Theoretical Support to the NOKO Experiments

Several approaches have been undertaken in a number of European countries to study and demonstrate the feasibility of innovative passive safety systems. The European BWR R&D Cluster combines experimental and analytical efforts that are mainly directed to the introduction of passive safety systems into boiling water reactor technology. The main objectives are to use the large scale European test facilities NOKO (Jülich, Germany), PANDA, LINX (Villingen, Switzerland), PANTHERS (Piacenza, Italy) as well as the demonstration power plant Dodewaard (Netherlands) for a synergistic experimental R&D program. The project should additionally contribute to validating and improving thermalhydraulic computer codes. In 1997 a BWR Physics and Thermohydraulic Complementary Action (BWRCA) was established to further assist and broaden the objectives of the EU BWR R&D Cluster. The work presented in this report is performed in the Workpackage Theoretical support to the NOKO experiments and divided into three tasks: Implementation of two phase flow instrumentation, ATHLET calculations of NOKO experiments and approach for the optimization of passive components. FZR is developing two phase flow measurement instrumentation (e.g. conductivity probes), which is installed in parallel to the emergency condenser test bundle. The experimental data are used for the validation of the improved ATHLET condensation model. The evaluation of these experiments shows an undesired accumulation of noncondensable gases in front of the water level and give valuable insights to the phenomena caused by noncondensables. An improved twophase flow instrumentation is suggested based on the collected experience and taking into account the detected problems.

Within the BWRCA FZR had performed 10 post test calculations and an additional blind test calculation of NOKO experiments. The results of these calculations are presented and discussed in detail. These calculations show that ATHLET is able to perform proper calculations. At least goals for an optimization of the emergency condenser are discussed. This is desirable because it allows the decrease of the pressurized BWR surface and the possibility of leakages.