Seong-Ho Hong

AN INVESTIGATION OF THE PARTICLE SIZE RESPONSES FOR VARIOUS FUEL-COOLANT INTERACTIONS IN THE TROI EXPERIMENTS

The TROI tests were analyzed in view of the particle size responses for various types of fuel-coolant interactions. This can provide an understanding about the relationship among the initial conditions, mixing, and explosion. First, several findings from the TROI experiments were considered. The results of the fuel-coolant interactions (FCIs) were dependent on the composition of the corium, the water depth, and the water area in the TROI experiments. Then, the difference between the explosive FCI and nonexplosive FCI was defined by comparing the final particle size. This analysis indicates that the explosive FCI resulted in a large amount of fine particles and a small amount of big particles. With this, the mixing size of the particles to participate in the steam explosion and the fine particle size produced from the steam explosion could be defined in the TROI test. And then, the parametric effects on the particle size were analyzed using the nonexplosive TROI tests. We note that the explosive test results cannot provide information on the mixing process. This analysis on the particle size response indicates that the explosive system includes large-sized particles to participate in the steam explosion, but the nonexplosive system includes less large-sized particles and more fine-sized particles. These particle size responses should be considered during a reactor safety analysis because the particle size will be an important parameter for explaining a steam explosion occurrence or steam explosion strength.

Results of the triggered steam explosions from the TROI experiment

Triggered steam explosion experiments have been carried out in the TROI facilities to investigate the energetics of the steam explosions. Two types of corium melt were used as a melt. One was eutectic corium at 70:30 wt% (UO2:ZrO2), and the other was corium at 80:20 wt%. The diameter of the water pool was 0.6 m, and the depth was varied from 0.67 to 1.3 m. An external trigger (PETN, 1.0 g) was applied just before contact of the melt and the bottom of the interaction vessel, which is believed to be the time of a possible spontaneous triggering. The external trigger led to triggered steam explosions in all the experiments. In the experiments with 70:30 corium, the maximum recorded dynamic pressure and the dynamic load were 17.0 MPa and 360 kN, respectively. Meanwhile, in the experiment with 80:20 corium, the maximum dynamic pressure and the dynamic load reached 7.7 MPa and 200 kN, respectively. The energetics obtained from the triggered steam explosion tests with 70:30 corium were greater than those from the triggered experiment with 80:20 corium. The strength of a triggered steam explosion was found to depend on the composition of the corium.

On the Fuel and Coolant Interaction Behavior of Partially Oxidized Corium

To simulate a fuel and coolant interaction phenomenon during a postulated severe accident in a nuclear reactor, a series of experiments were performed using a partially oxidized corium, which is a mixture of UO2, ZrO2, Zr, and stainless steel. The composition of the melt was chosen such that a separation of the oxidic liquid from the metallic liquid occurred due to the existence of a miscibility gap. A melting and solidifying experiment and two fuel and coolant interaction experiments to explore the possibility of an energetic steam explosion were performed in the TROI facility. The placement of a metal-rich layer consisting of U, Fe, and ZrO2 beneath the oxidic corium layer due to the existence of a miscibility gap was observed in the melting and solidifying experiment. An energetic steam explosion with a propagation of the dynamic pressure wave was observed in one test out of the two tests. The physical and chemical analyses were performed for the corium particles collected after the experiments. It is shown that U, Zr, and Fe formed a heterogeneous mixture and the morphology was in irregular shape with many pores at nonuniform sizes. In the case of nonenergetic interaction, where the melt temperature was lower than the energetic case, the mean particle size was bigger than that of the energetic case, and the melt-water interaction resulted in a substantial amount of hydrogen gas generation, while the amount of hydrogen gas generation was negligible in the case with an energetic steam explosion.

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.

Fuel-Coolant Interaction Test Results Under Different Cavity Conditions

Abstract Some advanced reactors adapt the in-vessel corium retention concept by cooing the outside wall of the reactor vessel in severe accidents. If a reactor vessel failure happens in this case, the molten corium in the reactor vessel is directly injected into the water in the reactor cavity without the process of a free fall. Experiments using ZrO2 and molten corium to simulate the conditions in which the reactor vessel is fully flooded were recently carried out at the Test for Real cOrium Interaction with water (TROI) experimental facility, and the results are compared with the data produced under conditions in which the reactor vessel is partially flooded. It was observed that the melt front velocity in the water under submerged reactor conditions is much faster than that under partially flooded reactor cavity conditions, and a large bubble was observed at the surface of the mixing zone under submerged reactor conditions. Accordingly, it is estimated that the breakup of the melt jet in the water during the fuel-coolant interaction (FCI) test under submerged reactor conditions would be different than that of the FCI test under partially flooded reactor cavity conditions.

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.

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.

The Triggerability and Explosion Potentials of Reactor Core Melt at Fuel Coolant Interactions

The Test for Real cOrium Interaction with water (TROI) experiments have been performed to reveal unsolved issues of a steam explosion using real core material at the Korea Atomic Energy Research Institute. 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 TROI tests were analyzed in view of a particle size response for various types of fuel coolant explosions. This can provide an understanding about the relationship between an initial condition, the mixing, and the explosion. The particle size distribution data from the TROI tests and a single-particle film boiling model were used for all these analyses. The difference between a quenched FCI and an explosive FCI was defined by comparing the final particle size. This analysis indicates that an explosive FCI resulted in a large amount of fine particles and in a small amount of large-sized particles. With this, the mixing size of the particles that participate in the steam explosion and the fine-particle size produced from a steam explosion can be defined in the TROI test. The particle size distributions of the quenched TROI tests were then considered. We note that the explosive test results cannot provide information on the mixing process. This analysis on the particle size indicates that a self-triggered system includes large-sized particles to participate in a steam explosion, but a non-self-triggered system includes smaller-sized particles and more fine-sized particles. Finally, the explosion potentials of the quenched TROI tests were compared to each other. Thus, the single-particle film boiling model based on the particle size distribution provides the explanation for the explosion behaviors of a variety of melts.