1.         NAME AND TITLE

STEX II: International Steam Explosion Experimental Data Base.


European Commission-Joint Research Centre, Institute for Systems, Informatics and Safety, Ispra, Italy:

Korea Atomic Energy Research Institute, Yusong-Gu, Daejon, Republic of Korea

Argonne National Laboratory, Argonne, IL, USA

University of Wisconsin-Madison, Madison, WI, USA

Sandia National Laboratories, Albuquerque, NM, USA

United Kingdom Atomic Energy Authority, Atomic Energy Establishment, Winfrith, Dorchester, Dorset, United Kingdom

Severe Accident Research Laboratory, JAEA, Tokai-mura, Ibaraki-ken 319-1195, Japan

Forschungszentrum Karlsruhe, Institut fur Neutronenphysik und Reaktortechnik, Karlsruhe, Germany

Released through OECD Nuclear Energy Agency Data Bank, Issy-les-Moulineaux, France.


None Listed.


STEX Database is a compilation of the experimental work conducted to investigate the phenomenon of “STeam Explosion,” an extensively studied problem in the area of nuclear safety. Steam explosion (also known as vapor explosion) is a physical phenomenon in which the internal energy of a hot liquid is rapidly transferred to a colder and more volatile liquid, which as a result vaporizes at high pressure and expands against the inertial constraint of the surrounding structure as well as the mixture. In the context of nuclear safety, the hot liquid is the molten ‘fuel’ and the colder more volatile liquid is the ‘coolant.’ A steam explosion is therefore a class of fuel-coolant interactions in which the timescale for heat transfer between the liquids is smaller than the timescale for pressure wave propagation and expansion in a local region of the fuel-coolant mixture.

Steam explosion experiments are numerous and can be categorized in different ways depending on the scale or the conditions of the experiment:

1- In-pile vs. out-of-pile experiments,

An in-pile experiment is one that is conducted inside a nuclear reactor core, where the fresh and irradiated fuels are used, whereas an out-of-pile experiment is one that is conducted in an experimental chamber outside the reactor. In an in-pile experiment, fuel-coolant interactions in addition to a number of other phenomena may be investigated, while out-of-pile experiments focus only on fuel-coolant interactions and hence allow for better measurement of initial conditions as well as better interpretation of the test results.

2- Small, intermediate or large scale experiments,

An experiment is classified as small scale when the amount of fuel (or coolant) is (either relatively or absolutely) very small when compared to the mass of the other liquid. While small scale experiments allow for a better understanding of the mixing, and fragmentation phenomena involved in fuel-coolant interactions, they fail to address the phenomenon of steam explosion as a safety issue, hence the need for large scale experiments.

3- Pouring, injection, or stratified contact mode,

Because direct contact between the fuel and coolant is essential for a steam explosion, experiments have employed various modes to introduce the molten fuel to the coolant. In the pouring mode, the fuel is poured into the coolant and as it falls, it fragments and mixes with the latter. In the injection mode, one of the liquids (usually the coolant) is injected into the other via a needle. In the stratified mode, a layer of one of the liquid lies on top of the other, i.e. both liquids are separated from one another due to density difference.

Currently, the STEX database is exclusively focused on out-of-pile, intermediate to large scale experiments in which the pouring mode has been employed to cause direct contact between the molten fuel and the coolant. Due to the large number of experiments that fit into this category, STEX focuses on experiments performed at six facilities: the FARO facility, the KROTOS facility, the TROI facility, the WFCI facility, the ZREX facility, and the FITS facility.

Phase II of the STEX database includes experiments from EXO-FITS, SNL Efficiency Scaling Steam Explosion Tests, ANL R-22 Vapor Explosion Tests, UKAEA Vapor Explosion Tests from Molten Fuel Test Facility (MFTF) & MIXA facility, QUEOS and ALPHA facilities.



The FARO quenching experiments aim at studying the characteristics of melt jet breakup, energy release, debris structure, and bottom plate thermal load. In addition, the importance of zirconium oxidation is also investigated by varying the initial melt composition.


1- The effect of melt physical and chemical characteristics (alumina vs. corium) on the energetics of the melt/water interaction has been explored.

2- Parameters that may cause suppression of a spontaneous steam explosion (eg. Degree of subcooling, environment pressurization) have also been studied.


The main objective of the TROI experiments is to determine whether the corium would lead to energetic steam explosion as a result of interaction with cold water at low pressure in a multi-dimensional test section. Based on these experiments, it is concluded that corium with a composition of 80% UO2 : 20% ZrO2 is less likely to trigger a spontaneous explosion compared to corium with the eutectic composition (70% UO2 : 30% ZrO2).


