MITIGATION OF EX-VESSEL RELEASE

Reactor accidents that progress to the point that core debris penetrate the reactor vessel and cascades into the reactor containment are, of course, quite severe. At this stage in a severe accident, only the containment provides a barrier to the release of radioactive materials from the plant into the environment. Indeed, if decay heat removal from the contaimnent could not be realized in a sufficient way, the failure of this last barrier becomes inevitable. Severe accident management efforts must, then, focus on  

Even if the strategy to maintain contaimnent integrity fails, this may lead to delayed containment failure. Delay in containment failure for even a few hours after core debris has penetrated the reactor vessel can have a profound effect on risk. Delay allows time for emergency preparedness measures to be put in place. It also allows time for natural and engineered processes to reduce the concentrations of airborne radionuclides within the reactor contaimnent. 

Unfortunately, it is also true that the containment could come under harsh assault at the time of vessel failure at high system pressure. This assault is then usually in the form of pressurisation and potential hydrogen combustion though in some cases it can include attack by expelled debris itself or even just elevated temperatures of the containment atmosphere. Accident management activities that can mitigate such potential threats to containment integrity are, of course, seriously considered, e.g. by primary system depressurisation in case of a high-pressure core melt sequence. 

The major radionuclide release processes associated with reactor vessel penetration by core debris 

These various processes that lead to radionuclide release into the contaimnent atmosphere are discussed individually in the subsections that follow. It should be remembered that while ex-vessel releases from core debris are taking place, there can be continuing releases from residual core debris in the reactor vessel and revaporisation releases of volatile radionuclides deposited in the reactor coolant system. 

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