Investigation of conditions for zinc particle release and transport in the reactor core

Background of the experimental and methodical work is a loss-of-coolant accident (LOCA) in a PWR. During a LOCA, the leakage water jet as well as the water in the containment can lead to corrosion of galvanized installations (e.g. grates). That means a change of consistency and chemical properties of the coolant during the sump recirculation operation, in particular an increase of the zinc concentration (Zn ions) and the pH-value. Furthermore, it leads to separation of layer-forming zinc compounds (e.g. zinc borate) at hot spots (fouling at fuel assemblies) and/or the crystallization of zinc corrosion products out of the coolant in hot areas. This in turn causes a decrease of pH-value and consequently an increase of the corrosiveness of the coolant flow. This can come along with a chemical transformation of zinc borate to nearly indissoluble zinc oxide by thermo hydrolysis. Spalling of parts of the coating on the surface of the fuel rods could lead to the release of particles into coolant water flow. In combination with released fibrous insulation material, which can be transported into the reactor containment, containment building sump and connected systems, this process leads to pressure build up at the strainers and the spacers of the fuel assemblies and a reduction of the pump performance. In this case, efficient heat dissipation cannot be guaranteed.
In a joint research project between the Institute of Process Technology, Process Automation and Measuring Technology (IPM) of the University Zittau/Görlitz and the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), the influence of different boundary conditions like the flow structures in the reactor pressure vessel, temperature distributions and the temperature gradient in the central core regions will be investigated. Therefore a test facility containing a 3x3 heated fuel rod dummy configuration was designed and constructed. The design and development of the test facility was accompanied by pre-test calculations and CFD simulations to get information about local surface and fluid temperatures under different flow conditions.
The paper includes a detailed description of the test facility and applied measuring techniques as well as an overview of all boundary conditions considered in the experiments. Main parts of the paper are the pre-calculation and the CFD simulation for the heat transfer in dependency of flow velocity and heat flux.
The investigations have been supported by the German Federal Ministry of Economy and Technology under grant nos.1501340 and 1501341.