Live experiments on melt pool heat transfer in the reactor pressure vessel lower head

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.The main objective of the LIVE program at Karlsruhe Institute of Technology (KIT) is to study the core melt phenomena both experimentally in large-scale 2D and 3D geometry and in supporting separate-effects tests in order to provide a reasonable estimate of the remaining uncertainty band under the aspect of safety assessment. Within the LIVE experimental program several tests have been performed with water and with non-eutectic melts (mixture of KNO3 and NaNO3) as simulant fluids to study the heat flux distribution in the conditions when the melt pool is covered by water from the top. The tests were performed in LIVE-3D and LIVE-2D facilities using different simulant materials and under different external cooling condition. The upward and downward heat transfer was compared between the 2D and 3D geometries. Although similar heat flux distribution through the vessel wall is observed for LIVE-3D and LIVE-2D tests, LIVE-2D test results have shown higher heat transfer from the top of the melt pool as compared to the LIVE-3D tests and to results from previous studies. Using water as simulant material resulted in a lower heat transfer both to the top of the pool and to the vessel wall. The outcomes of the LIVE top-cooling tests provide new insights for the evaluation of the established Nu-Ra correlations. The results of these experiments allow a direct comparison with findings obtained earlier in other experimental programs (SIMECO, ACOPO, BALI, etc.) and are used for the assessment of the correlations derived for the molten pool behavior. Besides the investigation of molten pool heat transfer behavior, melting process of debris in the reactor lower plenum after relocation of liquid melt in a large scale hemispherical geometry is also investigated in LIVE-3D facility using a noneutectic nitrate to simulate the debris bed material. Two experiments have been performed with different volume of the relocated liquid melt. The onset of melting, the form and the volume of the melt pool and the timing of important events during the melting process were identified.cf201

Test and simulation results of LIVE-L4 + LIVE-L5L. (KIT Scientific Reports ; 7593)

The objective of the LIVE program is to study the core melt phenomena during the late phase of core melt progression in the RPV both experimentally in large-scale 3D geometry and with CFD simulation. LIVE-L4 and LIVE-L5L experiments investigate the transient and steady state behaviors of the molten pool and the crust at the melt/vessel wall interface influenced by the several melt relocation numbers and different heat generation rate during external cooling. The melt pool behaviour and crust thickness in L4 test are calculated by CONV-code

Results of the LIVE-L3A Experiment. (KIT Scientific Reports ; 7542)

The sequence of a postulated core melt down accident in the reactor pressure vessel (RPV) of a pressurised water reactor (PWR) involves a large number of complex physical and chemical phenomena. The main objective of the LIVE program is to study the core melt phe-nomena during the late phase of core melt progression in the RPV both experimentally in large-scale 3D geometry in supporting separate-effects tests and analytically using CFD codes in order to provide a reasonable estimate of the remaining uncertainty band under the aspect of safety assessment. The main objective of the LIVE-L3A experiment was to investigate the behaviour of the mol-ten pool and the formation of the crust at the melt/vessel wall interface influenced by the melt relocation position and initial cooling conditions. The test conditions in the LIVE- L3A test were similar to the LIVE-L3 test except the initial cooling conditions. In both tests the melt was poured near to the vessel wall. In the LIVE-L3 test the vessel was initially cooled by air and then by water; in the LIVE-L3A test the vessel was cooled by water already at the start of the experiment. The information obtained in the test includes horizontal and vertical heat flux distribution through the RPV wall, crust growth velocity and dependence of the crust properties on the crust growth velocity and cooling conditions. Supporting post-test analysis contributes to the characterization of solidification processes of binary non-eutectic melts. The results of the LIVE-L3 and LIVE-L3A tests are compared in order to characterize the impact of transient cooling condition on the crust solidification characteristics and melt pool behaviour including interface temperature, time to reach thermal hydraulic steady-state and the steady-state heat flux distribution. The report summarizes the objectives of the LIVE program and presents the main results obtained in the LIVE-L3A test compared to the LIVE-L3 test

