Molten aluminum-water explosions

Besides the aluminum industry, the nuclear power industry has been interested in molten aluminum-water explosions due to the presence of aluminum metal in some boding water reactors. Certain accident scenarios lead to a meltdown of the reactor core with concomitant contact of molten aluminum and water. 

Results from extensive test programs on molten aluminum-water explosions have been reported by Long (1957), by Hess and Brondyke (1969), and by Hess et al. (1980). In almost all experiments, molten aluminum, usuaUy 23 kg, was dropped into water from a crucible with a bottom tap (see Fig. 9). In only a few tests was there instrumentation to indicate temperatures, pressures, delay times, etc. The test results were normally reported as nonexplosive or explosive—and if the latter, qualitative comments were provided on the severity of the event. A large number of parameters were varied, and several preventative schemes were tested. Over 1500 experiments were conducted. Some of the key results are summarized below. ... 

Extending this concept, we now consider those experiments which led to molten aluminum-water explosions without the presence of a wet, solid surface. In all of these there was an external shock applied to the system—usually in the form of an exploding wire or a detonator. As presumed by the investigators, these artificial shocks could be very effective in collapsing steam films. 

Dewing, E. W. (1980). The initiation of molten aluminum-water explosions. Memo., AIME Meet., 1980, Las Vegas, Nevada. 

Experimental test results for molten aluminum-water RPTs are described in Section V. Also shown is a tabulation of most documented aluminum-water explosive boiling incidents (see Table XIV). In many accidents, the quantity of water was quite small, e.g., some resulted when wet aluminum ingots were loaded into melting furnaces containing molten aluminum. In contrast, one notes that few, if any, serious events have ever been obtained when small quantities of aluminum were contacted with a large mass of water. Since laboratory tests were often carried out in the latter fashion, most of these have produced negative results. 

As with the smelt-water case, if an RPT did take place, the event was localized and rarely was dam e severe far from the site of contact. Modeling molten aluminum-water incidents (and, in fact, other molten metal-water explosions such as in the steel industry) has not been partic-... 

In Table XV, some data are given for molten metal-water explosions not involving aluminum. In most of these cases, the quantity of metal was large. Because only the more serious explosions are reported, these tend to illustrate the most damaging type of event. Again, it is interesting to note that usually only small quantities of water were involved. 

Two large blocks of aluminum, each weighing about 600 kg, were being loaded into a furnace. Later investigation ascertained that one block had a small amount of water (about a teaspoonful )- When this block entered the molten aluminum, an explosion occurred... 

Inadvertent contact of molten aluminum and water may lead to an event termed a thermal explosion. These incidents may produce little vapor, but they are accompanied by sharp, local shocks which are potentially damaging to personnel and equipment. Although not a major problem to aluminum producers, they do occur in casting plants, and it is important that preventative measures be enforced. The same general comments apply to other metal processing industries such as steel and copper. 

The lead in studying both the cause and prevention of molten metal-water thermal explosions has been taken by Alcoa. The Aluminum Association also sponsored several research programs both at Alcoa and at Battelle-Columbus. The Ai onne Natio Laboratory has carried out experiments with molten aluminum and water. 

Experiments also indicated that under circumstances where a significant quantity of molten aluminum could not contact a wetted surface, explosions were rare. Thus, small spills, or spills which required the metal to fragment or pass through considerable distances in air or water before contacting a wet, solid surface, seldom led to explosions. On the other hand, if molten aluminum were to contact wet, rusty steel (or steel with inorganic salts on it), explosions were more likely. 

The technique was then changed to entrap a small mass of water under a molten aluminum surface and simultaneously to overpressure the system. In this manner it was hoped to collapse steam films around the water. The actual procedure employed a small glass sphere containing water. The sphere was moved beneath the aluminum surface and broken by impulsively loading the system from a falling steel cylinder which impacted on a graphite toroid immediately above the molten aluminum. About 0.7 g of water was released into I kg of aluminum at 1170 K and pressurized to about 8 MPa. No explosions were detected. 

In the first set of experiments, the water vessels had rusted bottoms. Of the 21 tests, 14 produced explosions, but no correlation of explosion probability could be deduced. It was reported that, in all tests, molten aluminum reached the bottom of the vessel. High-speed movies showed that the entire explosion sequence between the first visible disturbance in the system to a full-scale chemical reaction was very rapid (on the order of 600 /Ltsec). Note that the word chemical was used in the quote. Lemmon suggests that chemical reactions play a key role in the explosion phenomenon, particularly for violent incidents. The proof that chemical reactions are important stems from the finding that strong explosions produced light and, also, limited spectrographic data indicated local temperatures in excess of 3000 K. The emphasis on chemical reactions was not stressed in the work of other investigators. 

