Prevention and intensification of melt-water explosive interactions

Anthony A Sansone, Purdue University

Abstract

The combination of a hot fluid (e.g., molten metals) and a cold vaporizing fluid (e.g., water) can undergo spontaneous or externally assisted explosive interactions. Such explosions are a well-established contributor to the risk for nuclear reactors exemplified by the infamous Chernobyl accident. Once fundamentals are understood, it may be possible to not only prevent but also, more importantly, control the intensity for useful applications 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, wherein materials turn superplastic with enhanced ductility). This thesis report 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 noncondensable gases such as hydrogen or air. It is found that under the appropriate conditions, spontaneous and energetic phase changes can be initiated within milliseconds if the hot metal is tin or Galinstan, including the timed feedback of shocks leading to chain-type reactions. Using 3–10 g of tin or Galinstan, shock pressures up to 25 bars (350 psig) and mechanical power over ∼2–4 kW were monitored about 4 cm from the explosion zone. The interaction could be intensified more than ten folds by dropping the melt through an argon atmosphere. A slow metal quenching interaction occurring over tens of seconds could be turned explosive to transpire within milliseconds if the thermal states are within the so-called thermal interaction zone. Such explosive interactions did not transpire with gallium or aluminum due to tough oxide coatings. However, by adding ∼10 w/o of salt in water, molten Gallium readily exploded. Similar additions 2.5–10 w/o of salt in the tin-water system revealed enhancements in the interfacial film heat transfer dynamics, subsequently leading to enhanced melt triggerability. Further studies on triggering enhancement were conducted using underwater detonations to artificially generate shock-like pressure traces. It was found that these pressure transients initiated film collapse in the otherwise thermally stabilized systems, leading to explosive interactions superior in nature to the best-case spontaneous explosions. It was also conclusively revealed that, for an otherwise spontaneously explosive interaction of tin-water or Galinstan-water, the inclusion of trace (0.3 w/o) quantities of aluminum has a radical influence on stabilizing the system and ensuring conclusive prevention of explosion triggering. However, inclusion of external trigger shocks readily initiated explosive interactions, rapidly reducing the original melt to micron-scale fragments and producing preliminary evidence that vapor explosions may act to mechanically catalyze the Al-water reaction. This thesis report compares and presents the results obtained in this study and draws analogies with industrial scale aluminum casthouse safety involving thousands of kilograms of melt. Insights are provided for enabling physics-based prevention, or, alternately, the intentional initiation of explosions with combined simultaneous chemical and thermo-mechanical energetic bursts and impulse hydrogen production. Finally, recommendations are provided for future studies impacting fields as diverse as nanoscale powder metallurgy, insensitive high energy density materials, impulse hydrogen production, supercooling, and nuclear and metals casting industrial safety.

Degree

M.S.

Advisors

Taleyarkhan, Purdue University.

Subject Area

Nuclear engineering

COinS