The LIVE-L1 and LIVE-L3 experiments on melt behaviour in RPV lower head

Die Experimente LIVE-L1 und LIVE-L3 zum Schmelzenverhalten im unteren Plenum des RDB Der Ablauf eines hypothetischen Kernschmelzunfalls in einem Reaktordruckbehälter (RDB) eines Druckwasserreaktors (DWR) beinhaltet eine große Anzahl komplexer physikalischer und chemischer Phänomene. Um das Verständnis über mögliche Ablaufszenarien von Kernschmelzunfällen bezüglich Kernzerstörung zu verbessern, wurde im September 2002 das LACOMERA Projekt am Forschungszentrum Karlsruhe gestartet. Das Ziel des Projektes war die Untersuchung von komplexen Prozessen während der Schmelzenseebildung und Verlagerung im RDB, Schmelzenausbreitung in die Reaktorgrube und Kernschmelze-Betonwechselwirkung und -Kühlung. Das LACOMERA Projekt mit einer Laufzeit von 4 Jahren war Bestandteil des 5. Rahmenprogramms der EU und eröffnete Forschungseinrichtungen der EU Mitgliedsländer und deren angegliederten Staaten den Zugang zu vier Großversuchsanlagen QUENCH, LIVE, DISCO und COMET am Forschungszentrum Karlsruhe. Innerhalb des LIVE Versuchsprogramms wurden zwei Versuche (LIVE-L1 und LIVE-L2) des LACOMERA Projekts durchgeführt. Das Experiment LIVE-L1 ist Bestandteil dieses Berichts und wurde in Kooperation mit der Technischen Universität Sofia, Bulgarien und dem Kernkraftwerk Kozloduy NPP, Bulgarien geplant und durchgeführt. Das Hauptziel des LIVE Programms ist es, das Verhalten der Kernschmelze während der späten Phase der Kernzerstörung und –Verlagerung im RDB sowohl experimentell in großem 3-dimensionalen Maßstab und in begleitenden Einzeleffektuntersuchungen als auch analytisch mit CFD Codes zu untersuchen. Dadurch soll eine bessere Einschätzung der Bandbreite der verbleibenden Unsicherheiten unter dem Aspekt der Sicherheitsbewertung ermöglicht werden. Die Experimente LIVE-L1 und LIVE-L3 untersuchen das Verhalten eines Schmelzensees und einer Schmelzenkruste mit Luftzirkulation an der äußeren Behälterwand des RDB mit nachfolgender Außenflutung des unteren Plenums. Die Anfangs- und Randbedingungen in beiden Versuchen waren bis auf die Eingussposition der Schmelze in den Versuchsbehälter fast identisch. In LIVE-L1 wurde die Schmelze zentral und in LIVE-L3 am Rand in den Versuchsbehälter eingegossen. Die aus den Experimenten gewonnenen Informationen beinhal-ten Wärmestromverteilungen durch die Wand des RDB in transienten und stationären Versuchsphasen, Krustenwachstumsgeschwindigkeit und die Abhängigkeit der Krustenbildung von der Wärmestromverteilung. Detaillierte Nachuntersuchungen tragen außerdem zur Charakterisierung von Erstarrungsprozessen von nicht-eutektischen Schmelzen bei. Die experimentellen Ergebnisse sollen weiterhin zur Entwicklung von mechanistischen Modellen verwendet werden, die das Schmelzenseeverhalten im Kern beschreiben sollen und dann in Systemcodes zur Analyse von schweren Störfällen wie z.B. ASTEC implementiert werden sollen. Der vorliegende Bericht fasst die Ziele des LIVE Versuchsprogramms zusammen und präsentiert die wichtigen Ergebnisse der Experimente LIVE-L1 und LIVE-L3

