Nuclear safeguards are a system of inspection and verification of the peaceful uses of nuclear materials, and the deterrence of diversion of nuclear materials through early detection. Uppsala University has for the past few decades been involved in developing safeguards instrumentation, techniques and analysis tools to support the international safeguards work. This project aims at studying and enhancing one of the instruments used by international safeguards inspectors, the Digital Cherenkov Viewing Device (DCVD). By measuring and quantifying the Cherenkov light emissions from spent nuclear fuel assemblies in wet storage, it is possible to verify that the assembly under study is a spent nuclear fuel assembly, as opposed to a non-radioactive item. The DCVD is one of very few instruments approved for partial defect verification, verifying that parts of an assembly has not been diverted. This project aims at further enhancing the DCVD performance to detect partial defects in assemblies, by studying the measurement setup and by investigating how various partial defects can be detected using the DCVD, and how various fuel parameters affect the Cherenkov light inteisty. One of the main difficulties in investigating partial defect in nuclear fuel assemblies is the lack of experimental data. Partial defect where a significant portion of the assembly has been removed and/or replaced in principle never occurs in a normal case. Furthermore, due to the number of possible diversion scenarios, and due to safety concerns when handling spent nuclear fuel assemblies, it is not possible to create a set of assemblies suffering partial defect that can be used to benchmark the performance of safeguards instruments. For this reason, these investigation require the use of Monte-Carlo radiation transport simulations, to investigate assemblies with various types of defects, and what impact the defect will have on a measurement of the fuel assembly. Computational resources provided by SNIC/Uppmax will allow this project to systematically simulate and investigate various partial defect scenarios, and to obtain sufficient statistics in the Monte-Carlo simulations to provide reliable information regarding the Cherenkov light intensity that can be measured from such assemblies. This data will form a basis for future work related to the analysis of the DCVD performance, based on the synthetic data obtained through the simulations. The simulation results will also be useful for benchmarking new analysis methodologies, and for identifying parameters of importance for predicting the Cherenkov light intensity of a spent fuel assembly.