Seong-Wan Hong

CORIUM BEHAVIOR IN THE LOWER PLENUM OF THE REACTOR VESSEL UNDER IVR-ERVC CONDITION: TECHNICAL ISSUES

Corium behavior in the lower plenum of the reactor vessel during a severe accident is very important, as this affects a failure mechanism of the lower head vessel and a thermal load to the outer reactor vessel under the IVR-ERVC (In-Vessel corium Retention through External Reactor Vessel Cooling) condition. This paper discusses the state of the art and technical issues on corium behavior in the lower plenum, such as initial corium pool formation characteristics and its transient behavior, natural convection heat transfer in various geometries, natural convection heat transfer with a phase change of melting and solidification, and corium interaction with a lower head vessel including penetrations of the ICI (In-Core Instrumentation) nozzle are discussed. It is recommended that more detailed analysis and experiments are necessary to solve the uncertainties of corium behavior in the lower plenum of the reactor vessel.

Steam Explosion Experiments Using Nuclear Reactor Materials in the TROI Facilities

A series of TROI steam explosion experiments was performed using various prototypic melts. The melt was pure zirconia, eutectic corium (70: 30 weight percent of UO2: ZrO2) or iron-added eutectic corium. In this series, an experiment with pure zirconia, two experiments with eutectic corium, and three experiments with iron-added corium were carried out. A steam explosion was found to be somewhat related to the melt composition and an external triggering. As with most of the previous tests, zirconia melt led to a steam explosion again. It is quite certain that a zirconia melt will more than likely result in an energetic steam explosion. Meanwhile, eutectic corium led to an energetic steam explosion by applying an external trigger, but it had a weak steam spike without an external trigger. The explosivity of eutectic corium cannot be ignored, as an external trigger led to an energetic steam explosion. Iron-added corium did not lead to an energetic steam explosion. The reason for this is likely to be a relatively low melt temperature (superheat) when compared to zirconia melt or oxidic corium melt, resulting from the melting method used in the TROI experiments, an induction heating applied to a cold crucible. The iron-added corium at a low temperature was solidified easily so a steam explosion did not occur.

An Investigation on Size Distribution and Physical Characteristics of Debris in TROI FCI Tests

Abstract Steam explosions by the interaction of molten corium with water have been studied extensively because they may have the potential to impact the integrity of the containment. Since breakup and fragmentation processes during premixing are important mechanisms that influence steam explosion behavior, the particle size distribution characteristics on fuel-coolant interaction (FCI) have been investigated in the TROI (Test for Real cOrium Interaction with water) test facility. The data characteristics indicate that FCI characteristics depend upon the composition of the prototypic corium material, and the particle size of the debris is related to the intensity of the dynamic pressure produced by an explosion. The mass mean diameters of the debris produced by explosive compositions were less than that of the nonexplosive compositions. A mass mean diameter of 2mm was found to be a boundary size produced by a steam explosion of corium. The particle sizes of the molten corium involving a steam explosion were shown to be mainly 3 to 6mm depending on the material and composition, but the size distribution shifted to smaller sizes if a steam explosion occurred. Small corium droplets of less than ~3mm did not seem to contribute to a steam explosion owing to solidification at an early stage before the explosion, but large droplets contributed due to their liquid state. Zirconia, with the largest fusion heat, has almost always exploded, and the explosions have been energetic, while the eutectic composition (UO2/ZrO2 = 70/30 at weight percentage) frequently exploded. On the other hand, noneutectic compositions rarely exploded, even though the heat of the fusion was very similar to the eutectic composition that frequently exploded. The main reason why noneutectic corium compositions do not explode seemed to be that they undergo solidification by forming a “mushy zone” with a small freezing temperature range. To determine whether noneutectic corium melts cooled down through the mushy zone, particles of this composition were analyzed from the surface inward using a scanning electron microscope, an electron probe microanalyzer, and X-ray diffraction. However, all particles were found to have a homogeneous solid solution. The large particles showed the typical solidification shapes of a general molten material. The small particles generally had only a few small pores and small cracks. The morphologies of the large and small particles were found to be similar.

Detailed Evaluation of the Natural Circulation Mass Flow Rate of Water Propelled by Using an Air Injection

One-dimensional (1D) air-water two-phase natural circulation flow in the “thermohydraulic evaluation of reactor cooling mechanism by external self-induced flow—one-dimensional” (THERMES-1D) experiment has been verified and evaluated by using the RELAP5/MOD3 computer code. Experimental results on the 1D natural circulation mass flow rate of water propelled by using an air injection have been evaluated in detail. The RELAP5 results have shown that an increase in the air injection rate to 50% of the total heat flux leads to an increase in the water circulation mass flow rate. However, an increase in the air injection rate from 50 to 100% does not affect the water circulation mass flow rate, because of the inlet area condition. As the height increases in the air injection part, the void fraction increases. However, the void fraction in the upper part of the air injector maintains a constant value. An increase in the air injection mass flow rate leads to an increase in the local void fraction, but it has no influence on the local pressure. An increase in the coolant inlet area leads to an increase in the water circulation mass flow rate. However, the water outlet area does not have an influence on the water circulation mass flow rate. As the coolant outlet moves to a lower position, the water circulation mass flow rate decreases.

