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CORE DEBRIS INTERACTIONS WITH COOLANT

Continued progression of reactor accidents beyond the stage of rapid steam oxidation of the zirconium alloy cladding leads to fuel melting. Inevitably this molten core debris will interact with liquid water. Attempts to arrest the accident progression will involve submerging the debris in water. Relocation of the molten debris from the core region will lead to interaction with residual water in the lower plenum of the reactor vessel. If accident management measures cannot arrest the accident, molten core debris will be expelled eventually into the containment where large quantities of water have accumulated from the pressure vessel blowdown and actions of the emergency core cooling system.

Core Debris Quenching

The interaction of hot, but still solid, core debris with water is expected to be benign in the sense that the interaction will not pose a prompt threat to the structural integrity of either the reactor pressure vessel or the reactor containment. Certainly, this was the case during the accident at Three Mile Island [28, 29]. QUENCH tests conducted in Germany also support this supposition [30]. Fragmentation of the core materials can lead to the formation of rubble beds with cooling characteristic quite different than those of intact fuel. The cooling of rubble beds depends on particle size and debris depth [31]. The dependence on debris size is shown in

FIG. 7.7

COOLABILITY OF CORE DEBRIS WITH INTERNAL HEAT GENERATION AS A FUNCTION OF DEBRIS PARTICLE SIZE [31, AND REFERENCES THEREIN]

Fig. 7.7. Fortunately, it is expected that quenching hot, solid debris yields rather coarse debris that is easily cooled provided there is a continuing source of coolant.

Molten core debris poses more challenges. Certainly, the accident at Three Mile Island demonstrated that molten core debris is more difficult to quench. If the molten material can be quenched, it need not fragment into fine debris. Sustained cooling requires a continued source or water and means to extract heat from the reactor containment. More problematical is that interactions of high temperature molten materials with liquid water can lead to explosive events that do pose threats to both the reactor coolant system and to the reactor containment. These steam explosions are discussed in the next section.

 
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