In the course of a small break LOCA in a Pressurized Water Reactor (PWR) the flow regime in the Reactor Cooling System (RCS) passes through a number of different phases and the filling level may decrease down to the point where the decay heat is transferred to the secondary side under Reflux-Condenser (RC) conditions. During RC, the steam formed in the core condensates in the Steam Generator (SG) U-tubes. For a limited range of break size and configuration, a continuous accumulation of condensate may cause the formation of boron-depleted slugs. If natural circulation reestablishes, as the RCS is refilled, boron-depleted slugs might be transported to the Reactor Pressure Vessel (RPV) and to the core. To draw conclusions on the risk of boron dilution processes in SB-LOCA transients, two important issues, the limitation of slug size and the onset of Natural Circulation (NC) have to be assessed on the basis of experimental data, as system Thermal-Hydraulic codes are limited in their capability to replicate the complex physical phenomena involved. The OECD PKL III tests were performed at AREVA’s PKL test facility in Erlangen, Germany, to evaluate important phases of the boron dilution transient in PWRs. Several integral and separate effect tests were conducted, addressing the inherent boron dilution issue. The PKL III integral transient test runs provide sufficient data to state major conclusions on the formation and maximum possible size of the boron-depleted slugs, their boron concentration and their transport into the RPV with the restart of NC. Some of these conclusions can be applied to reactor scale. It has to be mentioned, that even though this paper is based on PKL test results obtained within the OECD PKL project, the conclusions of this paper reflect the views of the authors and not necessarily of all the members of the OECD PKL project.
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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