Ex-vessel release

There are, then, several potential sources of radioactive material to the containment atmosphere. The most important of these are thought to be gap release, in-vessel release, ex-vessel release, and late invessel release. A recent assessment of the magnitudes of these releases in terms of fractions of the initial core inventories of important radionuclides has been prepared [S-2]. Results of this assessment are shown in Tables II-2 and II-3. It is important to view the results shown in these tables as somewhat conservative examples of both the timing and the magnitude of radionuclide release. These examples are not prescriptions of accident sources to any particular reactor contaimnent. 

S-15. J.E. Brockmann, Ex-Vessel Releases Aerosol Source Terms in Reactor Accidents , Progress in Nuclear Energy 79(1987)7. 

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. 

Fertile Particle Predicted Failure Core-Average Release/Blrth for Kr-8Sm Gore-Average Release/Birth for Xe-138 Outer Neutron Control Assembly Inner Neutron Control Assembly Control Assemblies Installed in Vessel Ex-Vessel Detectors and Location Plan View of Reactor Core Vertical Section of Reactor Core Control Rod Drive Mechanism... 

The PPIS receives signals from the ex-vessel detectors of the NCSS or other protection system sensors and interprets these signals to determine appropriate protective actions. In all DBEs except DBE-2, the control rod drives are tripped and their function is completed in the very early stages of the event. For DBE No. 2, which Includes shutdown by the reserve shutdown control equipment, the shutdown action (release of the reserve shutdown material) is delayed for a brief interval (i.e., minutes). However, if the shutdown action should take place after considerable delay (i.e., several hours), the RSCE is designed to withstand the most severe environment to which it might be exposed prior to completing the required action. 

Nuclide Gap Release 0.1-0.5 hours Early In-Vessel 0.7—3.0 hours Ex-Vessel 2—5 hours Late In-Vessel 10 hours... 

The capability to flood the reactor cavity prevents the failure of the reactor vessel given a severe accident. The vessel and its insulation are designed so that the water in the cavity is able to cool the vessel and prevent it from failing that is, in-vessel retention (IVR). By maintaining die vessel integrity, the core debris in the vessel eliminates the potential of a large release due to ex-vessel phenomena and its potential to fail the containment. 

There is no experiment (not even the TMI accident) which represents all features of a severe accident (i.e., primary system thermal/hydraulics in-vessd core damage fission product and aerosol release, transport and deposition ex-vessel core-concrete interaction containment thermal/hydraulics and hydrogen transport and combustion), and only the TMI accident is at full plant scale. It is therefore necessary for severe acddent codes to supplement standard assessment against experiment (and against simple problems with analytic or otherwise obvious solutions) with plant calculations that cannot be hilly verified, but that can be judged using expert opinion for reasonableness and internal self-consistency (particularly using sensitivity studies) and also can be compared to other code calculations for consistency. 

The large containment water inventory would be expected to prevent DCH, so the possibility of DCH was ruled out for this evaluation. However, the conditions are conducive to ex-vessel FCIs. Evaluations performed for NUREG-1150 indicated that when only a small amount of water is present on the drywell floor (a few feet), the FCI load would not be transmitted to the containment wall. However, the evaluations Indicated that with a large water inventory present, the load could be transmitted to the containment wall and possibly fall it. The largest uncertainty for ex-vessel FCIs is the amount of debris that would actually be released at vessel breach. This uncertainty dominates the probability of containment failure from an ex-vessel FCI. We used the same probability for occurrence of an ex-vessel FCI as used for In-vessel FCIs, and considered containment failure to occur for cases with an ex-vessel FCI. 

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