12.126. In an explosion, gases are formed so rapidly that the propa­gation front moves with supersonic velocity so that a shock wave is pro­duced. The resulting kinetic energy can be destructive. In severe accidents, two types of explosions are possible. One is a vapor explosion, or so-called steam explosion. The other is a result of a chemical reaction, or combustion, normally of hydrogen produced from zirconium oxidation. If the speed of the combustion wave is subsonic, we have deflagration or burning rather than the supersonic detonation.

12.127. The postulated vapor explosion during a severe accident is a result of the interaction of molten fuel with liquid water coolant. Several steps in this interaction take place, the details of which are important is assessing the load on the containment structure. Since both experimental and theoretical research on mechanisms is ongoing, we can only present some of the principles involved.

12.128. When the molten fuel contacts the water, steam is formed very quickly at the interface that separates the two liquids during a short met­astable period. This period may last from a few milliseconds to a few seconds, after which the film becomes unstable, resulting in very rapid fragmentation of the hot liquid fuel. The greatly increased interface surface area which quickly propagates throughout the molten fuel-water mixture then results in the formation of much more steam at a rate that can produce a detonation-like shock wave. Thus, adequate modeling requires a de­scription of the fuel-coolant mixing, so-called “triggering” when the met­astable condition is disturbed, and the subsequent explosion propagation step [35].

12.129. The role of hydrogen formed from the zirconium-steam re­action during a severe accident is reasonably well understood and is de­scribed by suitable modeling codes. Also, several countermeasures nor­mally prevent explosions in the containment. For example, thermal or catalytic recombiner devices combine the hydrogen and oxygen present so that the hydrogen concentration remains below flammable limits. Another approach is to “inert” the containment by substituting nitrogen for air during normal operation. A number of glow or spark plug ignitors distrib­uted throughout the containment deliberately burn the hydrogen in a con­trolled manner close to the point of formation, thus avoiding an explosion involving a large mass of reactant [36].