Project managers tasked with delivering safety-critical projects must demonstrate care,competence and confidence from the earliest stages of project inception, when levels ofuncertainty about the scope, delivery mechanisms and required project outcomes may be illdefinedand even bewildering. This paper builds on work by Saunders et al. (2015), which positedthe Uncertainty Kaleidoscope as a framework for conceptualising uncertainty in safety-criticalprojects. Our contribution here is to 1) refine and validate the Uncertainty Kaleidoscope and 2)explore the commonalities and differences between sources of project uncertainty across twosafety-critical industries: nuclear and aerospace. The findings are that the six key determinants ofproject uncertainty (Complexity, Environment, Capability, Time, Information and IndividualPerception) are broadly similar across both civil nuclear and aerospace projects. There arehowever sharper differences between the nature of projects with new build/new productintroduction projects having lower information uncertainty and suffering less from uncertaintiesin capability, whether internally or across the wider supply chain.