In the WFCI experiments the effect of the external trigger, axial constraint and temperature effects of both fuel and coolant on the energetics of the interaction have been investigated. In addition, the effect of coolant viscosity on the suppression of the steam explosion was studied by adding polymer additives to the coolant.


The effect of the melt zirconium content on the energetics of the explosion.


The main purpose of the FITS tests was to estimate the explosion conversion ratio as a function of ambient pressure, fuel composition, coolant-to-fuel mass ratio, coolant subcooling, and other initial conditions in an enclosed, fully instrumented chamber.

It was observed that only 1 ~ 3 % of the initial internal energy of the melt was converted to mechanical energy. In addition, it was found that spontaneous explosions could be suppressed by raising the pressure of the surrounding environment. However, if a powerful enough external trigger signal was provided, a steam explosion occurred even at elevated pressures.

It was also observed that use of saturated water did not spontaneously cause a propagating explosion to occur (at least at the studied melt mass scale). On the other hand, double explosions occurred in some of the FITS tests when subcooled water was used.


The MD series (included a total of 19 runs) is a melt-crucible delivery developmental set of experiments. The first 6 runs involved no water whereas the remaining 13 experiments were performed by dropping Al2O3 into water at an ambient pressure.

The MDC series (included 13 runs) was designed to gain familiarity with the preparation and delivery of corium A+R and all included an external trigger.

The MDF series, which included 5 scooping experiments, was developed to determine the effect of hydrogen generation during the mixing phase of a melt/coolant interaction on suppressing or enhancing the triggering of a steam explosion using Fe3O4.

The RC test series involved 2 runs that were conducted after replacing the lucite interaction water chamber with a 25 mm thick steel pipe 0.6 m in diameter to assess the effect rigid radial constraint on the FCI.

For the CM (coarse mixing) series, a total of 12 experiments were performed to investigate the mixing characteristics of an FCI to assist in development of the mixing phase models.

The OM (oxide melt) series involved 4 runs designed to contrast the behavior of metallic to oxidic melts and investigate the effect of melt composition on the probability of occurrence of spontaneous interactions and on the melt dispersal behavior and hence the extent of the reaction.

The key research question for the ACM series was whether or not this alternate contact mode (of pouring water onto the melt) would support an energetic steam explosion.

SNL Efficiency Scaling Tests

The OG series (known as the efficiency scaling test series) was conducted to determine the conversion ratio of the fuel thermal energy into mechanical work at large scale.

ANL R-22

The test series conducted under this experimental program aimed at investigating the tank and stream geometry, and the water temperature as the dominant system parameters.


MFTF tests, study the effect of release mode, which in essence limits the amount of coolant available for interaction, on the interaction.

Whereas the MIXA tests focus on the effect of dispersal characterisitcs on the interaction. This test series involved 7 runs. The MIXA tests used variable length skirts around the droplet former and therefore resulted in a range of pour shapes from one-dimensional (pour over most of the vessel width) to center pour (pour over a small region in the center of the vessel).


The specific objectives of these experiments are to phenomenologically understand steam explosions in a relatively large-scale geometry and to quantify the conditions to suppress spontaneous steam explosions and finally to obtain additional information to develop and/or validate analytical models. To this end, the experiments conducted under the ALPHA program were designed to investigate the effect of various parameters (melt mass, ambient pressure, water temperature, melt dispersion) on melt-water interaction.


The QUEOS test series focuses on the transient multiphase interaction of hot particles with the objective of establishing a database for testing the models of heat, mass and momentum transfer in multifluid codes and the codes' capability to correctly describe multiphase flows as they occur in the premixing phase of steam explosion.


FARO Programm: Experimental Investigations of Large Scale Melt Quenching in Water


The corium is melted at low pressure (0.1-0.2 Mpa) while the pressure in the interaction vessel is maintained at the required test pressure (2-5 Mpa). These low and high pressure regions are separated by the release vessel closing flap. At the end of the corium melting the main isolation valve is opened and the melt is released from the furnace to the catcher and then the main isolation valve is closed again. The release vessel serves as a pressure equalizer. Upon pressure equalization, the release vessel hinged flap is automatically opened and the melt is delivered by gravity to the water.


Several large scale quenching tests (L-06, L-08, L-11, L-14, L-19, L-20, L-24, L-29, L-31) have been conducted in either the FAT FCI vessel or the TERMOS vessel at the FARO facility using prototypic reactor material, UO2-ZrO2-Zr.

