Jun Fang

This report details the FY26 Computational Fluid Dynamics (CFD) modeling activities conducted for the water-based testing phase of the Natural convection Shutdown heat removal Test Facility (NSTF). Focusing on the reconfigured lower tank inlet piping configuration, the primary objectives of this study are twofold: (1) to evaluate the high-fidelity predictive capability of 3-D CFD for single-phase natural circulation and thermal mixing to complement 1-D system codes, and (2) to assess the current modeling capabilities and numerical stability of CFD codes for two-phase hydrostatic head reduction flashing in passive safety systems. For single-phase natural circulation, transient simulations were performed for the deep-pool baseline (Run089, 80% initial inventory fill) and the shallow-pool accident limit (Run093, 20% depleted inventory fill). The RANS-based models demonstrated outstanding numerical robustness, near-linear parallel scaling on High-Performance Computing (HPC) clusters, and excellent quantitative agreement with transient and spatial experimental thermocouple measurements. High-resolution flow fields illuminated the decisive influence of pool depth on buoyant jet trajectories, demonstrating unconstrained vertical thermal stratification in deep pools and revealing the potential for horizontal flow recirculation and thermal short-circuiting in shallow pools. For two-phase operations, demonstration simulations encompassing prescribed boundary conditions (Case 1), internally resolved Eulerian Multiphase (EMP) flashing models (Case 2), and scoping Mixture Multiphase (MMP) models successfully captured high-level complex multi-dimensional phenomena, including severe upper-pipe phase stratification, liquid acceleration beneath vapor blankets, and buoyant vapor plume venting in the primary tank. Despite these successes, modeling two-phase flash boiling presented severe numerical challenges driven by stiff interphase coupling and explosive volumetric expansion. Robust numerical stability was successfully achieved across the full integrated geometry in collaboration with Siemens engineering support—leveraging user-defined NIST REFPROP equation-of-state tables, non-reflecting boundaries, and under-relaxation scheduling. However, current stabilized two-phase predictions have notable limitations, as the modeled phase-change patterns deviate from test data and fail to capture the experimental downward propagation of the flashing front from the tank inlet into the vertical chimney. This study lays a solid foundation and provides recommendations to guide future research. Near-term priorities include systematically restoring higher-order spatial discretization schemes, calibrating interfacial drag and dispersion closures, tuning dynamic bubble interaction length scales, and extending computational domains to simulate full-duration inventory boil-off transients. Ultimately, integrating these multi-dimensional CFD insights with 1-D system-level codes (such as SAM and RELAP5-3D) will advance multiscale predictive capabilities for reactor passive decay heat removal systems.

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

This study presents a comprehensive benchmarking and verification effort of several thermal-hydraulic and multiphysics capabilities for high-temperature gas-cooled reactor applications. The first part of this effort focuses on the running-in verification of Griffin’s multiphysics capabilities, specifically for simulating the evolution of pebble-bed reactor cores from startup to equilibrium. Since Fiscal Year 2024, improvements and enhancements have been implemented in Griffin, including simplifying the process to specify streamlines and developing the online cross-section generation capability. In the absence of validation data, code-to-code comparisons are conducted with kugelpy, showing good agreement for integral quantities like k-eff predictions and predictions for maximum power density. However, accuracy issues are noted for more detailed quantities like the spatial distribution of fission rate densities which will require further work to address. The second part of this report presents an improved System Analysis Module (SAM) core channel model where the effects of cross flow are considered during the pressurized loss of forced cooling transient, resulting in an improved agreement of the predicted pebble temperature with respect to the predictions from the SAM 2D porous media model. Additionally, the wall channeling effect due to variable porosity at the near wall region of the core is also investigated. Furthermore, to demonstrate Griffin’s online cross-section generation capability, a Multiphysics simulation is performed by coupling Griffin to the SAM core channel model. In the third part of the report, as a part of the Organisation for Economic Co-operation and Development/Nuclear Energy Agency (OECD/NEA) thermal-hydraulic code validation benchmark activity for a high-temperature gas-cooled reactor, the High Temperature Test Facility (HTTF) is investigated first using the NekRS computational fluid dynamics (CFD) code to study the flow mixing phenomenon in the lower plenum of the facility. Then, code-to-code and code-to-data comparisons are performed for Test PG27, which is a pressurized conduction cooldown (PCC) test, using five different codes by six organizations from five countries. The different simulations show good agreements in terms of the general trend but there are differences in some results such as the peak temperatures of different regions and heat removal rate.