Critical flow break source term near the outlet of a slit

LBB (Leak-before-break) radioactive aerosol monitor in the nuclear power plant requires knowledge of the source term to build the relationship between monitored radiation activity and the leak flow rate. The leak flow rate could be used to evaluate the risk of LOCA (Loss of Coolant Accident). The source term, including initial diameter distribution, and initial concentration, comes from the break of the liquid phase in the critical flow existing in the crack. The relationship between the source term and the critical flow is complicated and requires dimensional analysis to be built. Experiments of water and R134a under lower stagnant pressure and lower stagnant temperature than the primary loop are conducted to measure the critical flow pattern, spray shape, and droplet diameter distribution near the outlet of the slit (simplified crack). Non-dimensional numbers based on the outlet physical variables are calculated by TFM-DEM (Two-fluid Model-Delayed Equilibrium Model), and the relation between diameter distribution coefficients and non-dimensional numbers is built by regression analysis. It is found that the critical flow patterns are similar and have no relationship with the break, while the spray shapes could be split into three groups, and the break relationships are different among them. Linear discriminant analysis is used to classify different spray shapes and different breaking modes. The relations are effective as long as the range of major non-dimensional numbers related to the breaking process covers those of actual industrial cases. The initial concentration is researched using random sampling and regression analysis so that it can be calculated using the gas phase volume fraction at the outlet. The source term equations, non-dimensional analysis, and random sampling process would be of great help in the development of more accurate model for the source term of LBB aerosol monitor with more complete source term measurement results in the future.