Between the 1940s and 90s, successive generations of nuclear specialists found themselves defined by conflicts. Their new expertise drew on both high physics and engineering heuristics. Their evolving jobs were defined alternately by the corporate industrial cultures of chemistry and of electrical engineering. And, despite the exuberance of the atomic age, their professional status and collective authority vascillated. Veiled by security, their new knowledge was guarded in national laboratories. Unlike their predecessors, the disciplinary borders, occupational roles and public status of nuclear engineers were defined largely by the State. As power generation developed as the principal application, nuclear engineers populated more stable job categories as designers and operators of reactors. But as ill-defined and often faceless experts, these practitioners’ characteristics became subsumed by those of corporate sponsors. In latter decades, these expert practitioners were delineated more often by accidents than by achievements. This paper examines factors shaping technical identity and argues that, as members of a perpetually ‘in-between’ or ‘Goldilocks’ profession, nuclear engineers had remarkably limited influence in defining their own identity. Based on archival and oral history research in the UK, USA and Canada and comparisons with earlier national studies, the history of their emergence can be characterised as transnational and yet largely transcending politics at the national scale.