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Investigating the Application of Self-Actuated Passive Shutdown System in a Small Lead-Cooled Reactor

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The application of passively or self-actuated passive safety systems in nuclear reactors allow to simplify the overall plant design, besides improving economics and reliability, which are among the high-level goals set out by the Generation IV International Forum. This thesis focuses on investigating the application of a self-actuated, passive shutdown system for a small, modular lead-cooled fast reactor, and on its implications on the dynamic response to an initiating event. The application of passive shutdown systems for a lead-cooled reactor is not studied extensively, due to the general consensus that lead as a coolant, is too dense to achieve any passive shutdown by gravity. On the contrary, dense liquid lead as a coolant is viewed to be extremely efficient in buoyancy-driven passive shutdown. Initially neutronic parameters were determined using a combination of Monte Carlo codes, OpenMC and Serpent, by carrying out sensitivity analyses on a critical, hot-state core at middle of life. The reactivity worths of the intended shutdown assemblies and control assemblies were then determined. According to a first-order approximation approach, the passive insertion of shutdown rods was assumed to be influenced by gravity, pressure drag and viscous drag due to flow against the assembly and finally the buoyant force. Sensitivity analyses were performed for a spectrum of models with varied drag coefficients, in addition to determining the effect of addition of ballast to the assembly and finally to assess the effect of changing coolant flow rate. The time of insertion of the shutdown assembly from its parking position in the core was determined for each of these scenarios. An optimised shutdown foot profile was designed to allow the quickest passive insertion and then implemented in BELLA multi-point dynamics code, in order to perform dynamic analyses of a transient overpower scenario. This study provides evidence for the viability and reliability of gravity-driven shutdown systems in a heavy liquid metal cooled reactor, and also providing specific data for buoyancy-driven insertion. Further studies could be carried out to investigate the application of such systems in different reactors cooled by, for instance, lead-bismuth or mercury, and also to improve the efficiency of safety systems in sodium cooled reactors.
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