The ongoing decommissioning of nuclear facilities in Germany requires a highly qualified workforce, yet the sector faces a growing shortage of trained personnel due to retirements, restricted facility access, and declining educational capacity. Immersive virtual reality (VR) provides an effective means of supporting competence development where safety or operational constraints limit real-world training. However, existing VR-based training applications for the nuclear power sector remain limited in scope and often lack participatory development involving domain experts. The present study reports on the participatory, design-based development of an immersive VR training system and presents findings from an evaluation with experienced nuclear professionals. Using a convergent mixed-methods design, quantitative data from standardized questionnaires were combined with qualitative insights from semi-structured interviews. Thirteen radiation-protection workers and project engineers tested two radiation-protection and two safeguards scenarios. Participants reported consistently high enjoyment, clear task structures, and substantial perceived didactic value. Spatial presence was rated at a moderate level. The VR training was perceived as particularly beneficial for novices, supporting conceptual understanding, procedural awareness, and safety culture. Qualitative findings also identified several areas for refinement, including enhanced preparatory guidance, improved interaction fidelity, richer contextual information, and the potential introduction of multi-user modes. Overall, the results indicate that immersive VR can serve as an accessible, engaging, and instructionally meaningful complement to existing training practices in the nuclear sector. The study provides empirically grounded design exemplars for site-independent VR training and highlights future directions for expanding scenario fidelity, validating learning outcomes, and embedding VR within structured training procedures.