Pre-accident weathering of impregnated carbons for iodine removal

The atmospheric degradation of carbons, measured by increased penetration of methyl iodide-131, is an important factor in the operation and replacement of carbon filters. The commercial carbons are distinguished by three types of impregnations: (1) KI + xI2, (2) KI and a tertiary amine, and (3) the tertiary amine alone. A degradation of these carbons has been observed in continuous flows of outdoor air and this degradation has been found to decrease in the sequence (1) to (2) to (3) over exposure times of up to one year. It is concluded that the choice of impregnation formulation in nuclear-grade carbons is an important factor towards high efficiency and long service life. The pattern of air contaminants observed in the above weathering at several locations was subject to considerable fluctuation. The atmospheric conditions, particularly relative humidity and dew point, are important factors when these are in a range in which the relative humidity is above 50%. The seasonal effect has been observed for monthly exposures of KI/sub x/ carbons. Weathering under laboratory conditions suggests that the expected life (24 to 30 months) of the carbon might be realized if only the normal contaminants of the air alone were present. In air flows above 50% RH, and when accidental solvent spills occur, the combined effect with the contaminants accelerates the degradation of the carbons. A determination of iodine-131 penetration is obviously the decisive factor in safety performance. The test results provide a valid indication of the condition of the carbon at the time of the test and if the carbon proves to be unacceptable, the filter system has already operated for a period of time with degraded carbon. It is recommended that some on-line measurement be made that would anticipate a drastic drop in trapping efficiency. A routine in-line chromatographic test procedure with methyl iodide-127 may be one possibility