30 Containment systems Design basis accidents

The containment system shall be designed so that the prescribed maximum leakage rate is not exceeded in design basis accidents. The primaiy pressure withstanding containment may be partially or totally surrounded by a secondary confinement for the collection and controlled release or storage of materials that may leak from the primary containment in design basis accidents. 

The atmosphere is the ultimate heat sink for the AP600 reactor system. Computer calculations have shown that under design basis accident scenarios the containment steel structure would provide sufficient cooling to prevent exceeding the containment design pressure. In performing this function, the heat is transferred through the containment vessel to the... 

The PCCS maintains the containment within its pressure limits for design basis accidents such as a LOCA. The system is passive with no components that move. The PCCS consists of three low pressure totally independent loops, each containing a steam condenser (passive... 

HMS components in containment are capable of sustaining normal operation and seismic loads. The HMS is not required to function in a design basis accident and is not safety-grade. However the system is expected to mitigate the effects of a degraded core accident and is designed to withstand the appropriate... 

As in a LOCA design basis accident (see Section 6.2.1.2.), the aerosol concentration in the containment atmosphere will also be reduced by the action of the containment spray system. Since larger aerosol particles are removed more readily by the spray droplets than are small particles, the removal efficiency to be expected in a severe accident is higher than in a design basis accident which has much lower aerosol concentrations in the containment atmosphere and, consequently, smaller dimensions of the aerosol particles (Pasedag et al., 1981). 

This group of characteristics specifies the structure around a reactor, designed to protect the reactor from outside intrusion and protect the outside from radiation effects, in case of a malfunction inside the structure. Except for the reactor, the containment usually contains all the reactor coolant system components. They are either designed to withstand the maximum pressure expected after a design basis accident (DBA), in which case they are termed full-pressure containments, or they may have installed systems to reduce the DBA containment... 

The containment building is an integral part of the overall contaimnent system. Its functions are to contain the release of airborne radioactivity following postulated Design Basis Accidents (DBAs) and to provide shielding for the reactor core and the RCS during normal operations. 

All identified design basis accidents shall be taken into account in the design of the containment system. In addition, consideration shall be given to the provision of features for the mitigation of the consequences of selected severe accidents in order to limit the release of radioactive material to the environment. 

Energy management features are incorporated that limit the internal pressures and temperatures within the containment envelope to values that are below the design limits for the containment system and the equipment that is needed inside the containment when a design basis accident occurs. Examples of energy management features include pressure suppression pools, ice condensers, pressure-relief vacuum-chamber systems, structural heat sinks, the free volume of the containment envelope, spray systems, air coolers, a sump or a suppression pool recirculation water-cooUng system, and the air extraction system for the annulus in double-containment systems. 

This maximum credible accident has, at various times, also been referred to as the design basis accident (DBA), the design-basis loss of coolant accident (LOCA), and the siting-basis LOCA. Given the rather prescriptive assumptions that evolved for demonstrating compliance with 10 CFR 100, the term design-basis LOCA is adopted here. This hypothetical accident is invariably initiated by the reactor-coolant system pipe break that would yield the highest containment pressure. 

The design-basis accident analyses take into account the reduction in the amount of radioactive material available for leakage to the environment by engineered safety features such as containment sprays and recirculating filtration systems. The amount of cleanup is evaluated for each system using conservative assumptions for parameters such as adsorption and filtration efficiencies. 

The analyses of system failures which could challenge the containment or lead to the release of radioactivity form the licensing process. The design basis analyses are deterministic, and degraded core accidents are not considered. PSA determines the probabilities of the numerous sequences that could lead to core degradation and how the core behaves. 

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