Radionuclide transfer in underground media, (2)

This paper describes adsorption coefficients (Ka) of elemental iodine gas, I2, to such typical ground-formation rocks in Japan as granite, tuff and sandstone, and also predicts the movement of I2 through the underground rocks for the purpose of risk assessment of radioiodine released from the underground nuclear station. Crushed rock samples were investigated by using the culumn technique at various steps of temperature and gas concentration. Main results are as follows. 1) The 1% breakthrough time of I2 through 100 m-long underground calculated with the packed bed theory, is proportional to the Ka value, and consequently the Ka is a main factor for the determination of the breakthrough time. 2) The Polanyi's adsorption theory is in good agreement with our experimental results and adsorption amounts per unit surface area of the underground rock are fit to the Polanyi equation. 3) The Ka value depends on temperature and varies as -3.2%/0C. 4) Humidity of I2 gas affects the Ka value and this effect corresponds to surface roughness of the particle. 5) A simple equation, which can predict the Ka value at a particular temperature and ground formation, is proposed on the basis of considering the size distribution. 6) The Ka value of the granite formation is estimated about 103 cm3/g with the abovementioned equation, and this value agrees with the value predicted with the hydraulic conductivity. (author)