OVERVIEW OF MODULE 2: SEVERE ACCIDENT CHALLENGES AND MITIGATION STRATEGIES

Module 2 describes the very complex physical and radiological phenomena during severe accidents. These include accident initiation and core uncovering, initial fuel damage, hydrogen generation and progression of the core melt, relocation of the core debris to the lower reactor vessel, reactor pressure vessel melt-through, interaction of the core debris with containment structures in the reactor cavity, ex-vessel generation of hydrogen, CO and CO2, water ingress (for specific reactors only), and concrete attack with possible basemat melt-through.

The description includes phenomena which can also be expected in the case of a loss of coolant in the spent fuel pool. The discussed phenomena are generic to all water cooled reactor designs, however design-specific features of BWR, PWR, PHWR and spent fuel pools can affect the degree to which these phenomena are involved - for example, spent fuel pool accidents also likely involve air ingression, and PHWR designs such as CANDU reactors have significantly different fuel arrangement compared to LWRs.

Also discussed in this module are potential threats that can challenge fission product barriers and lead to releases of radioactivity to the environment. Module 2 then identifies potentially available strategies to mitigate the consequences of those threats and/or releases; it identifies the hardware desired or needed and describes the main principles of how strategies are to be developed into procedures / guidelines. Finally, the potential consequences of radioactive releases from unmitigated or partially mitigated severe accidents are described.

This module concludes with a description of the analysis methods, computer codes used in estimating severe accident progression and resultant environmental and health consequences of severe accidents. In addition, embedded appendices provide greater detail on these topics for the interested reader. Two appendices are included that introduce plant damage state characterization and potential 'candidate' actions that can be used to mitigate accident progression impacts on the various fission product barriers.