Nuclear safeguards are a set of activities to verify that a State is living up to its international undertakings not to use nuclear programs for nuclear weapons purposes. Nuclear safeguards experts of International Atomic Energy Agency (IAEA) evaluate indicators by benefitting from several sources such as State declarations, on-site inspections, the IAEA databases, and other open sources. The IAEA expert evaluations are aggregated to make a final decision, which usually incomplete because of over 900 indicators, lack of expertise, and unavailability of information sources. In this study, a cumulative belief degree approach is introduced based on the belief structure. It is used to aggregate the incomplete expert evaluations that are represented with fuzzy linguistic terms. Moreover, a reliability index is employed to find the trustworthiness of the final result depending on the available evaluations. A numerical example illustrates the applicability of the proposed methodology.