Abstract
The containment atmospheric monitoring system (CAMS) readings for the drywell (D/W) and the suppression chamber (S/C) were obtained during the accident at the Fukushima Daiichi Nuclear Power Plant (1F) Unit-2, but the radioactive materials (RM) migration behavior within the D/W that caused the change in the readings is not clear. Therefore, an attempt was made to estimate the behavior of RM migration by analyzing the flow and radioactive source distribution in the D/W between 18:00 and 22:40 on March 14, 2011, when the core damage progressed.
The RM used to evaluate the migration behavior was assumed to be CsI aerosol, which was considered to be the representative radiation source during the relevant period. The deposition and entrainment of the aerosol to the D/W wall and internal structures were considered. The CsI aerosol distribution in the D/W obtained in this way was combined with the results of the preceding MCNP evaluation to estimate the D/W CAMS readings, and the amount of CsI entering the D/W was adjusted until it was roughly consistent with the actual measured value at 22:40.
As a result, the calculated D/W CAMS readings tended to increase due to the deposition of RM in the D/W atmosphere on the walls and other structures. It was suggested that the increase in the measured readings may have captured this effect. On the other hand, the rate of increase in the readings was underestimated. To quantitatively reproduce this rate of increase, further investigation is needed into the uncertainty of boundary conditions such as RM migration from the reactor pressure vessel (RPV) and the S/C, models such as RM deposition, entrainment, particle growth and their differences due to chemical forms, and the method of converting radioactive source distribution to CAMS readings.
