Experimental assessment of fluid-to-fluid scaling for critical heat flux investigations in rod bundles at low mass flow

Abstract

Critical heat flux (CHF) is an essential topic in nuclear technology and for that reason it has been investigated during several decades. Novel nuclear reactor designs, such as self-pressurized small modular integral reactors (SMiR), represent a new challenge regarding this issue since they are designed to work at conditions for which traditional tools and procedures may cause too large uncertainties. In addition, traditional fluid-to-fluid modeling techniques for CHF tests have been derived from experiments mainly performed at PWR conditions and therefore its use is doubtful at different conditions. In this work we make use of water and Freon (CCl2F2) experimental data points obtained at low mass flow condition and relatively low subcooling representing natural circulation CAREM-25 SMiR at nominal pressure and at 10 MPa. The test section consists of 19-rods in a hexagonal array mimicking 1/6 of CAREM-25 fuel bundle. Traditional fluid-to-fluid mass flux scaling rules (derived by Ahmad and Katto) are then used to quantify its performance by comparing water and Freon data tests performed at similar conditions in a dimensionless space. Such a space is defined by the subcooling number, the phase change number and the so-called mass flux number. As a result of this part of the study it is concluded that the use of Katto's scaling rule for the mass flux produces an average error in estimating water data of around 10 %. In addition, the use of Ahmad's scaling rule causes an average error of about 20 % when estimating water data. Due to these modest performances, a dedicated mass flux scaling rule was derived based on the dimensionless Weber and liquid Reynolds numbers. The new scaling rule succeeded in reducing the average difference between water and Freon test data to less than 3 %. The use of the proposed scaling rule allowed treating the Freon and water data as a single dataset, which in turn enlarged the experimental domain. The proposed mass flux scaling rule is thus recommended for fluid-to-fluid CHF tests at low mass flux conditions and reduced subcooling numbers.