In primary system, severe accident

The results of the VIP programme confirm the importance of proper severe accident management, as the presence of a small amount of water may be decisive. Also the avaUability of a voluntary depressurization of the primary system is essential, which removes the possibility of many possible scenarios of vessel rupture. The programme also confirmed the need to actively continue studies and research on the external cooling of the pressure vessel in case of severe accident. 

The primary depressurization eliminates at the source, all the severe accident sequences with a pressurized primary system (i.e. direct containment heating, destructive reaction forces due to perforation of the vessel, etc.). Moreover, in case of malfunction of the high pressure cooling systems, it allows the cooling of the core by intermediate pressure accumulators and low pressure systems. 

Saturation conditions are assumed in the primary system and, therefore, the initial phase of the pressurizer voiding during an accident cannot be simulated. This phase is not of great interest for the prevention of severe core damage which remains the field of deepest interest in the context for which the program has been written. The principal anal rtical instruments are the mass and energy conservation equations. 

Consequently, the reactor must be cooled even after shutdown and even in case of an accident. Therefore several independent cooling systems are incorporated. The worst accident that is considered (maximum credible accident MCA ) is the complete shearing off of the primary circuit line. If this should occur, then a mixture of steam and air would be blown into the containment for a time span of 12 s (blowdown phase). When the pressure falls below 26 bar, water tanks maintained at this pressure automatically would empty into the reactor vessel from both sides (high and low temperature) of the circuit. If this water should evaporate or leak out again, at a pressure of 10 bar, a pump would provide water from other reservoirs. Water leaking out from the primary circuit will have to accumulate on the bottom of the... 

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. 

According to the review paper by Bowsher (1987) on chemical reactions occurring in the primary system under severe accident conditions, the following types of interactions have to be considered ... 

Such interactions can only occur, however, when the volatile fission products and the primary aerosols appear simultaneously in the primary system, in spite of the large differences in their volatilization behavior. As was discussed above, uniform thermal-hydraulic conditions do not prevail within the reactor core during a severe accident (for example, the peripheral fuel rods may fail relatively late in the accident sequence, at a point when a large part of the central rods may already be molten) and it can be assumed that the broad time-envelope of significant release of structural aerosols will encompass the release of the volatile fission products. However, as was mentioned in Section 7.3.1.2., the amount of primary aerosols formed and the timing of their formation depend highly on the specific accident sequence this is particularly true for the control rod materials. 

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