Vapor explosion studies for nuclear and non-nuclear industries

Abstract

Energetic melt-water explosions are a well-established contributor to risk for nuclear reactors, and even more so for the metal casting industry. In-depth studies were undertaken in an industry-national laboratory collaborative effort to understand the root causes of explosion triggering and to evaluate methods for prevention. The steam explosion triggering studies (SETS) facility was devised and implemented for deriving key insights into explosion prevention. Data obtained indicated that onset of base surface-entrapment induced explosive boiling-caused trigger shocks is a result of complex combination of surface wettability, type of coating (organic versus inorganic), degree of coating wearoff, existence of bypass pathways for pressure relief, charring and non-condensable gas (NCG) release potential. Of these parameters NCGs were found to play a preeminent role on explosion prevention by stabilizing the melt-water steam interface and acting as a shock absorber. The role of NCGs was experimentally confirmed using SETS for their effect on stable film boiling using a downward facing heated body through which gases were injected. The presence of NCGs in the steam film layer caused a significant delay in the transitioning of film-to-nucleate boiling. The role of NCGs on explosion prevention was thereafter demonstrated more directly by introducing molten metal drops into water pools with and without NCG bubbling. Whereas spontaneous and energetic explosions took place without NCG injection, only benign quenching occurred in the presence of NCGs. Gravimetric analyses of organic coatings which are known to prevent explosion onset were also found to release significant NCGs during thermal attack by melt in the presence of water. These findings offer a novel, simple, cost-effective technique for deriving fundamental insights into melt-water explosions as well as for explosion prevention under most conditions of interest to metal casting, and possibly for nuclear reactor systems during severe accident conditions. Energetics of entrapment boiling induced shocks for explosive and non-explosive conditions were quantified using a modified zero-crossing technique. In honor of Professor R.T. Lahey Jr. the non-dimensional parameter “ L T” was proposed to delineate the explosion-onset boundary. Experimental evidence suggests that a system with L T above a threshold value of ∼65 leads to the growth of perturbations and onset of propagating melt-water explosions. The data appear to offer valuable insights into explosion prevention in nuclear reactors during beyond-design basis accident conditions. An unresolved issue concerns the potential for trigger shocks from chemical ignition reactions between reactive metals in contact with oxide coatings such as rust.