Efficiency of Passive Open Loop Heat Removal Over Wide Range of Loop Flow Resistances – PASI Loop Tests in the EU-PASTELS Project

Abstract

The PASI test facility, designed and constructed at LUT University, is a model of a containment cooling system using an open passive heat removal loop. The PASI facility consists of a pressure vessel simulating containment conditions and a circulation loop comprising an in-containment heat exchanger, riser and downcomer pipelines, a sparger, and a water pool as a heat sink / water reservoir. The loop flow resistance research with the PASI facility has been carried out within the PASTELS (Passive Systems: Simulating the thermal hydraulics with experimental studies) project with funding by the EU Horizon 2020 program. The experiments within PASTELS have been done to quantify the sensitivity of the heat removal capability versus loop flow resistance and to map the loop operating conditions where instabilities or flow oscillations appear. During each experiment, loop flow resistance was varied by a throttling valve, keeping the heating power to the containment constant. Experiments were done with different heating powers. It was observed that the system removed heat efficiently over a wide range of additional loop resistance. Loop flow rate was stable until saturation was approached. With close to saturation conditions in the loop, geysering-like flow oscillations set in. Heat removal capability remained consistently good despite the oscillations. Decreasing heating power decreased the oscillation frequency of the loop flow, and the oscillation amplitude stayed almost constant. After each increase in the flow resistance, void fraction in the riser increased and the system found at once a new quasi-steady state where heat was adequately transferred.