Unit 3 Nuclear Power Plant Accidents 3-13 It does not require a "meltdown" for sufficient fission products to be released from the fuel to pose a threat. It does require loss of the many redundant systems designed to keep the core covered and cool (by removing the decay heat). These systems are designed to maintain cooling even under severe accident conditions such as a total break in the largest pipe in the system. As discussed earlier, excess heat is normally removed from the reactor by the primary coolant system. If cooling water flow cannot be maintained, the control rods are automatically inserted into the core to stop the fission process and thus shut down the reactor (called a scram). However, the radioactive fission products remaining in the core would continue to decay. As previously stated, this decay process yields radiation and heat (called decay heat). To prevent increased temperatures and damage to the reactor core, the decay heat must continually be removed, even after shutdown. Numerous systems and back-up
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