Evaluation of prompt nucleation of bubbles in annular fuel elements during the initial depressurization transient of a DEGB LOCA

摘要

In the first moments of a DEGB LOCA, the depressurization wave may result in the system pressure falling below the saturation pressure of the coolant resulting in bubble formation. Immediate formation of bubbles might result in flow instability,with the affected fuel element unable to survive the LOCA. Nucleation of bubbles can occur by either pressure undershoot, or liquid superheat in the boundary layer adjacent to hot surfaces. This investigation developed a method for evaluating thepossibility of occurrence of bubble formation as a result of this initial depressurization. The method developed was based on bubble nucleation models and experimental results. The results show that the superheat required for prompt nucleation, within the initial period between the depressurization wave and the re-pressurization wave, is much greater than the available superheat in the subject fuel element, either measured experimentally or calculated by RELAP. Accordingly, prompt nucleation would notoccur in the subject fuel element in this initial time period.