Annulus air extraction system

The upper hemisphere of the steel shell is surrounded by a shielding made of reinforced concrete with a wall thickness of about 2 m. This shielding protects the nuclear part of the plant against any external impact (e. g. gas explosion, military aircraft crash) it also significantly reduces the likelihood that radionuclides will escape to the environment. The interspace between the steel shell and the secondary containment is held at sub-atmospheric pressure, so that any radionuclides penetrating the steel shell via leaks in the event of a loss-of-coolant accident would be transported by the annulus air extraction system to the standby filters and retained here, thus preventing release to the environment. 

These results mean that penetration of fission products (except the fission product noble gases) through operational leaks in the containment shell in the course of a severe reactor accident will be very small. The largest fraction of the penetrating fission products will be plated out in the adjacent rooms and compartments with the condensing steam, while the still airborne fraction will be transported to the filters of the annulus air extraction system. Fission product iodine plated out into the water pools formed in the annuli will be subjected to the reactions to be discussed in Section 7.3.4.3. 

Finally, the standby filters of the annulus air extraction system will act as an additional barrier, preventing the release of radionuclides from the annuli to the environment. The comparatively high humidity of the off-gas under accident conditions does not adversely affect the function of the aerosol filters, but potentially reduces the efficiency of the iodine filters. Nonetheless, a retention efficiency of >99% for I2 and of >90% for CH3I can be assumed, even under such unfavorable conditions. 

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. 

A continuous circulation of groundwater is generated in the area surrounding the remediation well, as aquifer waters replace the annulus water. The circulation thus delivers new contaminants to the stripping zone. Volatile contaminants dissolved in the groundwater are transferred from the liquid to the gas phase and are extracted from the groundwater surface via a double-cased screen. Soil air from the unsaturated zone is also extracted and transported to the off-gas treatment system. 

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