Prevention & Intensification of Melt-Water Explosive Interactions

ABSTRACT

The combination of a hot fluid (e.g., molten metals) and a cold vaporizing fluid (e.g. water) can be made to undergo spontaneous or externally assisted (e.g., via trigger shock) onset of explosive interactions (via destabilization of the interfacial vapor layer) and resulting in rapid heat transfer, phase change, pressure buildup and melt fragmentation. Energetic melt-water explosions are a well-established contributor to the risk of nuclear reactor systems such as the infamous Chernobyl Accident. The prevention of onset of such interactions in nuclear systems is of paramount importance. However, once the fundamentals are understood, it may be possible to not only intensify but more importantly, to control the intensity of the interaction. The control and intensification of explosive interactions can become of considerable importance in the areas covering variable thrust propulsion with tailored pressure profiles, for enhancing rapid heat transfer, and also for powder metallurgy (i.e., supercooled powder production in which the resulting materials may turn superplastic with enhanced ductility and strength). This paper discusses results of experiments conducted with various molten metals specifically, tin, gallium, galinstan and aluminum interacting with water (with and without salt), and with and without non-condensable gases such as hydrogen or air. It is found that under the appropriate combination of conditions, spontaneous and energetic liquid water to vapor phase changes can be readily initiated within milliseconds if the hot metal fluid is tin or galinstan (but not for aluminum) including the timed feedback of shocks generated from earlier explosions leading to chain-type reaction fronts propagating through mixtures. Using 3-10 g metal masses of tin or galinstan spontaneously exploding in water, shock over-pressures up to 12 bar (175 psig) were monitored about 4cm from the explosion zone, accompanied with mechanical shock power levels of about 2 kW. The same reaction could be made to intensify more than ten-fold by dropping the melt through an inert gas. A previously slow phase change process (viz., normal metal quenching) occurring over tens of seconds could be turned explosive to transpire within milliseconds for melt-water thermal states within the so-called thermal interaction zone (TIZ). Such explosive interactions did not transpire with gallium or aluminum due to tough oxidic coatings. With about 10 w/o of salt in water, molten Gallium readily exploded. However, it was also conclusively revealed that, for an otherwise spontaneously explosive combination of Tin-Water or Galinstan-Water, the inclusion of even trace (0.3) quantities of aluminum (which generates a non-condensable gas in the interfacial layer) is found to have a radical influence on stabilizing the interfacial vapor layer between hot fluid and cold fluid, thereby ensuring conclusive (100% of time) prevention of explosion triggering for all cases tested. The same effect has also been noted if air was bubbled through the water [21]. This paper compares and presents the results obtained in this study along with insights into energetics, with gram quantity melt droplets and draws analogies with data taken for industrial scale aluminum casthouse safety conditions involving thousands of kilograms of melt. Insights drawn for adaption to industrial settings, including for nuclear reactor safety are provided for enabling physics-based prevention, or, alternately, the intentional initiation of such explosive interactions.

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