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

摘要

Generic experimental and methodical investigations were carried out aiming at the systematic elucidation of physico-chemical mechanisms and their influence on thermo-hydraulic processes, which can occur during the sump circulation operation after a loss-of-coolant accident (LOCA) in PWR. In such a case, borated coolant with dissolved zinc, formed by corrosion of zinc-coated containment installations, may reach core regions of higher temperature (hot spots). Experimental studies were done at semi-technical scale focused on the influence of such zinc-containing coolants on thermo-hydraulics at heating configurations similar to those inside the PWR core. The impact of physico-chemical mechanisms on thermo-fluid-dynamical behavior of coolant inside a 3×3 configuration of heating rods, which act as fuel rod simulators with spacer segments, was determined. As a test parameter, borated coolant with dissolved zinc was used with an initial fluid temperature in the range of 45...50°C. The initial boric acid concentration in the experiments is based on the accident scenario in german PWR. In the case of a LOCA, emergency cooling water (containing 2,500 ppm boron) from two refueling water storage tanks is injected into the primary cooling circuit during LOCA. During heat-up of zinc-containing coolant in a circuit, an increasing turbidity of the coolant fluid caused by forming colloids was observed first. This was followed by the formation of solid corrosion products consisting of zinc borates (identified by means of Raman spectroscopy [2]). In dependence of the temperatures of fluid and heatable surfaces, the solids showed various attributes concerning their mobilization potential, their density and their ability to form deposit layers. 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 released solid particles were done by means of microscope and particle analyzer. Subsequently, investigations were expanded considering original zinc sources (zinc-coated gratings) and a 16×16 shortened PWR fuel assembly dummy with a centered 8×8 heating rod configuration. Achieved experimental results allow conclusions about the solubility behavior of zinc corrosion products in borated coolant as well as about the formation of solids and the consequences thereof. Beside processes of deposit layer formation and particle release, further effects like outgassing of dissolved air and local subcooled boiling phenomena were observed, which can accelerate the remobilization of formed solids and may strengthen the mentioned mechanisms.