Summary of the Results of Computational and Experimental Studies of Water Reflood of FA in Support of Modeling of Severe Accidents using the SOCRAT Code

Abstract

Based on modern theoretical knowledge and the results of representative experimental studies, the phenomenology of reflooding of fuel assemblies is considered. The parameters (test section pressure, water subcooling, peak cladding temperature at the start of the flooding, bundle power, etc.) maintained in the experiments under consideration are close to those expected when implementing measures to manage hypothetical severe accidents at pressurized water reactors. A list of processes accompanying the reflood of fuel-rod assemblies has been formulated, and specific effects have been established that can lead to a change in the local conditions of heat exchange between the cladding of fuel-rod simulators and the steam-water mixture and affect their quenching. A comparison of the results of experimental studies showed the influence of cooling water flow rate on the spread of measured values of quench time in the upper part of the fuel assembly. The view of reflood physics allowed us to analyze the results of validation of the SOCRAT code in experiments of varying phenomenological complexity (in an intact core, with an intense steam-zirconium reaction, formation of a melt). The analysis showed that the SOCRAT code correctly predicts the temperature histories of the fuel-rod simulator claddings, the quench time, and the total mass of hydrogen released during the experiment with a tendency toward slight underestimation; the modeling results do not contradict the experimental data. During validation, it was established that the thermal hydraulics model makes the greatest contribution to the assessment of the model error in calculating the quench time and the total mass of hydrogen production when modeling experiments of varying phenomenological complexity. Good predictive capabilities of the SOCRAT code confirmed the applicability of a one-dimensional approach to modeling the reflooding of fuel assemblies.