After earthquake, the discharge of radioactive materials became evident early on the morning of March 12 when the air dose rate measured by a monitoring car near
MP-6(monitoring post No. 6 in the site of Fukushima Dai-ichi NPS) increased. It can be estimated that there was a leakage of radioactive materials from the PCV and a discharge of such materials to the air, as a slight decrease in the PCV pressure was observed in Unit 1 after an abnormal rise at this point. According to an analytical result, that fuel meltdown had already started.
Monitoring measurements performed afterwards at the same point found that the dose rate had increased until the noon of March 12, and D/W pressure had not significantly decreased until around 14:00 despite the venting operation that continued in Unit 1. It could be considered that non-condensable gases, such as noble gases, continued to be discharged from the melted fuel in the reactor into the environment through the S/C.
TEPCO judged at 14:30 on March 12 that venting succeeded and D/W pressure decreased. At this point, it is believed that radioactive materials including iodine, which was neither deposited on the reactor vessel and others, nor absorbed by the S/C, were discharged to the air and, as a result, due to a plume effect, a reading of about 1 mSv/h was observed from a measurement made near MP-4. In addition, a reading of 20 μSv/h was observed from a measurement made at the joint government building of City of Minami Soma by the Fukushima prefectural government that started in the evening, and it is believed that the plume was first blown south by a weak northerly wind and then diffused to the north by a strong southerly wind.
From 08:00 to 09:00 on March 13, the dose rate near MP-1, 4 and 6, increased significantly, and it is estimated that this was caused by the vent operation of Unit 3 performed after its fuel was exposed due to a decrease in the reactor water level. Also, this plume is assumed to have spread to the north under the weather conditions prevailing during this period, in which a weak westerly wind turned southerly. A measurement by Minami-soma City indicated a rise of about 1 μSv/h in the dose rate. A significant rise in the dose rate was confirmed near MP-1, 4 and 6 corresponded to the multiple decreases in the D/W pressure of Unit 3.
A rise in multiple dose rates was confirmed in the morning of March 14, but no information was obtained on events that might have been related to the discharges from each plant. For this reason, although causes of the dose rate increases are uncertain, it is plausible to consider that one of the causes can be the re-floating of deposited radioactive materials because the background dose rate increased at each measuring point due to radioactive materials discharged up to March 13.
An air dose rate of about 3 mSv/h was measured near MP-6 at 21:00 on March 14. This rate decreased once but increased again after 06:00 on March 15, and a dose rate of about 12 mSv/h was measured at 09:00 on the same day. In Unit 2, a decrease in D/W pressure was observed due to a wet venting at 21:00 on March 14, and it is estimated that radioactive materials were discharged from Unit 2 because of a blast sound from the unit at around 06:00 on March 15 and a subsequent S/C pressure decrease. At around the same time, however, an explosion occurred in the reactor building of Unit 4, thus a clear distinction cannot be made between them. Since wind often blew from the north in this period, the plume was very likely to have blown to the south, and agencies including the Japan Atomic Energy Agency (JAEA) in Tokai village, Ibaraki prefecture observed a rise in the dose rate and detected radioactive iodine, etc. in the atmosphere.
In addition, an increase in the air dose rate was observed near MP-6 at 23:00 on March 15 and at 12:00 on March 16. D/W pressure decreases were observed in Unit 3 and Unit 2 at respective times. It is estimated, therefore, that discharges occurred from Unit 3 and Unit 2 at these respective times.
