Neutron imaging based vapor fraction measurements in an inductively heated boiling packed bed

Abstract

During a loss of coolant accident, the formation of porous heat-generating debris bed causes concerns with the further progression of accident through inadequate removal of the decay heat. The sustainable heat removal or cooling of the porous debris bed using emergency coolant water poses limitations on the cooling mechanisms by imposing a maximum removable heat flux, also known as the dryout heat flux. Modeling debris bed cooling and the dryout heat flux requires the use of parameters that are non-linear functions of the vapor fraction in the two-phase flow. These models need high resolution vapor fraction data for validation, which is currently lacking in literature. This paper presents first of a kind high-resolution vapor fraction measurements in an inductively heated debris bed using a non-optical and non-intrusive neutron imaging technique. The neutron beam from TRIGA Mark II reactor at Kansas State University was used to image the packed bed test section and gather vapor fraction data. The vapor fraction measurements are presented for a range of heat fluxes up to the limiting dryout heat flux. Images showing the formation of a dryout zone for heat fluxes higher than the limiting dryout heat flux are also presented. The measured vapor fractions are compared with predictions from the classical 1D Lipinski model. The mean absolute deviation between the model predictions and the measured vapor fractions is found to be less than 10%, providing the first validation of the classical 1D Lipinski model.