FY26 Report on Water NSTF Testing: Various Test Scenarios with Lower Tank Inlet

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Abstract
The Natural convection Shutdown heat removal Test Facility (NSTF) at Argonne National Laboratory completed its eighth and final year of water-based testing in 2026, marking the conclusion of a multi-configuration experimental campaign that first began in 2018. This report documents results from the past year of testing activities, detailing nine tests across pressure and loss coefficient parametric conditions, beyond design basis accident scenarios, and studies at full dryout conditions. Over this past year, the NSTF logged 370 hours of heated operation, producing data sets from four scoping and five accepted DataQuality tests under the NQA-1 quality assurance program. The pressure parametric testing finalized a multi-year comparison between mid-tank and lower-tank inlet configurations across a range of system pressures. The effects of pressure primarily influence the local parameters such as liquid temperatures and oscillation damping behavior, with relatively minor influences on the fundamental instability mechanism. Tank inlet elevation, however, was found to strongly affect system behavior, with significant influences on flashing instability propagation, duration of large amplitude flow oscillation, and quasi-steady-state heat removal performance. The lower tank inlet configuration consistently produced lower loop flow, higher riser and panel temperatures, and stronger persistence of flow oscillations than the mid-tank inlet configuration, reflecting the reduced chimney height and resulting in higher hydrostatic head. A full-inventory depletion test, the first to combine a near-full initial inventory level with continuous accelerated drain to complete depletion, successfully characterized the system response across the entire usable inventory range in a single continuous transient. Comparison with prior discrete inventory parametric cases showed that thermal structure conditions at boiling incipience, not inventory level alone, is a primary driver for determining startup and initial transient flow oscillation behavior, especially below 30% fill levels. This test concluded in flow stagnation and geysering, after which cold inventory replenishment via the primary water tank was initiated to allow loop recovery and re-establishment of natural circulation flow. Two throttling tests examined variations in loss coefficient along the network piping for loss factor coefficients (Kvalve) up to 423 at the inlet header and up to 522 in the upper chimney. Expanding on previous testing, results from the throttle parametric series showed that heat removal performance remains effective across a wide range of throttling conditions, with only minor degradation observed even when the loop experienced significant flow instabilities. The testing indicates that upper chimney throttling can create destabilization conditions at loss coefficients lower than header throttling, but did not progress to full geysering, instead producing irregular non-stagnant oscillations. Reduced power scoping tests were performed to evaluate the potential of developing natural air circulation cooling within the empty water pipes under dry out conditions. Measurements indicate that minor amounts of heat removal by air circulation do, in fact, develop within the channels of the empty loop. A single-phase repeatability and rapid evacuation test then showed excellent reproducibility with prior baseline data (differences under 3% across key parameters) and resulted in the first direct measurement of NSTF thermal response to a nearly instantaneous, nearly total inventory loss. With the completion of this test matrix, the experimental campaign using the current water-based NSTF configuration is concluded. Across eight years of water-based testing, NSTF performed 94 tests over 1,978 hours of operation, generating a robust dataset that has been shared with multiple U.S. based organizations. These data sets have directly supported code validation, licensing-relevant dat
Funding Information
  • US Department of Energy (AC02-06CH11357)
  • USDOE Office of Nuclear Energy - Office of Nuclear Reactor Technologies - Advanced Reactor Technologies (ART)