Live experiments on melt behavior in the reactor pressure vessel lower head

Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.Behavior of the corium pool in the lower head is still a critical issue in understanding of Pressurized Water Reactor (PWR) core meltdown accidents. One of the key parameter for assessing the vessel mechanical strength is the resulting heat flux at the pool-vessel interface. A number of studies [1]-[3] have already been performed to pursue the understanding of a severe accident with core melting, its course, major critical phases and timing and the influence of these processes on the accident progression. Uncertainties in modeling these phenomena and in the application to reactor scale will undoubtedly persist. These include e.g. formation and growth of the in-core melt pool, relocation of molten material after the failure of the surrounding crust, characteristics of corium arrival in residual water in the lower head, corium stratifications in the lower head after the debris re-melting [4]. These phenomena have a strong impact on a potential termination of a severe accident. The main objective of the LIVE program [5] at Karlsruhe Institute of Technology (KIT) is to study the core melt phenomena both experimentally in large-scale 3D geometry and in supporting separate-effects tests, and analytically using CFD codes in order to provide a reasonable estimate of the remaining uncertainty band under the aspect of safety assessment. Within the LIVE experimental program several tests have been performed with water and with non-eutectic melts (mixture of KNO3 and NaNO3) as simulant fluids. The results of these experiments, performed in nearly adiabatic and in isothermal conditions, allow a direct comparison with findings obtained earlier in other experimental programs (SIMECO, ACOPO, BALI, etc.) and will be used for the assessment of the correlations derived for the molten pool behavior. The information obtained from the LIVE experiments includes heat flux distribution through the reactor pressure vessel wall in transient and steady state conditions, crust growth velocity and dependence of the crust formation on the heat flux distribution through the vessel wall. Supporting posttest analysis contributes to characterization of solidification processes of binary non-eutectic melts. Complimentary to other international programs with real corium melts, the results of the LIVE activities provide data for a better understanding of incore corium pool behavior. The experimental results are being used for development of mechanistic models to describe the incore molten pool behavior and their implementation in the severe accident codes like ASTEC. The paper summarizes the objectives of the LIVE program and presents the main results obtained in the LIVE experiments up to now.mp201

A wake-active locomotion circuit depolarizes a sleep-active neuron to switch on sleep

Sleep-active neurons depolarize during sleep to suppress wakefulness circuits. Wake-active wake-promoting neurons in turn shut down sleep-active neurons, thus forming a bipartite flip-flop switch. However, how sleep is switched on is unclear because it is not known how wakefulness is translated into sleep-active neuron depolarization when the system is set to sleep. Using optogenetics in Caenorhabditis elegans, we solved the presynaptic circuit for depolarization of the sleep-active RIS neuron during developmentally regulated sleep, also known as lethargus. Surprisingly, we found that RIS activation requires neurons that have known roles in wakefulness and locomotion behavior. The RIM interneurons-which are active during and can induce reverse locomotion-play a complex role and can act as inhibitors of RIS when they are strongly depolarized and as activators of RIS when they are modestly depolarized. The PVC command interneurons, which are known to promote forward locomotion during wakefulness, act as major activators of RIS. The properties of these locomotion neurons are modulated during lethargus. The RIMs become less excitable. The PVCs become resistant to inhibition and have an increased capacity to activate RIS. Separate activation of neither the PVCs nor the RIMs appears to be sufficient for sleep induction; instead, our data suggest that they act in concert to activate RIS. Forward and reverse circuit activity is normally mutually exclusive. Our data suggest that RIS may be activated at the transition between forward and reverse locomotion states, perhaps when both forward (PVC) and reverse (including RIM) circuit activity overlap. While RIS is not strongly activated outside of lethargus, altered activity of the locomotion interneurons during lethargus favors strong RIS activation and thus sleep. The control of sleep-active neurons by locomotion circuits suggests that sleep control may have evolved from locomotion control. The flip-flop sleep switch in C. elegans thus requires an additional component, wake-active sleep-promoting neurons that translate wakefulness into the depolarization of a sleep-active neuron when the worm is sleepy. Wake-active sleep-promoting circuits may also be required for sleep state switching in other animals, including in mammals