Carbon filters--adsorbers are used in the third and fourth power-generating units of the second phase of the Leningrad nuclear power plant for catching gaseous and aerosol forms of radioactive iodine. They are installed in VTs-4 and -8 ventilation systems, where air enters from the space below the equipment and enclosures of the bottom water pipes, blowoff from the stub of the unloading-loading machine and the tank for emptying it, as well as cases with nonhermetic fuel assemblies. The airflow into ventilation systems equals 2300-8300 m3/h or, on the average, 6000 m3/h [I]. Iodine filters are installed three to four filters in parallel. The temperature of the filtered air equals 60-70~ and the relative humidity equals 90% (at 20 ~ The AUI-1500 carbon iodine absorber has the following characteristics: capacity (with velocity in the free cross section 0.4 m/sec) 1500 m3/h, height of the adsorption layer 40 cm, carbon mass 200 kg, absorber resistance 2.35-2.55 kPa, working temperature <60~ and dimensions 131.5 x 115 x 81.25 cm. Impregnated SKT-3I activated carbon is used as the adsorbent. The rated efficiency of radioiodine removal equals 98-99% [2]. However, this carbon is not produced commercially. For this reason, prior to August 1996 in the case of the fourth power-generating unit and up to now in the case of the third power-generating unit, AUI-1500 filters were filled with nonimpregnated SKT-3 carbon with the following characteristics [3]: bulk density 0.472 g/cm 3, apparent density (no voids) 0.75 g/cm 3, specific surface area 1100 m2/g, and fractional composition 3.6-1 mm (97%).In designs, the iodine carbon absorbers must meet several requirements. The adsorber must work in the entire temperature and humidity interval characteristic for normal operation and operation during an accident. Ordinarily, the maximum relative humidity is taken to be 98 %. The most important criterion is the duration of the contact (ratio of carbon volume to volume velocity of the flow or the height of the fill to the linear velocity), which should equal 1 sec and longer but not shorter than 0.5 sec. The purification efficiency increases with contact duration [4].The possibility of carbon dust being carried out of the filter imposes an upper limit on the velocity of the gas: the average gas velocity should not exceed 0.6 m/sec. The mass of the carbon fill must be calculated for absorption of iodine in all of its chemical forms. Furthermore, the geometry and dynamic resistance of the fill (the layer must be at least 30-cm high), the exposure dose from radionuclides accumulated in the filter, the ease of replacing carbon in the filter, and the possibility of leakage must all be taken into account.The efficiency of radioiodine absorption in carbon filters in VTs-4 and -8 ventilation systems was determined at the Leningrad nuclear power plant in 1993-1995 (Table 1). The 131,133I concentration in air was measured by the standard method by a team from the OOT and TB Office. The 131'133I removal factor calculated from the ratio K = Cin/Cou...
