Passive iodine removal

The purpose of this question is to find out whether any passive sink for the iodine has been provided to supplement the natural iodine removal mechanisms, like deposition, adsorption, chemical reaction, mass transfer into the water pool or into the droplets, and pool scrubbing, etc. Table 6 presents a summary of the responses. Except the borax used in the ice condenser of the Loviisa units (Finland) no other passive means have been reported. 

The purpose of this question was to find out whether any additional iodine mitigation measures have been planned or already implemented other than using additives in the spray and containment sump water and using controlled containment venting with a filter. Other than these measures, if already implemented, there are no other mitigation measures reported by the participating organizations. 

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Plant Name Passive iodine removal Iodine mitigation measures implemented/ planned Measurement of airborne iodine (gaseous and particulate form) activity in containment Measurement of controlled iodine (gaseous and particulate form) release into environment Other AM measures to mitigate airborne iodine (in gaseous/particulate form) activity in containment Comments... 

No containment venting filters, no passive iodine removal, no controlled iodine release into environment, no other AM measures to mitigate airborne iodine activity in containment... 

Dukonavy 4 units no intentional passive Iodine removal Hydrazine addition to the spray solutions (implemented) Measurement of aerosol deposition on filters and of gaseous iodine No measurement of corttrolled iodine release No other dedicated AM measures ... 

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 benefits of containment spray systems regarding removal of airborne radionuclides from the atmosphere are not undisputed. Condensing steam in the atmosphere as well as reactions with the walls ensure an effective removal of elemental iodine, whereas organoiodides, as far as they are produced under the prevailing conditions, are decomposed by ionizing radiation. Thus, even without the action of containment spray systems, radioiodine is removed from the containment atmosphere rapidly and effectively by passive reactions. 

When compared to the solid state filters, liquid-phase retention systems prove to have several advantages. The heat introduced by the venting flow as well as by the decay of the absorbed radionuclides is passively removed by boiling of the water phase, the load capacity for aerosols is virtually unlimited, and the retention of volatile iodine compounds can be guaranteed by appropriate chemical conditioning of the water phase. An important prerequisite for high retention efficiency, besides the thermal and radiation stability of the chemicals applied, is a very intimate contact between the gas-steam flow and the liquid phase of the retention system, in order to obtain a fast and effective exchange of matter. 

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