When a liquid comes into contact with another liquid and the first liquid is at a temperature much greater than the boiling point of the second liquid, rapid vaporization of the second liquid may occur as the first liquid cools. Under some circumstances, this rapid vaporization may cause a detonation. Such det­onations have been observed in metal foundries where vats of molten metal have been accidentally poured into vessels of water, or vice versa. They may also occur if room-temperature water is brought into contact with liquid natural gas; in this case, the detonation may be followed by a fire as the gas cloud burns. The potential for an energetic interaction between molten uranium fuel and the water coolant may also exist if molten fuel is jetted into water. This can occur as

• molten fuel is ejected into the coolant when the cladding fails during a

severe power excursion (cf. Chernobyl).

• the lower core support plate fails and molten fuel is jetted into a pool of water in the vessel lower head

• the lower head of the pressure vessel fails and molten fuel falls into a water-filled reactor cavity

The circumstances arising in a fuel-coolant interaction and leading to a vapor explosion are illustrated in Figure 6.3. The molten fuel is initially above the pool of coolant (Figure 6.3a) and then falls into it (Figure 6.3h), giving rise to coarse mixing between the fuel and the coolant with a dispersion of large elements of the molten fuel as illustrated. These elements might be 1 cm in diameter. They transfer heat relatively slowly to the water, since a thin vapor film forms around them and insulates them from the water coolant. The third stage is that of trig­gering a shock wave. This is often postulated to occur at the surface of the ves­sel (Figure 6.3c) and might be caused by a small, localized vapor explosion or impact. This shock wave then passes through the coarse fuel-coolant mixture and breaks up the fuel into small elements, which may transfer their stored en-

ergy rapidly to the coolant. This energy release strengthens the shock wave, which continues to propagate through the mixture in an explosive manner (Fig­ure 6.3d).

The energy stored by the molten fuel on release into the coolant pool is partly converted to energy in the shock wave. The extent of this conversion is obviously very important in considering the effects of the resultant shock wave on the reactor system. Experimental studies indicate that the efficiency of con­version from the stored energy in the fuel to the energy within the explosion is about 1.5%.

This would result in an explosion of roughly 1 GJ (or 200 kg TNT equivalent) if all the fuel in a P’^TC, say, reacted simultaneously.

There is still considerable discussion about the precise mechanism by which the shock wave propagates through the fuel-coolant mixture. One theory sug­gests that associated with the shock, there is spontaneous formation of vapor bubbles, giving rise to rapid transfer of energy from the fuel to the coolant. An­other theory suggests that in the shock itself the mechanism of heat transfer is quite different, with the fuel being shredded to small elements by the shear forces in the shock and these elements transferring their energy rapidly to the coolant behind the shock. As we have discussed earlier, a high-pressure im­pulse resulting from a steam explosion is transmitted into the coolant pond. This accelerates a slug of coolant, which impacts the upper head of the vessel and might induce failure. The influence of steam explosions in reactor systems is still a subject of debate, and no final judgments can be made at this time.