Removal of gaseous iodine by filters

Pathways for venting the containment atmosphere may be provided for a number of reasons, and these pathways may be equipped with filters to remove iodine from the vented gas. Filters for iodine removal can be present in both passive systems (in which flow continues only as long as there is a pressme difference) and active systems (in which there is a continuous forced flow at a controllable rate). Dry filters intended for the removal of aerosol particles are not likely to be effective for the removal of gaseous forms of iodine, especially organic iodides. Even if gaseous iodine will absorb on the filter mediiun, heat loads on the filter medium caused by radioactive decay can lead to revaporization of the absorbed iodine. Filters that involve water must be maintained at high pH to avoid the formation of volatile forms of iodine by processes identical to those that occur in reactor containment smnps. 

The effectiveness of active filter systems is known to be high, based on system tests, research results and simulations. The effectiveness of filters in passive systems as a means of limiting the potential for iodine releases is not known. Backfitting a filter system to an existing containment may be more expensive and less effective than some combination of other mitigation techniques discussed below. A drawback for all charcoal filter systems is the high cost associated with periodic testing of the effectiveness of charcoal. 

---