6.2 The Reactor Pressure Vessel Is Expected to Be Rapidly Cooled in a Severe Accident

Fig. 6-4 Conceptual diagram of in-vessel debris coolability experiments in the ALPHA program
The equipment simulates a lower head of the pressure vessel in a reactor containment. Its pressure and temperature are controllable. Experiments start with pouring molten Al2O3 as a simulant of debris into the lower head experimental vessel. Temperatures are monitored at the middle of the debris simulant and on the outer surface of the vessel.

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Fig. 6-5 Temperature change of debris simulant
The temperatures of both debris simulants decrease similarly at the middle in spite of a big difference in weight.

Fig. 6-6 Temperature change on the outer surface of the lower head experimental vessel for 50 kg debris simulant
The temperatures of the vessel wall rise sharply with the introduction of molten debris simulant, reach a maximum, and then decrease rapidly. The temperature decrease proceeds from the periphery to the center of the vessel bottom.

JAERI has promoted the so-called Assessment of Loads and Performance of containment of Hypothetical Accident (ALPHA) program to evaluate the integrity of a reactor pressure vessel and its containment under severe accident conditions. The following is recent, useful data on debris coolability obtained in large-scale experiments of the program.
During a severe accident, the reactor core is molten, and falls through the water and accumulates on the lower head of the pressure vessel. Figure 6-4 shows the large equipment for simulating such phenomena. The core melt is called debris and is simulated here by molten Al2O3 produced by the exothermic reaction between aluminum and iron oxide (thermit reaction).
Figure 6-5 shows that the debris simulants (about 30 and 50 kg) poured into the experimental vessel are cooled similarly from the initial temperature, about 2,700 K, although there is a difference of a factor of about two in volume (weight).
In the case of the 50 kg debris simulant, the outer wall of the vessel was rapidly cooled (Fig. 6-6), while the simulant still kept a high temperature of about 2,000 K (Fig. 6-5). This observation implies that an interfacial gap was formed between the simulant and the vessel wall which allowed water to penetrate, causing an effective cooling of the lower head experimental vessel. The ultrasonic technique applied after the cooling experiment confirmed the existence of a gap ranging from 1 to 2 mm.
This information is important because of the implication that a reactor pressure vessel keeps its integrity in the presence of water during a severe accident.

Reference
Y. Maruyama et al., Studies on In-Vessel Debris Coolability in ALPHA Program, NUREG/CP-0517 (2), 161 (1997).

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