KROTOS FCI Experiments: Alumina versus Corium Melts


The test starts by loading the furnace with the melt of desired composition. After the melt reaches the desired temperature, the crucible containing the melt is released from the furnace and falls by gravity through the release tube. During its fall, the crucible strikes a retainer ring and a conical shaped spike pierces the bottom of the crucible, allowing the melt to be released into the water. High speed videos of the melt as it through the water are captured, in addition to the measurement of the dynamic pressures during the melt/water interaction.

The KROTOS experiments were conducted using either an alumina/water system, or a corium/water system. In both sets of experiments, the effect of the water condition (either highly subcooled or near saturation) was investigated. In addition, the effect of melt superheat and initial pressure on the energetics of the interaction was also explored.


Several tests have been conducted in the KROTOS facility with different simulant fuels. Initially, tin melt was used, while later tests employed alumina or corium melts.

TROI Steam Explosion Experiments in a Multidimensional Geometry


In the TROI experiments the fuel/coolant interaction is initiated between the melt and water by releasing the crucible from the furnace -by gravity- into a pool of water in the test section.


Several tests have been conducted in the TROI facility to answer the fundamental question about the explosivity of reactor materials. Some of these tests employed ZrO2 as melt whereas others employed a mixture of UO2-ZrO2 or UO2-ZrO2-Zr.

WFCI Vapor Explosions in Large Scale 1-D Geometry


The test starts by allowing the melt to reach the desired temperature in the furnace which is then tilted to an almost horizontal position to pour the melt into the transfer vessel. The transfer vessel is then moved by a motor on a transport rail system to a position over the funnel above the test chamber. The transfer vessel has a spring loaded stainless steel plug at its bottom to keep the melt from falling. At the appropriate moment, the pin that holds the loaded spring at the bottom of the transfer vessel is removed by a pneumatic cylinder thus allowing the melt to pour into the test section.


A total of 37 tests in eight different series (A through H), have been conducted with tin as a simulant fuel and water as coolant. Molten tin with initial temperature ranging from 491 deg.C to 983 deg.C was poured in water (pure or with additives) at a temperature ranging from 25 deg.C to 93 deg.C. In these tests the degree of coolant subcooling ranged from 7 deg.C to 75 deg.C, while the coolant to fuel mass ratio varied from ~ 2 to ~ 12.

ZREX: ZiRconium Explosion


In both ZREX and ZR/SS tests, the melt was prepared by induction heating in a graphite crucible. When the melt was ready, the tests started by dropping either 1-kg batches of zirconium/zirconium dioxide (Zr/ZrO2) or 1.2-kg batches of zirconium/stainless steel (Zr/SS) mixtures in a column of water.

The primary objective of the tests was to determine the effect of the content of Zr in the melt, hence the melt composition was varied from 60 to 100 % for the Zr/ZrO2 mixtures and from 0 to 100 % in the Zr/SS mixtures. A total of 14 tests (9 of which were externally triggered) were performed using Zr/ZrO2 mixtures, while a total of 8 tests (5 of which were externally triggered) were performed using Zr/SS mixtures.


Several tests have been conducted with Zr/ZrO2 mixtures whereas others have been conducted using Zr/SS mixtures. In addition, a number of scoping experiments were performed to develop the experimental techniques.

FITS: Fully Instrumented Test Series


The test started by delivering the melt to the water. This was accomplished by allowing the melt to fall under gravity into the water by removing the bottom of the crucible (a separate graphite disk). A fall height of approximately 1.2 m was used to achieve terminal velocities of about 4.5 ~ 6.0 m/s, which was deemed necessary for proper dispersion of the melt in the water.

High speed cameras were used to observe the melt as it fell into the water as well as the subsequent mixing and propagation leading to a possibly explosive event up to the detonation point. The velocity of melt dispersion and subsequent reaction speeds were then inferred from the captured images using some reference length in the test section.


Four test series have been conducted (A through D) with two different fuel types: iron-alumina thermite (Fe-Al2O3) and corium A+R (53 w/o UO2, 16 w/o ZrO2, 2 w/o NiO, 27 w/o SS and 2 w/o Mo).



The EXO-FITS tests were conducted by dropping ~ 5-20 kg melt into the water lucite chamber. For these experimental series, the water temperature ranged from ambient to the saturation temperature at the ambient pressure of ~ 0.082 MPa.


Several large scale tests have been conducted in the EXO-FITS test facility: MD, MDF, MDC, RC, CM, OM, ACM and NPR/FCI test series.

SNL Efficiency Scaling Steam Explosion Tests


The open geometry test series were intended to be scoping in nature. These tests were designated “open geometry” experiments because they performed in an open vessel with minimum instrumentation. Most experiments used thermatically generated fuel simulant, Fe-Al2O3 since it is non-hazardous, easy to produce and inexpensive. A few of the experiments used Corium A+R (Stainless Steel 59%, UO2 23%, ZrO2 15%, NiO 3%).

