The Spanish Centralized Interim Storage facility will provide temporary storage for all spent fuel and high-level waste from Spanish nuclear power plants. The most critical process is performed in the hot cell, where the casks of spent fuel are unloaded and encapsulated into canisters. To validate the control system design and the operation of the equipment used in the unloading process, Tecnatom has developed an engineering simulator. The simulator has both the Human Machine Interface (HMI) for the remote operation of the equipment replicating the one of the control systems, as well as a virtual reality environment of the hot cell. A series of functionalities have been introduced to analyze the operation an detect aspects to be improved in the design. Thanks to this, the HMI has been properly redesigned, potential issues of the control system has been early detected, and a better understanding of the system achieved. In addition, the virtual reality environment of the hot cell helps decision-making for other aspects of the design, such as the location of video cameras. Finally, the simulator can be used in the future as a training platform and be the basis of a digital twin of the facility.