Spontaneous Steam Explosions Research Articles

Sensitivity analysis of simulated premixed layer and vapour explosion in stratified configuration

Experiments of fuel–coolant interaction in stratified geometry at the PULiMS and SES (KTH, Sweden) test facilities resulted in spontaneous steam explosions. Prior to the explosion, a premixed layer of ejected melt drops in the water layer was observed in the experiments. Based on the experimental and analytical knowledge, we have recently developed a model for premixed layer formation in the Fuel-Coolant Interaction code MC3D and applied it to estimate steam explosion energetics.In the paper, a sensitivity study of this model is performed on the three main uncertain parameters of the premixed layer formation model, which define the melt fragmentation rate, the size of the ejected melt drops and the ejected melt drop velocity. The analysis is performed against the SES S1 and the PULiMS E6 experimental results. The chosen tests were selected as in both of them the same material was used, the geometry was similar, and both of them resulted in a spontaneous steam explosion. The effects can be observed in all the performed analyses and they are consistent in simulations of both experiments, affecting the premixed layer height as well as the explosion strength and duration. Some uncertainty of the experimental results is assessed, with the main limitation related to the visual observations. The combination of both analyses provides us with the assessment of future work necessity and prioritization.The presented sensitivity analysis of the premixed layer formation model enables a more reliable assessment of the stratified vapour explosion risk and its uncertainty in nuclear power plants and other industries.

An experimental study on droplet quench and steam explosion in boric acid solutions

Boric acid (H3BO3) is widely adopted as an additive in the coolant of light water reactors for reactivity control, but its effect on fuel coolant interactions (FCI) during severe accidents (especially on steam explosion) was rarely investigated. To examine the effect of the boric acid additive in coolant on steam explosion, a series of molten droplet-coolant interaction tests using H3BO3 solutions (with concentration ranging from 0–3.2% by weight) is carried out in the present study. The characteristics of melt-coolant interactions are the occurrence probability of typical phenomena (no fragmentation, minor fragmentation, or spontaneous steam explosion), lateral deformation ratio, quench depth, pressure impulse and debris particle size distribution. The statistical data of such characteristics are obtained through repeating 20 runs of the same test category. The experimental results show that the H3BO3 addition in coolant has various impacts on the above-mentioned characteristics of melt-coolant interactions, depending on the H3BO3 concentration. In particular, the probability of steam explosion sightly decreases as the H3BO3 concentration increases from zero to 1.2 wt.%, but significantly increases as the H3BO3 concentration further increases to 3.2 wt.% trough 2.2 wt.%. Namely, the inhibiting effect of boric acid on steam explosion is diminishing with increasing H3BO3 concentration beyond 1.2 wt.%. It is also found that both melt and coolant temperatures are crucial parameters impacting the likelihood and energetics of steam explosion.

An experimental study on the effect of chemical additives in coolant on steam explosion

In assessment of severe accident risk in light water reactors (LWRs), steam explosion is a nonnegligible phenomenon following a relocation of core melt (corium) into coolant, and thus various research efforts have been paid to steam explosion. There had been numerous studies showing that the occurrence of steam explosions is influenced by several factors such as melt and coolant temperatures, melt materials, non-condensable gasses, etc. However, most of the existing experiments used deionized (DI) water or tap water as coolant, with little consideration of the effect of chemicals (e.g. boric acid, sodium hydroxide, sodium phosphate) commonly applied in reactor coolant. To examine the effect of the chemical additives in coolant on steam explosion, the present study performs a series of molten Tin droplet-coolant interaction tests using DI water and different chemical solutions, including H3BO3 solutions, NaOH + H3BO3 neutral solutions, and Na3PO4 + H3BO3 neutral solutions. The experimental results show that adding NaOH and Na3PO4 in boric acid solution significantly affects the occurrence probability of spontaneous steam explosion, because of the presence of PO43− and H+ ions. When different solutions have equivalent concentrations of H3BO3, the peak pressure values of the spontaneous steam explosion of Sn droplets are similar among various solutions. Compared with those in DI water, steam explosion in the chemical solutions occurs predominantly within a narrow range of depth from 28 mm to 40 mm and produces a much higher peak pressure. This implies that more energetic steam explosions may occur in the chemical solutions.

