If it is not possible to cool the debris bed within the containment building, the debris begins to react with the concrete floor of the building and penetrates this and also the bedrock on which the reactor is built. This gradual downward pen­etration of the molten pool has colloquially been referred to as the “China Syn­drome," it being imagined that the pool could ultimately penetrate through to the other side of the earth, which in the case of the United States is imagined to be China. Actually, this imagined situation is impossible: the pool would miss China by a long way and could only pass outward from the center of the eaith if gravity mysteriously became negative. However, penetration of the molten material is limited.

Figure 6.5 shows an overall diagram for the containment for a PWR. Turland and Peckover (1979) calculated the behavior of a molten pool arising from a 3- GW(t) reactor core. There are two extreme situations.

First, if the melt consists mainly of oxide, it is likely to be miscible with the base concrete and rock. A molten pool would be formed of limited depth (around 3 m) and with a diameter of about 13 m (Figure 6.5). This pool will re­main for a period of up to several years. Figure 6.5 illustrates the situation after 1 year and shows the temperature profile in the rock-concrete around the pool. The heat generated by fission product decay within the pool is dissipated into the surrounding rock due to the temperature gradients illustrated.

Second, if in the melting process molten steel is produced, this may dissolve fission products from the fuel. If this molten steel is oxidized, the melt pool will be miscible with the concrete-rock base and a pool such as that illustrated in Figure 6.6 will be formed. If the steel is not oxidized, the steel-fission product solution will not be miscible with molten fuel and concrete-rock and will itself penetrate the base rock much farther. Calculations by Turland and Peckover (1978) are illustrated in Figure 6.7. It shows that a molten metal, immiscible pool of this type could penetrate to a maximum depth of about 14 m.

The two melt pools illustrated in Figures 6.6 and 6.7 are drawn in scale in the diagram of the containment shown in Figure 6.5.

It is noteworthy that the interaction between the molten fuel and the con­crete-rock will result in the release of significant amounts of vapor and gas as a

result of the chemical reaction. This may result in pressurization of the contain­ment building over a long period of time, particularly if no cooling is available.

As the fission products in the pool of material decay, the molten fuel gradu­ally solidifies. Calculations indicate that the pool of molten material under the reactor might reach a maximum size equivalent to a hemisphere about 27 m in diameter. Because a considerable amount of concrete is mixed with the fuel, it has been suggested that the final form of the solidified mass is likely to be a glasslike substance that would immobilize the fission products and limit their subsequent migration.

As we have seen above, even the worst case of fuel meltdown and failure to cool would lead to an acceptable situation provided there is no failure of the containment.