Report on Year-3 of Water NSTF Matrix Testing: Two-Phase Parametric Test Series

Abstract

The Natural convection Shutdown heat removal Test Facility (NSTF) is large scale test facility constructed at Argonne National Laboratory to generate NQA-1 qualified validation data for passive decay heat removal systems in advanced reactor concepts. With support from the Department of Energy (DOE) offices of Advanced Reactor Technologies (ART) and Advanced Reactors Regulatory Development (RD), this facility reflects key features of a ½ scale, water-based, Reactor Cavity Cooling System (RCCS) and is intended to study the behavior, bound performance, and ultimately guide design decisions for passive decay heat removal systems. Since the facility also retains all aspects common to a fundamental boiling water thermosiphon, the generated experimental data sets are well poised to advance the understanding of fundamental natural circulation boiling and two-phase flow phenomena. This report serves as a summary of the experimental activities conducted during the programs third year of water-based test operations. Two-phase testing performed in prior years was conducted using a transient mode of facility operation, where the test progression results in a gradual reduction of system level due to loss of inventory from the system by natural boil-off. In this year’s testing, a static mode of facility operation was introduced where the system level was maintained at a constant level by continuous refill of boil-off condensate back into the system. This modification allowed the facility to reach a true steady-state mode of operation where single-parameter variations could be studied for their influence on liquid temperatures, system pressures, flow behaviors, and other system parameters. With established confidence in the ability to repeat test conditions and maintain steady-state mode of facility operation, this third year of testing then embarked on a number of two-phase parametric test cases to further understand the thermal hydraulic behavior, two-phase instability phenomena, and decay heat removal performance of this scaled water-based RCCS test facility. Over a 12-month period, 250 hours of active heating was logged and 11 total matrix tests attempted; eight classified as Successful per NQA-1, and three as Trending. In continuation from the previous year’s campaign, testing was first performed within the inventory parametric series to examine the role of tank level. Testing resulted in the generation of experimental data at two-phase flow conditions with starting inventory levels of 80%, 70%, and 60%. A fourth test was performed using an initial inventory level of 55%, and through an accelerated drain process, allowed to extend into a depleted condition in an attempt to observe potential flow stagnation and cessation of heat removal function. Due to large voiding in the chimney and swelling of the heated leg, flow of the primary coolant continued to function even as the tank level fell 35-inches below the chimney discharge port within the tank. The test series examining the role of system pressures was then initiated, which varied the system pressure by altering the resistance of the steam discharge line off the gas space of the primary tank. Testing within the power parametric series was also performed which examined the role of integral power at levels corresponding to various ranges of prototypic decay heat loads in a prototypic full-scale reactor. Finally, investigations were made on the role of loss coefficients along the primary piping, where a throttling valve was used to impose varying degrees of flow restriction at the inlet to the heated test section. With the facility operating in a static mode of operation with continuous refill of condensed boil-off, flow restrictions of up to 83% of the nominal flow area were imposed on the loop while operating at steady-state two-phase conditions. Initially, the increasing restriction at the header inlet resulted in a stabilization of system flow oscillations, however with fur