Modelling of premixed layer formation in stratified fuel–coolant configuration

In the recent fuel–coolant interaction experiments performed in the stratified configuration at the SES and PULiMS facilities (KTH, Sweden), a premixed layer of ejected melt drops in water was clearly visible and was followed by strong spontaneous steam explosions. The purpose of our research was to improve the knowledge, understanding and modelling of the fuel–coolant interaction phenomena in the stratified configuration. In the paper, models for the premixed layer formation, based on the visual observations and some available mechanisms from the literature, are presented. Due to the uncertainties and the lack of detailed information, two different approaches were followed in the modelling. Both models and experimental observations were discussed and joined into the third model, which describes the considered phenomena the best. The developed model was implemented into the MC3D code (IRSN, France) and validated against the experimental results. The presented analyses demonstrate the model capability to describe the premixed layer formation in a qualitative agreement with the available experimental data. The indirect comparison of the simulated premixed layer with the SES S1 and PULiMS E6 experiments via the strength of vapour explosion shows underestimation in simulations’ explosion strength. This indicates possible contributions to the vapour explosion from the classical melt jet breakup and additional mixing during the explosion itself.

Study on thermal fragmentation characteristics of a superheated alumina droplet

In the frame of the European Commission FP7 SAFEST project, IRSN proposed to experimentally investigate the steam explosion triggering mechanisms of a superheated alumina droplet falling into water, through a set of experiments in the Micro Interactions in Steam Explosion Energetics facility (MISTEE) at KTH. Since thermal fragmentation is considered to be a likely process for the triggering of Steam Explosions in the KROTOS tests (performed at CEA) with alumina, the ability of a single droplet of such material to undergo thermally induced fine fragmentation is studied on the MISTEE facility with a close-up visualization.A series of experiments were conducted, where droplets of molten alumina were discharged into a water pool and potentially exposed to a small pressure wave. The intense interactions were recorded with a high-speed camera along with the pressure in the droplet vicinity.The ability of alumina to undergo thermal fragmentation is expected to be firstly contingent on the stability of the vapour film enshrouding the melt droplet. The water and melt temperatures may then play a crucial role on the vapour film stability, and therefore on the observation of a steam explosion. Indeed, under high to moderate water sub-cooling conditions, experimental observations indicate that fine fragmentation of the melt can occur when the droplet is exposed to even a weak pressure wave, in the range of 0.15 MPa. In contrast, melt fine fragmentation is suppressed at low water sub-cooling conditions (less than 30 °C), where the formation of a thick vapour film (and large wake) is observed, and which is probably too stable to be destabilized by the weak pressure wave.The effect of the melt temperature on thermal fragmentation is also assessed. This parameter influences the solidification of the droplet and the strength of the explosion as it determines the available heat energy. In the present conditions, fine fragmentation of melt occurred even at quite low melt superheat (≈60 °C). For a high melt superheat (above 200 °C) a very energetic spontaneous steam explosion was observed. A physical analysis on the debris particles acquired indicates a mass median diameter of ≈100 µm, comparable to the one observed in the KROTOS alumina experiments.The MISTEE experimental results are finally used to assess the heat and mass transfer modelling of the coolant during the fragmentation process in the FCI code MC3D.

Ex-vessel fuel-coolant interaction experiment in the DISCO facility in the LACOMECO project

In the frame of the LACOMECO (large scale experiments on core degradation, melt retention and containment behavior) project of the 7th European Framework Program, a test in the DISCO (dispersion of corium) facility was performed in order to analyze the phenomena which occur during an ex-vessel fuel–coolant interaction (FCI). The test is focused on the premixing phase of the FCI with no trigger used for explosion phase. The objectives of the test were to provide data concerning the dispersion of water and melt out of the pit, characterization of the debris and pressurization of the reactor compartments for scenarios, where the melt is ejected from the reactor pressure vessel (RPV) under pressure. The experiment was performed for a reactor pit geometry close to a French 900 MWe reactor configuration at a scale of 1:10. The corium melt was simulated by a melt of iron–alumina with a temperature of 2400 K. A containment pressure increase of 0.04 MPa was measured, the total pressure reached about 0.24 MPa. No spontaneous steam explosion was observed. About 16% of the initial melt (11.62 kg) remained in the RPV vessel, 60% remained in the cavity mainly as a compact crust. The fraction of the melt transported out of the pit was about 24%.

