An Experimental research on the effect of the axial power distribution on the critical power

Liu, W.; Kureta, Masatoshi   

R&D project to investigate thermal-hydraulic performance in the tight-lattice rod bundles for Innovative Water Reactor for Flexible Fuel Cycle (FLWR) has been progressed at Japan Atomic Energy Agency (JAEA). The basic thermal-hydraulic characteristics about the critical power and the pressure drop, under both steady and transient conditions, have been ascertained with using 37-rod tight lattice bundle. The effect of the spacer on the critical power has also been ascertained. However, as to the axial power distribution, the severest one - which is called as the basic heating case in this research - is selected in these experiments, with no considering to the change of the axial power distribution with the proceeding of the burnup. Therefore, an experimental research on the effect of the axial power distribution on the critical power is necessary. In this paper, an experimental research on the effect of the axial power distribution on the critical power is performed under the atmospheric pressure with using a circular tube, which is axially divided into 3 insulated levels and is heated separately by 3 DC power supplies. The test section is capable to simulate the power distribution in the double-humped core. Axial uniform heating condition, the upper 3 layers simulation and the center 3-layers simulation of the basic heating case are selected as the cases for the detailed evaluation. The effects of relative power ratio on the critical power, critical quality and critical boiling length under different heating conditions are ascertained for the detailed evaluation cases. With using the experimental data, existed correlating methods for the boiling transition are evaluated. As the result, a combination of the critical quality - boiling length method and the critical heat flux - critical quality method is found a good correlating to the present data with a changed axial power distribution.