For the study of the Heterogeneous Inherent Boron Dilution transient in a Pressurized Water Reactor, a Small Break Loss Of Coolant Accident (SB-LOCA) is postulated. Natural Circulation (NC) may be interrupted and, under Reflux-Condenser (RC) conditions, the steam formed in the core condensates in the Steam Generator (SG) U-tubes: a boron-depleted slug may accumulate in the crossover leg and in the SG outlet chamber. If NC restarts as the Reactor Cooling System (RCS) is refilled, boron-depleted slugs might be transported to the Reactor Pressure Vessel (RPV) and to the core. The mixing of the boron depleted slug with the borated water in the Cold Legs (CLs), downcomer and lower plenum after Restart of Natural Circulation (RNC) is quantified by means of Computational Fluid Dynamics (CFD) analyses. The CFD code STAR-CD is used to perform this analysis. Boundary conditions for this calculation — especially the boron-depleted slug size and the NC restart mass flow rate — are extrapolated from PKL experimental findings. The initial conditions are derived from an overall plant analysis performed with the CATHARE system code. Buoyancy effects, both in the cold leg and in the downcomer, are very significant phenomena for the evaluation of the slug transport and mixing: the hot (saturation temperature) boron-depleted water slug tends to accumulate in the upper parts of the cold legs and in the upper part of the downcomer (above the cold legs), before being pushed and dragged down. The boron concentration distribution at the core inlet during the transient, evaluated with STAR-CD, is compared with a critical value in order to check that boron concentration at the core inlet is always above the threshold necessary for the core to remain subcritical.

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