Thermal-hydraulic aspects of FCIs in TROI corium/water interaction tests

This paper aims to clarify the thermal-hydraulic effect on the results of TROI corium/water interaction tests. The comparisons were done within the same material melt of 70:30 corium to exclude the material effect. Thermal-hydraulic aspects of corium/water tests are similar to past melt/water tests. The spontaneous steam explosion is a stochastic phenomenon. The depth has a negative effect on a spontaneous steam explosion, while the water pool area and water subcooling have a positive effect on a spontaneous steam explosion. The external triggering event always creates a steam explosion, eliminating the negative thermal-hydraulic effects to prevent such an explosion. The deep and narrow water pool effect, however, works on corium-water tests even with the external triggering event, and deters a triggered steam explosion.

Triggered Steam Explosions with Corium Melts of Various Compositions in a Narrow Interaction Vessel in the TROI Facility

Three triggered steam explosion experiments using corium melts of various compositions were performed in the TROI facility. The interaction vessel was 0.3 m in diameter. The melt compositions were 70:30 (UO2:ZrO2) corium, pure zirconia, and partially oxidized corium (UO2:ZrO2:Zr:SS = 53.91:23.09:12.00:11.00 in weight percent). The test with 70:30 corium was performed with a 0.95-m-deep water pool under an elevated pressure of 0.205 MPa, while the others were performed with a 1.3-m-deep water pool under atmospheric pressure. The water temperature was maintained at room temperature. The melt mass released to the water pool was ˜10 kg for each test. The test with 70:30 corium resulted in a triggered steam explosion, considering the long duration of the dynamic pressure and the large amount of fine debris. The dynamic pressure trace from the steam explosion seemed to be superimposed on that from the external trigger. The test with pure zirconia led to multiple spontaneous steam explosions before any external triggering. The zirconia melt confirmed its explosivity. The spontaneous steam explosion with pure zirconia seems not to be affected by the water depth and diameter of the interaction vessel. The test with partially oxidized corium also resulted in a spontaneous steam explosion before an external triggering. These results are different from the previous TROI tests with 80:20 corium in a narrow interaction vessel of 0.3-m diameter, in which no spontaneous steam explosions occurred. The geometry of the interaction vessel used in these tests does not seem to influence the occurrence of a steam explosion, but the corium composition does affect the triggerability of it.

Analysis of TROI-13 Steam Explosion Experiment

The prediction of steam explosion inducing loads in nuclear power plants must be based on results of experimental research programmes and on simulations using validated fuel-coolant interaction codes. In this work, the TROI-13 steam explosion experiment was analysed with the fuel-coolant interaction MC3D computer code. The TROI-13 experiment is one of several experiments performed in the TROI research program and resulted in a spontaneous steam explosion using corium melt. First, the TROI-13 premixing simulations were performed to determine the initial conditions for the steam explosion simulations and to evaluate the melt droplets hydrodynamic fragmentation model. Next, a number of steam explosion simulations were performed, varying the steam explosion triggering position and the melt droplets mass participating in the steam explosion. The simulation results revealed that there is an important influence of the participating melt droplets mass on the calculated pressure loads, whereas the influence of the steam explosion triggering position on the steam explosion development was less expressive.

