Topics: behavior of radioactive iodine in power reactor building

Some findings were obtained a few years ago in the examination on the behavior of radioactive aerosol such as radioactive iodine around the pool which has been used for fuel inspection or replacement above the reactor in the reactor building at the time of the regular inspection for a power reactor. The gaseous radioactive materials in the reactor seems to be released and diffused in the building quickly after opening the lid of the reactor. Then, a May-Pack type air sampler was made, and the concentration of radioactive iodine 131I was determined by measuring activity in each layer separately through absorbing air by installing it under the venting duct. The iodine concentration level little increased during several hours after opening the lid, but greatly increased its level in air after the lid was completely removed and moved. With this, it was found that the radioactive iodine was released as the aerosol in the condition of considerably strong particle property. Comparison of 131I change with that of 54Mn in course of time after opening the lid allows the estimation that the mechanism of generation of both aerosols is similar, and the generating rates of both aerosols are approximately constant, while short-lived 131I is decaying naturally and faster than 54Mn. It was also found in this examination that the radioactive iodine can become the main hazard in regular inspection only for about a month in the reactor building environment, and after that, 54Mn is rather the predominant contaminant. (Wakatsuki, Y.)