ASTRID safety design: Progress on prevention of severe accident

ASTRID is the Advanced Sodium Technological Reactor for Industrial Demonstration which is intended to prepare the Generation IV reactor, with strong improvements in safety and operability. In order to meet the objectives of the Generation IV reactors and comply with the related specifications, the ASTRID project integrates innovative options. In the earlier phase of ASTRID project, a specific safety approach was set and its main guidelines were agreed by the French Nuclear Safety Authority. This basic safety design guide is currently applied as reference for the choices of the design options. The paper presents the main applications, in terms of design provisions, to prevent any severe accident, as far as reasonably possible. In particular, the design measures for ⁿeutron reactivity control are presented. These measures are based on efficient, reliable, redundant and diversified means for reactor shutdown. In addition, core features and inherent reactor behavior are enhanced and supported if needed by innovative devices. The design approach includes design criteria for assuring safe reactor shutdown states. As concerns the ^decay heat removal safety function, its loss must be practically eliminated in order to prevent, with a very high level of confidence, a severe accident that could lead to a cliff edge effect. The method applied to reach this objective is described and the resulting heat removal systems and design means are presented. In the frame of ASTRID safety studies, analyses presented in the paper are devoted to well define: - A domain of accidental sequences with very low occurrence frequency for which severe accident can reasonably be prevented thanks to appropriate design provisions, - A few hypothetical situations, consequences of which could not reasonably be mitigated, requiring robust safety demonstrations, in terms of prevention. They have to be practically eliminated. Despite the high level of prevention of severe accident implemented in ASTRID, its safety approach postulates also a hypothetical severe accident, in order to comply with the defence-in-depth principle (fourth level) and to check that the induced potential consequences are suitably mitigated. (author)