A physical and chemical analysis of fast quenched particles of UO 2 and ZrO 2 mixture

An interaction between molten fuel of a nuclear reactor, which is called corium and mainly consisted of UO 2 and ZrO 2, and sub-cooled water may result in a steam explosion. It is one of the outstanding reactor safety issues. To investigate the fundamental mechanism behind the recent experimental observation that the composition of the material highly affected the strength of the steam explosion, a physical and chemical analysis for the fast quenched particles of UO 2 and ZrO 2 mixture at different compositions was performed. Six cases were selected for the study, in which the melt composition was changed, while other initial and boundary conditions of the molten fuel and water interaction tests were maintained the same. It was observed that the cases at eutectic composition resulted in a spontaneous steam explosion, while the cases at non-eutectic composition did not result in a spontaneous steam explosion. Electron probe microanalysis (EPMA) was performed for fast quenched particles along a cross-section. Results demonstrated that the UO 2 and ZrO 2 mixtures formed a solid solution of U 1− x Zr x O 2. The mechanism for the hydrogen generation during the molten material and water interaction was examined by thermogravity analysis (TGA), X-ray diffraction (XRD) and hydrogen reduction analysis. It was demonstrated that the hydrogen generation was not directly related to the oxidation of UO 2. Morphologies observed by scanning electron microscopy (SEM) indicated that the particles from the eutectic mixture had many holes, while the particles at non-eutectic mixture did not. The existence of mush phase for the non-eutectic mixture is suggested to be the reason for the non-explosive nature.

On some salient unresolved issues in severe accidents for advanced light water reactors

Some of the outstanding issues of the severe accident are evaluated from the synthesis of the results from the research programs of the simulation of naturally arrested thermal attack in the vessel (SONATA), test for real corium interaction with water (TROI), and the test for the performance of a quenching screen for a hydrogen burn. The strategy of the in vessel retention (IVR) by an external reactor vessel cooling (ERVC) is evaluated for the Korea Standard Nuclear Power Plant (KSNP) and Advanced Power Reactor-1400 (APR-1400). The favorable and adverse effects of the proposed strategy and the uncertainties of the relevant phenomena including the gap cooling and heat removal capability of the EVRC are discussed. It is shown that the thermal margin for the failure of the reactor vessel is challenged for these reactors. The observation of the energetic spontaneous steam explosions in the TROI suggests that proper measures should be taken to avoid and/or lessen the risk of an in-vessel or ex-vessel steam explosion, when we employ an ex-vessel cooling strategy and/or an in vessel injection strategy. To address the issue that multiple burns induced by the hydrogen igniter can pose a threat to the safety equipment, the use of a quenching screen is suggested. The results of the laboratory scale experiments for the performance of the quenching screen indicated the feasibility of using a quenching screen to protect the essential equipment by stopping the flame propagation.

The Effect of the UO2/ZrO2 Composition on Fuel/Coolant Interaction

A series of experiments on fuel/coolant interaction (FCI) was performed in the TROI facility, where the composition of the mixture was varied. The compositions of the UO2 and ZrO2 mixture in weight percent were 50:50, 70:30, 80:20, and pure ZrO2. The responses of the system including the temperature of the pool of water in the test vessel, pressure and temperature of the containment vessel, and dynamic pressures and force were measured. In addition, high-speed movies were taken through the windows. The tests using corium with a 70:30 composition and pure zirconia resulted in a spontaneous energetic steam explosion, while the tests with other compositions did not lead to an energetic FCI. The debris size distribution and pressure and temperature responses clearly indicated the cases with an energetic explosion and the cases without an explosion. The high-speed movie taken during the FCI through the visible window clearly disclosed the outstanding phases of the FCI, which were the melt entry phase, the triggering phase, and the continued melt jet and expansion of the mixing zone phase.

Insights from the Recent Steam Explosion Experiments in TROI

The paper discusses the results of steam explosion experiments of TROI-13, TROI-14, and TROI-15, which were performed under the research program named “Test for Real cOrium Interaction with water (TROI).” TROI-13 and TROI-14 used corium, which is a mixture of UO2 and ZrO2 at a 70:30wt%, while TROI-15 used ZrO2. These three cases resulted in spontaneous steam explosions. It is an important observation from the aspect of the explosivity of prototypic material, as it was not observed in the previous experiments. Various aspects of the test results including cold crucible melting, hydrogen generation, melt temperature measurement, debris morphology, size distribution of the debris, dynamic pressure, dynamic force, shape of the melt jet, location of the trigger, response of the vessel pressure, and the response of the water pool temperature are discussed. The potential contributors to the explosivity of corium are suggested.

