The United Kingdom Atomic Energy Authority are involved in the design and manufacture of the diagnostic windows for ITER. ITER is an international project, with 35 nations collaborating to design, construct and operate a prototype controlled nuclear fusion reactor in southern France. As well as providing line of sight for diagnostics, the windows also form part of the reactor primary containment boundary and are consequently classified as nuclear Safety Important Class 1 (SIC-1) components. The windows will be the first SIC-1 components in the world which are non-metallic – which makes them unique. The current manufacture process involves diffusion bonding a glass window to an Inconel 625 ferrule via an aluminium interlayer. This presentation aims to discuss this diffusion bonding process and detail the specific challenges related to component qualification for the intended nuclear SIC-1 application.