Scaling analysis of the Loop Seal Plugging and Clearing phenomena in a SB-LOCA transient on the BETHSY facility

Abstract

The objective of this work is to investigate what controls the duration between the Loop Seal Plugging (LSP) and Clearing (LSC) phenomena occurring in a 6% cold leg break LOCA transient carried out on the French BETHSY facility. This influences the duration of a possible core uncovery and the height of the associated Peak Clad Temperature (PCT). This work presents these phenomena, which are complex and difficult to predict by the codes and a scaling analysis is performed on experimental data of the 6.2-TC test. The Fractional Scaling Analysis (FSA) method is applied to a pressure difference equation between an upper control volume and a lower control volume of the BETHSY primary circuit. The relative magnitude of the processes that control the pressure difference between the two control volumes is evaluated. Through a phenomenological analysis, the bottom-up approach correlates the scaling analysis results (i.e. the magnitude of the obtained scaling criteria) with the behaviour of the transient at the system scale. Sensitivity analyses on key parameters are performed in order to establish hypotheses on the code difficulty to predict these phenomena. Finally, conclusions are drawn regarding the behaviour of LSP and LSC, and recommendations on the design of integral tests with respect to the best preservation of these phenomena are made.