Occurrence of the Accident and Release, Transport, and Washout of the Radiation Plume

From the severe-accident analysis based on the MAAP or MELCOR code, it is reported that the core damage incident for each unit happened approximately at the period listed in Table 3.1.

Figure 3.7 shows the temporal evolution of the ambient dose rate observed inside the 1F site, in nearby and distant cities, together with the wind conditions

Table 3.1 Core damage progression simulated by MAAP and MELCOR codes

Hours from scram (March 11, 2011, 14:46 JST)

Fig. 3.7 Temporal evolution of the ambient dose rate at a 1F monitoring post, c nearby, and d distant, b wind direction indicated by sine and cosine components (data from the University Tokyo and those publicly released by TEPCO and MEXT)

at the 1F monitoring posts (MPs). Note that in 1F, a monitoring car was used because the MPs were not working due to the power failure. The direction and speed of the wind were recorded with a 16 point compass, e.g. north (N), northnorthwest (NNW), northwest (NW), west-northwest (WNW), south (S), east (E), etc., at the same time as the radiation dose rate at the monitoring posts/car in 1F. In order to compare the temporal evolution of the wind vector with other events, we represented the wind direction θ as the sine and cosine components of the direction (orienting to the east being 0°, while orienting to the north being 90°).

In the present analysis, sin(θ) > 0 corresponds to the direction from the land to the ocean, while cos(θ) < 0 corresponds to the direction to the south (toward Tokyo).

At the 1F site, the dose rate began to increase from 4:04 on March 12 (13 h after scram), which presumably coincides with the incidents in 1F1 (Fig. 3.7, A). First, venting and the following hydrogen explosion was presumed to be the cause of the increase in the dose rate in Minami-sōma, 26 km north of 1F, on March 12 (Fig. 3.7, B). Note, however, that precise data recorded every 20 s (telemeter system) disclosed in November 2013 revealed that the rapid increase of dose rate at Kamihatori 6 km north west of 1F coincided with the attempt to vent around 14:30 while the hydrogen explosion at 15:36 did not cause apparent increase in the dose rate around 1F.

The core damage incident in 1F3 occurred on March 13. The venting of the PCV of 1F3 was operated several times in the depressurizing procedure of the RPV during March 13 and 14, and the hydrogen explosion occurred at 11:01 on March 14 (68 h after scram). The fact that the wind was directed to the east (sea direction) during this period was, so to say, one consolation in the disaster (Fig. 3.7, C). However, the release of the radioactive materials in this event was considerably smaller than the following incident in 1F2.

The incident at 1F2 caused the most serious release of radioactive nuclides. The suspected leakage of the PCV caused the release of radioactive gases around 21:30 on March 14 (Fig. 3.7, D1), which was several hours before detection of the sound of the explosion or rupture at the suppression chamber (SC) of 1F2 (at 6:10) (Fig. 3.7, D2). Note that for that time, we have not enough evidence to tell whether the event was an explosion or a rupture. On October 2, 2011, it was reported that the accident investigation commission of Tokyo Electric Power Company (TEPCO) determined from the signals recorded on a quake meter that the hydrogen explosion might not have occurred in 1F2. It is more likely that the sound was delivered from the hydrogen explosion at 1F4, presumably caused by the escaped hydrogen from 1F3 through a duct.

The radioactive leakage from 1F2 in this period (Fig. 3.7, D1), presumably caused by opening the safety relief valves (SRVs) followed by the leakage through the damaged PCV, initiated the radiation plume toward the South direction, and the increase in ambient dose was observed as the plume propagated and passed through the locations at a speed of about 10 km/h (Fig. 3.7, E). The radiation plume was observed even in Tokyo (SW 230 km of 1F) and Shizuoka (SW 360 km).

Note that by using SRVs to depressurize the RPV, external water injection becomes possible. However, flash boiling (see Appendix B) can accelerate the core exposure. SRV operation after core damage can therefore cause a significant transport of radioactive materials out of the RPV into the SC.

Figure 3.8 shows the temporal evolution of the ambient dose observed at different locations after the initial prominent radioactive release on March 15.

[North <50 km from 1F]

The plume on March 15 (Fig. 3.7, D1) soon passed and the ambient dose rate decreased rapidly, particularly in distant locations. However, the plume initiated by the SC rupture (Fig. 3.7, D2), propagated to the Northwest direction and caused fallout/washout/rainout due to rainfall and/or snowfall. This contributed to the significant increase in dose rates in these areas, such as Iitate-mura (NW 40 km) (Fig. 3.8, F).

Although the origin of the later peaks at the main gate (Gate M) of 1F, indicated in Fig. 3.7 as D3 and D4, has not yet been rigorously identified, the release of radioactive materials still continued even after March 16. As a result, rainfall over a wide area to the south washed out the plume into the soil, leading to a significant increase in the ambient dose rate. This time the decrease in the dose rate was dominated by the radiation decay of the radioactive nuclides. On March 18 and 19, the wind blew toward the North direction, and several dose rate peaks were observed in Minami-sōma (N 30 km). However, presumably because there was no rainfall, these plumes did not deposit material onto the ground (Fig. 3.8, G).

On March 21, although rain fell in Fukushima, the plume did not deposit material onto Minami-sōma, because the wind was heading south (Fig. 3.8, H).

This suggests that ground contamination occurred due to both the plume and rainfall.

[South 50–100 km from 1F]

Ibaraki prefecture, located south of Fukushima, was subjected to a considerable degree of washout/rainout on March 16 and 20, that can be seen from the increase of the baseline of the ambient dose rate, having a decay timescale of 131I, 8.02 d (Fig. 3.8, I).

Just after the delivery of the plume, the decay of the short-lifetime radioactive nucleus was also observed, such as 135I (6.7 h), or 132I in radiative equilibrium with 132Te (78 h) (Fig. 3.8, J).

[South >100 km from 1F]

In Tokyo, rain on March 21 washed out the plume and increased the radiation dose rate, which led to a minor panic when 131I was detected from the tap water source (Fig. 3.8, K).

In Shizuoka, at 360 km from 1F, one can see from the time difference between the rain and the increase in the dose rate that the plume arrived during rainy weather (Fig. 3.8, L).

This suggests that the plume remains no longer than a few days when new plumes are not delivered.

This speculation agrees with the observation of the radioactive material level of fallout in Tokyo per day [10]. Usually the fallout lasted around 3–4 days in

Fig. 3.8 Temporal evolution of the ambient dose rate of distant locations. Right axis corresponds to rain. Note Data for office 0.5 km from 1F divided by 10 to scale its temporal behavior consistent with that of main gate (GateM) and west gate (GateW)

March and April. For the purpose of protection from the radioactive exposure, it is preferable to watch the dose rate near one's location and the rain for a few days after passage of the plume. In particular, the rain causes cesium deposition onto soils, while removal of the deposited cesium is difficult. Therefore, we think that covering playgrounds with plastic sheets before it rains might be effective as an emergencyprotectionagainstgroundcontamination—evenamattressorblanket is better than nothing. Some prefectural offices and nuclear power plants provide real-time dose rates. It might be preferable if one could watch these data together with rain and wind speed given in weather forecasts. At the same time, it might be required that not only the government but also scientists provide appropriate information about how to interpret the monitored data.