A beyond design basis accident in which the core is substantially damaged. Phenomena include fuel and cladding melting, hydrogen production from zirconium oxidation, relocation of molten material to the vessel bottom, possible vessel failure and molten core interaction with concrete. Management shifts from emergency operating procedures to severe accident management guidelines, whose priorities are cooling debris, protecting containment and limiting releases.
Core exit thermocouples over 1200, we're transitioning to the severe accident guidelines.
Hydrogen from zirconium oxidation is a hazard in its own right, which is why containments carry recombiners or igniters to remove it before it reaches an explosive concentration.
Also heard as
- core damage accident
- core melt accident
Related terms
Beyond design basis accident
An accident more severe than those the plant's safety systems were explicitly designed to handle.
Containment
The massive, leak-tight structure around a reactor that holds in radioactive material and steam if the coolant system fails.
Source term
The amount, type, timing and chemical form of radioactive material released, or assumed released, in an accident.
CDF
Core damage frequency: the estimated likelihood per reactor-year of an accident that damages the core.