The fuel melt was generated in an insulated cylinder which was held above the water tank, and the melt was then poured into the water. Melt quantities ranged between 1-20 kg at a temperature of ~ 3000 K. Regular tap water was used in quantities of approximately 175-840 kg at a temperature of about 300 K. In a few experiments, the water was heated using immersion-type electric heaters. In one experiment, the water was subjected to air injection to simulate a highly vigorous boiling situation. The interaction vessel was coated by lard to eliminate/reduce the probability of spontaneously initiated explosions.

The mechanical work generated by the explosion was determined by measuring the impulse delivered downward to crushable honeycomb blocks and by estimating the potential energy of the upward ejected water and/or fuel.


Almost 60 large scale tests have been conducted in the OG test series at Sandia National Laboratory. 49 tests used Fe-Al2O3as melt while the remaining 11 tests were performed using Corium A+R.

ANL R-22 Vapor Explosion Tests


Tests were performed by dropping a constrained and an unconstrained mass of R-22 into water. For the unconstrained runs, R-22 was allowed to fall freely into water whose temperature was varied from 55-100 deg.C. The constrained runs were conducted to test the hypothesis that high explosion pressures are generated as a result of the inertial constraint of the water. The idea was to suddenly release R-22 into water when it is totally submerged. Several mechanisms were attempted to suddenly release the R-22 charge under the water. Ultimately a cylindrically rubber and mesh container was used to hold the R-22 as it penetrates into the water. The event is initiated by rupturing the R-22 container using a scalpel blade positioned near the bottom of the water tank. 26 runs were conducted with this mechanism using water temperatures ranging from 0-100 deg.C.


About 50 large scale tests have been conducted at Argonne National Laboratory using R-22/water pair.

UKAEA Vapor Explosion Tests


The two series of experiments performed at MFTF facility, SUW and WUMT, used thermite generated uranium dioxide/molybdenum melt (81% UO2, 19% Mo) at an initial temperature of 3600 K in quantities up to 24 kg released into water.

Overall, twelve experiments were performed in the SUW series. Nine of which were conducted by droping 24 kg of melt simulant into the coolant in free release mode, while the remaining three were in the restricted mode. Some of these tests investigated the effect of increased system pressure and reduced cover gas volume on the melt dispersion. While the rest investigated the effect of water subcooling on the interaction. For the last two experimental runs in the SUW series, the melt mass was reduced to 8 kg and were carried out in the restricted mode at pressures of 0.1 and 1.0 MPa and 60 deg.C subcooling. After each experiment, the solidified melt debris was dried and the particle size distribution measured by sieving.

In the WUMT series of experiments 24 kg of melt was poured into a square-section mixing vessel containing water. These experiments were carried out to examine mixing; thus various combinations of melt/water mass ratio, vessel size, water depth and melt pour diameter were used in the experiments.


Intermediate to large scale tests have been conducted at Winfrith, UKAEA. Test series SUW and WUMT were performed in the MFTF facility, while MIXA test series were performed at the MIXA facility.

ALPHA Program Vapor Explosion Tests


The first test series, designated by STX-series, was performed to study the effects of ambient pressure, fuel mass, and melt dispersion on the energetics of the FCI. A total of 21 experiments were conducted in the STX-series. Another test series, designated by MJB (melt jet breakup), studied the jet breakup and fragmentation of a molten lead-bismuth eutectic alloy into a deep pool of water.


Several large scale tests have been conducted in the ALPHA facility at JAERI to investigate the fuel-coolant interaction phenomenon which would threaten the nuclear containment integrity under severe accident conditions.



In QUEOS, hot solid particles are used instead of molten material. To simulate the melt jet as closely as possible, the hot spheres are released as one or three cylindrical jets into a three dimensional test vessel. Three types of spheres with two different densities (zirconia and molybdenum spheres were used) and three different diameters are used. With masses ranging from 7-14 kg for spheres made of zirconia, and 10-20 kg for spheres made from molybdenum, this leads to numbers between 2300 and 49000 spheres per run. To capture the radiation effects in a prototypical situation, the temperature of the spheres were up to 2200 K (for zirconia spheres) and 2500 K (for molybdenum spheres).


Approximately 60 large scale tests have been conducted in the QUEOS facility. A few of the tests used steel as melt, whereas the majority of the tests used wither molybdenum or zirconium dioxide as melt.


System is an html indexed and hyperlinked table.


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10.        REFERENCES




The package is transmitted on a single DVD which contains data, reference material and documentation


August 2011