Containment conditions during severe accidents

Radionuclides are released to the containment as gases and as aerosol particles by a variety of processes during severe accidents. Modem, mechanistic analyses of these radionuclide releases and the subsequent behaviour of aerosols and vapours under reactor accident conditions strive to be realistic. This realistic approach contrasts with the deliberate attempt to be conservative (which may not have been successful) in the definition of radionuclide behaviour for the design of nuclear power plant safety systems. A discussion of the various radionuclide release processes during severe reactor accidents is presented in Chapter II. Of primary interest in these discussions of release is the potential magnitude of radionuclide release and the radionuclides of most concern. Factors that most affect radionuclide release but can also be affected by accident management measures are discussed. 

Review of the major predicted phenomena during FP transport and deposition in the RCS and containment building under severe accident conditions. 

During a severe accident it would be very difficult or impossible to predict with confidence the performance of the containment. However, based on NUREG-1150, the conditional probability of containment failure and a release requiring protective action under EPA PAG ranges may be as high as 0.5 for some plants. The conditional probability of a release that could result in early health effects is much lower, about 0.001 or less. 

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