The first wall (FW) of a water cooled ceramic breeder (WCCB) blanket uses ferritic/martensitic steel (FMS) as structural material and tungsten (W) as armor material. The FW is exposed to fusion reaction fuels: deuterium (D) or tritium (T) plasma; and HT, T-2, or HTO produced in breeding reactions. Titanium (Ti) is required as an interlayer in a W/FMS welding joint formed by hot isostatic pressing (HIP). However, Ti is sensitive to hydrogen (H) embrittlement which can result from the concentration and diffusion of H isotopes at welding interfaces. Qualitative analysis of the effect of D concentration on the W/Ti/FMS bonding quality was performed. The experiments demonstrated that D can affect the microstructure and mechanical properties of the W/Ti/FMS joints at lower D concentrations. Excessive TiD2 will turn the Ti interlayer into powder, and finally lead to joint failure at higher D concentrations. The H isotope concentration in the joints shall be strictly controlled to ensure steady operation of the fusion reactor.