RAMI analysis of the Collective Thomson Scattering system front-end – Part2 – reliability block diagram analysis

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ITER is an experimental tokamak nuclear fusion reactor with the goal of achieving afusion energy gain factor of 10. Nuclear fusion presents an extreme environment for materials during operations while requiring continuous uninterrupted operation.The Reliability Block Diagram (RBD) is a graphic methodology for system reliability modelling which is constructed from the reliability-wise relations between different subsystems/components and their respective reliability and maintenance data. In this paper, we construct andanalyse the RBD for the Collective Thomson Scattering (CTS) front-end both in terms of fulfilling its functions and its effects in the ITER tokamak operations. Our RDB analysis results were then used to propose mitigation actions which include design changes and operational procedures to deal with the identified failure modes.Our initial results indicate that the system's mean availability at the end of its lifecycle of 20 years is no greater than 82.55% which was due to low Mean Time Between Failures (MTBF) of a critical system component – theSplit Biased Waveguide (SBWG) – aswell as several thermally loaded components. After mitigation actions, which aimed at controlling the system's exposure to heat loads, the MTBFs increased and the resulting mean availability achieved values over 97%, in conformity with design specifications. Furthermore, considering the CTS failure modes with impact on ITER operations, the expected value for the mean availability is of 99.995%. Therefore, we concluded that these failure modes did not require mitigation actions.In this article, we cover the second half of the RAMI analysis of the CTS diagnostic, the RBD analysis.