Severe reactor accidents that go well beyond the design basis of a nuclear reactor involve extensive reactor fuel degradation and even extensive melting. Core debris may be expelled from the reactor coolant system. Though reactor containments are not designed to withstand the rigors of all severe accidents, they are the last physical barrier preventing the release of radioactive material from the plant into the public environment. The worth of modern reactor containments for protection of the public was demonstrated well during the accident at Three Mile Island. Equally so, the dire consequences of prompt failure of this final barrier was demonstrated by the reactor accident at Chernobyl. It is too early to know the exact roles containment played in the reactor accidents at Fukushima Daiichi. Current analyses certainly suggest that the containments and engineered safety systems within these containments were important, while they were functional in limiting the extent of radionuclide release to the environment .
Severe accidents are expected to be very rare events , but there is now ample evidence that they can occur. In this chapter, severe accident processes that threaten the containment integrity are examined. Also, in this chapter, processes within the containment that affect the inventory of radioactive material suspended in the containment atmosphere and available for release should there be a loss of structural integrity are examined.
Accident phenomena that threaten containment integrity early in accident progression before emergency response measure can be fully implemented are, of course, of primary concern. Three such accident phenomena are examined here:
- • Combustible gas explosions
- • Explosive interactions of core debris with water
- • Direct containment heating
In addition, the interaction of core debris with structural concrete represents a longer term threat to structural integrity and is also considered in this chapter.
Radionuclides released from containment will be present as gases, primarily noble gases, and aerosolized, solid particles. Isotopes of iodine can transform between these two physical forms and is particular consequential should it escape the containment. In this chapter, physical and chemical processes that affect the radioactive gases and aerosol concentration in containment will be examined with particular attention given to the behavior of radioactive iodine.