Fuel coolant interaction experiments in TROI using a UO 2/ZrO 2 mixture

This paper discusses the results of steam explosion experiments using reactor material carried out under “Test for Real cOrium Interaction with water (TROI)” program. About 4–9 kg of corium melt jet is delivered into a sub-cooled water pool at atmospheric pressure. Spontaneous steam explosions are observed in four tests among six tests. The dynamic pressure, dynamic load, and morphology of debris clearly indicate the cases with steam explosion. The initial conditions and results of the experiments are discussed.

Insights from the Recent Steam Explosion Experiments in TROI

The paper discusses the results of steam explosion experiments of TROI-13, TROI-14, and TROI-15, which were performed under the research program named “Test for Real cOrium Interaction with water (TROI).” TROI-13 and TROI-14 used corium, which is a mixture of UO2 and ZrO2 at a 70:30wt%, while TROI-15 used ZrO2. These three cases resulted in spontaneous steam explosions. It is an important observation from the aspect of the explosivity of prototypic material, as it was not observed in the previous experiments. Various aspects of the test results including cold crucible melting, hydrogen generation, melt temperature measurement, debris morphology, size distribution of the debris, dynamic pressure, dynamic force, shape of the melt jet, location of the trigger, response of the vessel pressure, and the response of the water pool temperature are discussed. The potential contributors to the explosivity of corium are suggested.

Corium melt quenching tests at low pressure and subcooled water in FARO

The first two corium melt quenching tests in subcooled water performed in the FAT FCI vessel of the FARO facility are described. The tests, referenced as L-29 and L-31, involved 39 and 92 kg of 80 wt% UO 2+20 wt% ZrO 2 melt, respectively. The molten material was poured into a 1.5-m depth pool of water at 0.2 MPa and ambient temperature. No spontaneous steam explosion occurred. The quenching process was different between the two tests, producing a conglomerate debris bed in L-29 and a particulate debris bed in L-31, with higher thermal load in L-29 than in L-31. The difference in the melt initial temperature is believed to be the cause of this dissimilarity.

Steam Explosion Experiments using ZrO$_2$

Korea Atomic Energy Research Institute (KAERI) launched an intermediate scale steam explosion experiment named Test for Real Corium Interaction with water (TROI) using reactor material to investigate whether the molten reactor material would lead to energetic steam explosion when interacted wish cold water at low pressure. The melt-water interaction experiment is performed in a pressure vessel with the multi-dimensional fuel and water pool geometry. The novel concept of cold crucible technology, where powder of the reactor material in a water-cooled cafe is heated by high frequency induction, is firstly implemented for the generation of molten fuel. In this paper, the lest facility and cold crucible technology are introduced and the results or the first series of tests were discussed. The 5 kg of molten ZrOjet was poured into the 67cm deep water pool at 30 ∼ 95 . Either spontaneous steam explosions or quenching was observed. The morphology of debris and pressure wave profiles clearly indicate the differences between the two cases.

Results of recent KROTOS FCI tests: alumina versus corium melts

Recent results from KROTOS fuel-coolant interaction experiments are discussed. Five tests with alumina were performed under highly subcooled conditions, all of these tests resulted in spontaneous steam explosions. Additionally, four tests were performed at low subcooling to confirm, on one hand, the suppression of spontaneous steam explosions under such conditions and, on the other hand, that such a system is still triggerable using an external initiator. The other test parameters in these alumina tests included the melt superheat and the initial pressure. All the tests in the investigated superheat range (150–750 K) produced a steam explosion and no evidence of the explosion suppression by the elevated initial pressure (in the limited range of 0.1–0.375 MPa) was observed in the alumina tests. The corium test series include a test with 3 kg of melt under both subcooled and near saturated conditions at ambient pressure. Two additional tests were performed with subcooled water; one test was performed at an elevated pressure of 0.2 MPa with 2.4 kg of melt and another test with 5.1 kg of melt at ambient pressure. None of these tests with corium produced a propagating energetic steam explosion. However, propagating low energy (about twice the energy of the trigger pulse) events were observed. All corium tests produced significantly higher water level swells during the mixing phase than the corresponding alumina tests. Present experimental evidence suggests that the water depletion in the mixing zone suppresses energetic steam explosions with corium melts at ambient pressure and in the present pour geometry. Processes that could produce such a difference in void generation are discussed.