The final set of experiments employed external sources to provide a shock wave in the water during or immediately following a spill of molten aluminum. Usually the shock was caused by the detonation of a small charge of explosive in the water. 

A crane holding a bucket of scrap aluminum and sows (10,000 kg) was loading into a holding furnace which contained some 12,000 kg of molten aluminum. Upon opening the clam shells, an immediate explosion occurred. The furnace was cooled and most sows were recovered intact. There was no evidence of foreign material in the charge. It was considered likely that one sow had a shrinkage cavity which contained some water... 

An aluminum ingot which had just been cast was submerged below water. The skin covering the top apparently broke, thereby exposing molten metal. An explosion resulted... 

A vessel holding about 0.14 m of water was contacted with molten aluminum during a bleed-out from an ingot. An explosion took place... 

Leak in a reduction pot allowed molten aluminum Mild. 3 x 3 m section of to flow into basement. Leak stopped, explosion concrete floor lifted occurred about J hr later. Water of unknown origin contacted pool ... 

During casting of 37-cm ingots, due to a failure of equipment, molten aluminum was allowed to flow into water (depth unknown). An explosion resulted. The equipment was coated with an approved material, but some bare areas may have been present. Also there were external shocks from equipment falling against the vessel with aluminum and water Failure of a flow-metering rod in a casting operation led to the flow of aluminum into a pool of water. Two separate explosions resulted... 

The spout on a holding furnace broke during casting and molten aluminum fell into a pool of water. An explosion resulted An aluminum-manganese alloy was being melted in a refractory crucible. Due to an accident to the crucible, molten metal spilled into the furnace pit which contained water. An explosion resulted... 

Ingots 0.3 X 1 m in cross section were being cast when one ingot stuck and water was trapped in the hanging section. Molten aluminum flowed into the cavity and an explosion resulted... 

One experiment which does not seem to fit into the network of the salt-gradient theory was that of Wright and Humberstone (1966), who impacted water on molten aluminum and obtained explosions. These results are at variance with those of Anderson and Armstrong, but the latter worked at 1 bar whereas the former used a vacuum environment. It might be possible that, under vacuum, it is much easier to achieve intimate contact between the aluminum and water and, under these conditions, there may be sufficient reaction between the aluminum and water to allow soluble aluminum salts to form. This salt layer could then form the superheated liquid which is heated to the homogeneous nucleation temperature and explodes. 

Krause, H. H., Simon, R., and Levy, A. (1973). Smelt-Water Explosions, Final reports to Fourdrinier Kraft Board Institute, Inc. Biittelle Laboratories, Columbus, Ohio. Lemmon, A. W. (1980). Explosions of molten aluminum and water. In Light Metals 1980 (E. McMinn, ed.), p. 817. (Proceedings of Technical Sessions Sponsored by TMS Light Metals Committee at 190tb AIME Armuat Meeting.)... 

Lipsett, S. G. (1966). Explosions from molten metals and water. Fire Technol. 2, 118. Long, G. (1957). Explosions of molten aluminum in water-cause and prevention. Met. Prog. 71(5), 107. 

EXPLOSION and FIRE CONCERNS nonflammable gas, but strong oxidizer NFPA rating Health 4, Flammability 0, Reactivity 0 reacts explosively with acetylene, ether, turpentine, ammonia, fuel gas, hydrogen, and finely divided metals reacts violently with many alcohols explodes on contact with molten aluminum, ammonia, benzene, bromine pentafluoride, diborane, and many others combines with moisture to form reactive hydrogen chloride gas use water spray or fog for firefighting purposes. 

There are two molten materials speciflcally listed in the Hazardous Materials Tables in 49 CFR molten aluminum and molten sulfur. Molten aluminum has a four-digit UN identification number of 9260. The NAERG refers to Guide 169 for hazards of the material. Molten aluminum is the only material that refers to this guide. The guide indicates that the material is above 1300°F and will react violently with water, which may cause an explosion and release a flammable gas. 

Table II summarizes the resulting damage from an 80 kg equivalent explosion caused by 26 kg of water interacting with molten aluminum. Table II clearly indicates the inadvisability of placing control rooms next to furnaces, as graphically illustrated in the recent deaths of 6 people at a ferroalloy facility [6].

Because the plutonium-burning reactor proposed in this report is assumed to use a metal or oxide fuel, (such as Pu-Al, Pu-Zr02, or Pu-ZrH).6) the potential for an energetic steam explosion is of some concern, provided an accident sequence can be identified that leads to large quantities of molten fuel and cladding. The purpose of this section is to discuss some of the steam explosion concerns involving aluminum-water and zirconium-water in relation to the proposed low power density, low flow plutonium-burning reactor. 

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