Generic thermohydraulic and physicochemical analyses for the implementation of an ATHLET module for the simulation of thermohydraulic consequences of zinc borate deposits in the PWR core. Subproject: Development and validation of an ATHLET module for the simulation of thermohydraulic consequences of zinc borate deposits in the PWR core

The main goal of the present project is to allow for the simulation of zinc borate deposits and their thermohydraulic consequences in the PWR core after a loss-of-coolant accident (LOCA) with the system code ATHLET. The phenomena of zinc dissolution and formation of zinc borate particles occur because of flow-induced corrosion processes caused by a direct fluid contact or wetting with zinc-coated installations inside the containment sump. A new module called "Zinc borate" has been developed and implemented to extend the area of application of ATHLET to the simulation of such chemical processes. The "Zinc borate" module contains three sub-models (or closure models) for simulating the processes of zinc release, the precipitation/accumulation of zinc borate par ticles as well as the transport of zinc ions and/or zinc borate particles. To develop and validate each sub-model of the "Zinc borate" module, data and correlations from experiments on two different test facilities were used, which were designed and operated by the project partners University Zittau/Görlitz (HSZG), Helmholtz Zentrum Dresden-Rossendorf (HZDR) und Technische Universität Dresden (TUD). Thermal hydraulic models of both the integral test facility ZFT ("Zittau Flow Tray") and the test facility THETIS ("Twofold HEaTIng rod configuration for core Simulation") were developed in ATHLET for the validation of the sub-models as well as of the overall "Zinc borate" module. After the validation of the input deck against specific trial operations, selected tests on the release of zinc and on the formation and accumulation of zinc borate were simulated and compared with experimental data. The comparison of the simulation results with the available experimental data has shown that the correlations developed by the project partners and implemented in ATHLET are able to describe the zinc release, the precipitation of zinc borates and the deposition of zinc borate on the fuel cladding surfaces. However, further upgrading of specific correlations is necessary in order to close the actual model gaps such as for the calculation of the deposition of zinc borate particles on spacer grids and thus to improve the ATHLET module "Zinc borate".