Experimental and calculational results of the QUENCH-05 test

The QUENCH experiments are to investigate the hydrogen source term that results from the water injection into an uncovered core of a Light-Water Reactor (LWR). The test bundle is made up of 21 fuel rod simulators with a length of approximately 2.5 m. 20 fuel rod simulators are heated over a length of 1024 mm, the one unheated fuel rod simulator is located in the center of the test bundle. Heating is carried out electrically using 6-mm-diameter tungsten heating elements installed in the center of the rods and surrounded by annular ZrO 2 pellets. The rod cladding is identical to that used in LWRs: Zircaloy-4, 10.75 mm outside diameter, 0.725 mm wall thickness. The test bundle is instrumented with thermocouples attached to the cladding and the shroud at 17 different elevations with an axial distance between the thermocouples of 100 mm. The hydrogen that is produced during the experiment by the zirconium-steam reaction is analyzed by three different instruments: two mass spectrometers and a "Caldos 7 G" hydrogen measuring device (based on the principle of heat conductivity). This report describes the results of test QUENCH-05 performed in the QUENCH test facility at the Forschungszentrum Karlsruhe on March 29, 2000. The objective of the experiment QUENCH-05 was the investigation of pre-oxidized rod cladding on cooldown by steam. This experiment could be compared to test QUENCH-04 which was conducted without pre-oxidation. The experiment consisted of a heatup phase to temperature plateau of around 900 K, a pre-oxidation phase at -1473 K, a transient phase, and a cooldown phase. All phases were conducted in an argon/steam atmosphere. At the beginning of the transient phase the test bundle was ramped at around 0.32 K/s in the temperature range 1445 - 1740 K to the temperature excursion which led to a maximum rod cladding temperature of 2272 K at the 750 mm level (TFS 5/11). For cooling the test bundle, steam was injected at the bottom of the test section at a rate of 50 g/s. The steam injection led to undelayed cooling of the rods; all rod cladding temperatures began to drop within one second. The total amount of hydrogen released during the QUENCH-05 experiment was 27 g. Of the 26 g, -4 g is estimated for the first heatup phase, -12 g for the pre-oxidation phase, 9 g for the transient phase, and only -1 g for the cooling phase. After the test the shroud exhibited a localized failure zone between 900 mm and 1010 mm elevation. Only a limited local melt formation was observed in the test bundle itself. The maximum oxide layer thickness measured at 950 mm elevation amounted to -420 μm (mean value of all fuel rod simulators). Prior to the cooldown by steam, i.e. in the middle of the transient, the ZrO 2 layer thickness had a maximum of -160 μm at the same level. The latter data were obtained from corner rod B which was withdrawn from the test bundle at 1620 K. Calculations with the FZK code CALUMO were performed to study the oxidation behavior of fuel simulator cladding and shroud. To achieve a satisfactory agreement between the calculational results of the CALUMO code (with respect to the evolution of the oxidation) and the experimental findings of QUENCH-05 it was necessary to considerably increase the heat transfer values of fuel rod simulator/coolant below 750 mm.