Investigation of the release of zinc in the reactor sump and the behavior of dissolved zinc at hotspots in the reactor core after a loss of coolant accident

Generic experimental and methodical investigations were carried out aiming at the systematical elucidation of physico-chemical mechanisms and their influence on thermo-hydraulic processes, which can occur during the sump circulation operation after loss-of-coolant accidents in PWR. In such cases, boric acid containing coolant with dissolved zinc, which is formed by corrosion of zinc-coated containment internals, may reach core regions with higher temperature (hot-spots).
The experimental studies done at semi-technical scale were focused on the influence of such zinc containing coolants on thermo-hydraulics at heating configurations similar to those inside the core of PWR. The impact of physico-chemical mechanisms on thermo-fluid-dynamical behavior of the coolant inside a 3×3 heating rod configuration with spacer segments was determined. As an initial condition, boric coolant with dissolved zinc was used at a fluid temperature in the range of 45...50°C.
During the heating of zinc-containing coolant, an increasing turbidity of the fluid caused by formed colloids was observed first, followed by the formation of several solid corrosion products consisting of zinc borates. In dependence of the temperatures of fluid and heatable surfaces, the solids showed a different behavior concerning their mobilization potential, density and ability to layer formation. Deposits occurred at the rod surfaces as well as at the spacer segments. They effected a hindered heat transfer from the rod surfaces to the fluid, an increasing head loss at the spacers and some changes of the flow distribution. In addition, quantifications of the formed solid corrosion products including a characterization of the released particles were done. Subsequently, investigations were expanded considering original zinc sources (zinc-coated gratings) and a 16×16 fuel rod dummy with a centered 8×8 heating rod configuration.
Summing up, achieved experimental results allow conclusions about the solubility behavior of zinc corrosion products in boric coolant as well as about the formation of solids and the effects thereof. Beside processes of deposit layer formation and particle release, effects like outgassing of air dissolved in the coolant and local subcooled boiling phenomena were observed, which can contribute to the remobilization of formed solids and may strengthen the mentioned mechanisms.