Nuclear analyses in support of the water-cooled lithium lead breeding blanket design development: a prospective strategy to achieve the tritium self-sufficiency

The development of a closed tritium fuel cycle is essential for the sustainable operation of future fusion power plants. Within the EUROfusion roadmap, the Water-Cooled Lithium Lead (WCLL) breeding blanket is a key candidate for the Demonstration Fusion Power Reactor (DEMO). It uses pressurized water as coolant and a liquid lithium-lead (LiPb) eutectic alloy as both breeder and neutron multiplier. The current WCLL design features modular inboard and outboard Breeding Units (BUs) composed of helical Double Wall Tubes (DWTs) immersed in LiPb. This configuration improves heat removal and reduces the water inventory in the Breeding Zone, thereby potentially increasing tritium generation. This work presents an assessment of the WCLL DEMO nuclear performances in terms of shielding effectiveness and tritium self-sufficiency, based on 3D radiation transport simulations performed using the Monte Carlo MCNP code and JEFF nuclear data libraries. A detailed MCNP model has been developed, including a fully heterogeneous representation of the Breeding Zone and first wall (FW) water channels. Radial profiles of fast and total neutron fluxes, along with nuclear heating, are evaluated at the equatorial level, providing 3D maps of these quantities as well. The total Tritium Breeding Ratio (TBR) has been evaluated, accounting for contributions from both the BZ and LiPb manifolds. To further enhance tritium generation and support the long-term viability of the WCLL concept, design modifications to the latest layout have been explored through a prospective approach. The effectiveness of these modifications has been assessed by means of parametric studies and discussed in light of their potential impact on overall reactor performance.