Due to the complexity of nuclear decommissioning projects, developing a risk assessment model to manage the project risk is paramount. The current study is an attempt to identify all the possible risk factors that can arise in a nuclear decommissioning project. A literature review and the expert judgement method were used to identify and analyse all the possible risk factors. The identified factors are then further refined, and the most important risk factors are selected to be included in the final risk assessment model. This work presents the criteria developed for the simplification, refinement, and assessment of risk factors. The criteria used considered the availability of data, the clarity, and the measurability of the selected factors. Risk factors with a lower rating are eliminated, and those with a similar focus of attention are grouped, enabling the selection of a simplified final list of risk factors for the model. This work also proposed a method of assessment for the remaining factors to allow the quantification of individual risk factors within the model. From the 81 risk factors initially identified, 18 consolidated factors are considered for the development of the risk assessment model. The selection process and risk rating approach presented in this work serves as a critical foundation for the development of a robust nuclear decommissioning risk assessment and management.