Steam explosions of single drops of pure and alloyed molten aluminum

Studies of steam explosion phenomena have been performed related to the hypothetical meltdown of the core and other components of aluminum alloy-fueled production reactors. Our objectives were to characterise the triggers, if any, required to initiate these explosions and to determine the energetics and chemical processes associated with these events. Three basic studies have been carried out with 1–10 g single drops of molten aluminum or aluminum-based alloys: untriggered experiments in which drops of melt were released into water; triggered experiments in which thermal-type steam explosions occurred; and one triggered experiment in which an ignition-type steam explosion occurred. In untriggered experiments, spontaneous steam explosions never occurred during the free fall through water of single drops of pure A1 or of the alloys studied here. Moreover, spontaneous explosions never occurred upon or during contact of the globules with several underwater surfaces. When Li was present in the alloy, H2 was generated as a stream of bubbles as the globules fell through the water, and also as they froze on the bottom surface of the chamber. The triggered experiments were performed with pure A1 and the 6061 alloy. Bare bridgewire discharges and those focused with cylindrical reflectors produced a small first bubble that collapsed and was followed by a larger second bubble. When the bridgewire was discharged at one focus of an ellipsoidal reflector, a melt drop at the other focus triggered only very mildly in spite of a 30-fold increase in peak pressure above that of the bridgewire discharge without the reflector. Experiments were also performed with globules of high purity A1 in which the melt release temperature was progressively increased. Moderate thermal-type explosions were produced over the temperature range 1273–1673 K. At about 1773 K, however, one experiment produced a brilliant flash of light and bubble growth about an order of magnitude faster than normal; it destroyed the chamber. The exceptional vigor of this latter interaction was attributed to ignition of the metallic A1. In both the triggered steam explosions with drops of high purity A1 and untriggered experiments with drops of A1Li, only a few tens of cm3 of hydrogen per gram of metal were generated; the extent of metal-water reaction was only a few percent at most. The information obtained from these studies with triggered and untriggered drops of molten aluminum-based alloys can be used directly in certain safety studies where hypothetical core or component meltdown occurs in a production reactor. Moreover, this information is consistent with recent advances in the understanding of single-drop steam explosion triggering and concepts of the development of larger scale steam explosions.

Phenomenological studies on melt-coolant interactions in the ALPHA program

Two series of experiments to investigate melt-coolant interactions have been performed as part of the ALPHA program at JAERI. In the melt drop steam explosion experiments, melt simulating a molten core was dropped into a pool of water. Volume fractions of the melt, water and steam in the mixing region prior to the occurrence of spontaneous steam explosions were quantified. Other characteristics of melt-coolant interactions were evaluated for settling velocity of the melt in water, propagation and expansion velocities, energy conversion ratio and debris size distribution. It was found that the probability of the occurrence of spontaneous steam explosions could be reduced by using a melt dispersion device. Measurement of void fraction in the mixing region clearly showed that the melt dispersion device enhanced steam generation. However, one experiment indicated that the use of the dispersion device could possibly result in a more energetic steam explosion. It was found that the mixing of non-condensable gas in the steam phase of the mixing region during melt dispersion played an important role for the suppression of the spontaneous steam explosion. Knowledge of the parametric effects of melt mass, ambient pressure and water temperature was extended. In the melt coolability experiments, water was poured onto the melt to investigate melt-coolant interactions in a stratified geometry where water overlies on a melt layer. Melt eruptions which could induce an explosive interaction were observed when the subcooled water was poured through a pipe nozzle. The eruption was not observed when the water was near the saturation temperature or supplied through a spray nozzle. The explosive interaction in the stratified geometry was found to be much smaller in magnitude than the steam explosion in the melt drop configuration.