Project managers tasked with delivering safety-critical projects must demonstrate care, competence and confidence right from the earliest stages of project inception, when levels of uncertainty can be very high. Based on interviews with 30 project management practitioners in civil nuclear and aerospace sectors, this paper builds on work by Saunders et al. (2015), who posited the Uncertainty Kaleidoscope as a framework for identifying uncertainties.Our findings are that the six determinants of project uncertainty are similar across both civil nuclear and aerospace projects. The most commonly mentioned determinant of project uncertainty was the Environment, followed by Complexity, Capability and Information. The impact of Time on project uncertainty and Individual perceptions of uncertainty were mentioned less frequently by respondents.Our key contribution is to validate the Uncertainty Kaleidoscope over a larger data set, thereby enriching our understanding of the sources of project uncertainty in these two important and highly-consequential project environments.