The replenishment of tritium fuel in a breeding blanket is fundamental for the operation of commercial DT fusion reactors. Accurate modelling of hydrogen transport and inventories within the breeding blanket will be essential for safety issues and economic sustainability. One of the proposed breeding blanket concepts for DEMO, the Water Cooled Lithium-Lead (WCLL) concept is modelled using The open-source hydrogen transport code FESTIM. Multi-material and multi-physics 3D simulations of the WCLL design have been conducted to investigate the significance of tritium inventories and permeation into cooling channels. Trapping effects are considered in the solid domains, in addition to how trapping properties alter as a results of neutron damage over time, subsequently affecting tritium inventories. A fluid dynamics model is implemented to simulate the flow of the liquid metal LiPb in the blanket, accounting for MHD effects. The resulting velocity field was coupled with FESTIM to accurately simulate hydrogen transport in both the liquid and structural domains of the model. A novel method for modelling permeation barriers is presented by modification of the conditions at the discontinuous boundaries between material domains. Simulations have been conducted assuming DEMO operates at steady-state, for a full power year. The presence of a magnetic field is shown to severely disrupts the flow regime of the liquid metal breeder. The inclusion of trapping mechanisms has been shown to increase tritium inventories by 15%. However, when considering the neutron damage effects, inventories increase by several orders of magnitude and result in localised build ups closest to the sources of tritium.