = 0.02 relative to the TBR\({}_{98}\) in all cases of the WLCB geometry layouts shown in Table 3. Thus, TBR = TBR\({}_{98}\) + \(\Delta\)TBR\({}_{\text{DZO}}\) = 1.16 (case #5, Table 3) could be achieved by switching to the heavy water coolant. The crucial disadvantage of this coolant is the parasitic tritium generation due to the \({}^{2}\)H(n,\(\gamma\))\({}^{3}\)H nuclear reaction.