Effect of melt water interaction configuration on the process of steam explosion

ABSTRACT Steam explosion experiments are performed at various modes of melt water interaction configuration using prototypic corium melt. The tests are performed to simulate both melt water interaction in a partially flooded cavity and melt water interaction in a cavity with submerged reactor. The tests are performed using zirconia and corium melts. The behavior of melt jet fragmentation during the flight in the air and fragmentation and mixing of melt jet in water is investigated by a high-speed video visualization and by comparison of debris size distribution and morphology of debris. Strength of steam explosion is estimated by measuring dynamic pressure and dynamic force.

Detailed Evaluation of RCS Boundary Rupture during High-Pressure Severe Accident Sequences

A depressurization possibility of the reactor coolant system (RCS) before a reactor vessel rupture during a high-pressure severe accident sequence has been evaluated for the consideration of direct containment heating (DCH) and containment bypass. A total loss of feed water (TLOFW) and a station blackout (SBO) of the advanced power reactor 1400 (APR1400) has been evaluated from an initiating event to a creep rupture of the RCS boundary by using the SCDAP/RELAP5 computer code. In addition, intentional depressurization of the RCS using power-operated safety relief valves (POSRVs) has been evaluated. The SCDAPRELAP5 results have shown that the pressurizer surge line broke before the reactor vessel rupture failure, but a containment bypass did not occur because steam generator U tubes did not break. The intentional depressurization of the RCS using POSRV was effective for the DCH prevention at a reactor vessel rupture.

A Mechanism for the Suppression of a Steam Explosion in Real Core Melt and Water Interactions

TROI experiments1) have been performed to reveal unsolved issues of a steam explosion by using real core material at the Korea Atomic Energy Research Institute (KAERI). One of the findings from the TROI experiments is that the results of a fuel coolant interaction (FCI) are strongly dependent on the composition of corium, which is composed of UO2, ZrO2, Zr, and steel. The TEXAS-V simulation for the TROI experiments indicated that a relatively low void fraction seems to have resulted in a strong steam explosion, and the low-voided mixture seems to have been induced by large-sized particles. The particle sizes in the nonexplosive TROI tests were analyzed because the explosive tests do not represent the particles during mixing. The analysis of particle sizes indicated that the debris size reflected the material difference, and the order of the particle size for each melt material was the same as that in the TEXAS-V simulation. The TEXAS-V calculation for the alumina/water system indicated that thermal conductivity is also related to the material effect on the FCI result. A heat loss evaluation using a single-sphere, filmboiling model showed that reliable values for thermal conductivity and particle size provide a reasonable estimation for the FCI result. The steam explosion in corium/water interactions is suppressed by a smaller particle size and an induced larger heat loss during mixing.

Suppression Features of a Vapor Explosion with Prototypic Reactor Materials

The suppression of a vapor explosion is reviewed from a void fraction point of view from previous research results and the results of an experiment and analysis for TROI using a prototypic reactor material. In a tin-water system, a high fraction of air which played the role of steam reduced the peak pressure of a steam explosion. According to the sensitivity analysis that was carried out with an increase in vapor volume fraction, an energetic vapor explosion hardly took place in a mixture with a high void fraction. Under higher vapor fraction conditions (αv > 0.3), the vapor explosion was very weak. The prototypic corium showed a relatively high void fraction compared to ZrO2, which is known as an explosive material, because this corium system generates many smaller particles compared to the ZrO2 system. Also this corium system showed a relatively low explosivity compared to the ZrO2 system because the high void fraction of the corium system prevents contact between water and hot melt drops in the triggering stage. When considering the experimental results for the role of air instead of steam, an air supply system to provide a high volume fraction during a premixing process can radically prevent and/or mitigate a steam explosion.

Analysis of Steam Explosion under Conditions of Partially Flooded Cavity and Submerged Reactor Vessel

A steam explosion in a reactor cavity makes a mechanical load of the pressure pulse, which can result in a failure of the containment isolation. To prove the integrity of the containment during the ex-vessel steam explosion, the effects of water conditions on a steam explosion have to be identified, and the impulse of a steam explosion has to be exactly assessed. In this study, the analyses for steam explosions were performed for the conditions of a partially flooded cavity and a submerged-vessel in a pressurized water reactor. The entry velocity of a corium jet for the scale of the test facility was varied to simulate the two plant conditions. The TEXAS-V code was used for simulating the phases of premixing and explosion, and the load of a steam explosion was estimated based on the pressure variation. The impulse of a steam explosion under the condition of a corium jet falling into water without a free-fall height is bigger than that under a free-fall height. The fragmented mass of corium in an explosion phase and the distribution of steam fraction are the main parameters for the total load of the steam explosion. This study is expected to contribute to analyses of a steam explosion for a severe accident management strategy.

Flame-quenching model of the quenching mesh for H2–air mixtures

A deflagration to detonation transition (DDT) occurrence is one of the most important issues concerning safety during severe accidents in nuclear power plants because it can damage the integrity of the containment. It is possible to arrest the acceleration of a flame which can cause DDT by installing quenching meshes between the compartments. To evaluate the applicability of a quenching mesh to nuclear power plants, it requires a means to evaluate a flame arrest of a quenching mesh under a given combustion condition. The flame-quenching models developed by previous researchers were derived to fit the experimental geometry and to consider various thermal boundary conditions from a flame to the mesh wall. Flame-quenching tests were carried out at the 10% hydrogen concentration in a dry air by changing atmospheric pressure to 2.2 bar as the initial pressure. The quenching criterion of a quenching mesh with a 0.3 mm gap distance for hydrogen–air mixtures is established by using the experimental data. The flame-quenching models are also evaluated by using the experimental data. A flame-quenching model that can be used to evaluate a flame arrest for various hydrogen–air mixtures in nuclear power plants is proposed.