4.138. Water pools or tanks through which the containment atmosphere is bubbled for steam condensation should be considered a valuable means for the removal of radioactive products. However, care should be taken in evaluating the efficiency of such a process, since it is dependent on the thermodynamic conditions of water and steam. For example, the degree of subcooling of the water and the consequent efficiency of steam condensation have a significant effect on the scrubbing efficiency of a suppression pool.

Ventilation and venting systems

4.139. Where ventilation systems are used for cleaning exhaust air to mitigate the consequences of an accident, filters should be so designed and maintained as to preclude any loading of the filters with pollutants beyond authorized limits prior to their use in relation to an accident.

4.140. The ventilation system should, if necessary, be provided with equipment (such as moisture separators and preheaters before the filters) to prevent the temperature from dropping below the dew point at the air filter inlet.

4.141. The efficiency of the sorption material in iodine filters should be demonstrated in laboratory tests under simulated accident conditions as deemed appropriate. Provisions should be made to test periodically the filter system in situ.

4.142. Ventilation systems are often used to collect, filter and discharge air from the interspace of double containment systems or from a secondary confinement, which may become contaminated with airborne radionuclides in accident conditions as a result of leakage from the containment. For such cases the recommendations in paras 4.139-4.141 apply.

4.143. Where containment venting systems are installed, the discharge should be filtered to control the release of radionuclides to the environment [15]. Typical filter systems include sand, multi-venturi scrubber systems, HEPA or charcoal filters, or a combination of these. HEPA, sand or charcoal filters may not be necessary if the air is scrubbed in a water pool.

4.144. Noble gases cannot be filtered out, but consideration should be given to the use of systems to delay their release until further radioactive decay has occurred.