Radioactive contamination processes during March after the Fukushima accident: What does atmospheric electric field measurements tell us? M. Takeda 1, M. Yamauchi 2, M. Makino 3, T. Owada 4, and I. Miyagi 3 EGU (XL355, ) (1) Kyoto University, Japan (2) Swedish Institute of Space Physics (IRF), Kiruna, Sweden (3) National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan (4) Kakioka Magnetic Observatory, Japan Meteorological Agency, Ishioka, Japan

Total release: Bq for 131 I & Bq for 137 Cs ~ 15% of Chernobyl Accident over > 100 km radioactive plumes

Detection principle Figure 2: Radiation from the radioactive dust (grey hatched area) increases ion density (~ electric conductivity), re-distributing potential drop between the ionosphere (200 kV) and ground (0 V) to the higher altitude. Blue dashed lines denotes equi-potential surfaces. Large-scale monitor & combine PG (Fig.2 & 5) Ion density n: dn/dt = q - αn 2 - βnN Estimate initial I/Cs ratio Confirm dosimeter result Figure 1: Three different types of measurements

Figure 3: (a) Red: downward atmospheric electric field (PG) at Kakioka measured by in- house sensor. The data gap (11-13 March) is due to the power failure. Blue: rainfall at Kakioka. (b) Radiation dose rate at the nearest location from Kakioka (cf. Fig 5 for location). PG dropped to near-zero (ion density increase by more than factor 10) twice, at 14 March and at 20 March (green arrows). Radiation dose rate and the PG show independent behavior. Figure 4: PG drops after nuclear test (Harris, 1955) and Chernobyl Accident (Tuomi, 1988) Rain 24 hours 2 weeks Helsinki, 1986 Tucson, 1952

Figure 5: Locations of the failed Fukushima Dai-ichi Nuclear Power Plant (FNPP-1), Kakioka (about 150 km SSW), and other measurements. Figure 6: Since PG reflects altitude distribution of the ion, the same radiation dose level gives different PG for different forms of radioactive contamination (black area). Inversely, contamination form can be estimated from the combination of PG and dose rate.

(a) PG at Kakioka. (b) radiation dose rate. No rain at Kakioka except 20 UT on 15 March. The PG-drop at 21 UT on 14 March is traced back to the increase of the dose rate at 20 UT (Mito), 19 UT (Kitaibaraki), and 17 UT at Dai-ni, as indicated by light-red arrows (cf. Fig 5 for location). However, on 16 March, the PG increased after 2 UT when the first strong wind (> 5 m/s in average) blew although the radiation dose rate at Mito increased. The finite PG continued afterward (20-30 V/m). The backup instrument showed the same PG pattern.  Wind most likely brought radioactive material to few km, to re- redistribute the PG. (a) (b) Activity of nuclear reactor Figure 7:

(a) PG and rainfall at Kakioka. (b) radiation dose rate and rainfall at Mito (cf. Fig 5 for location). Final settlement to PG = 0 V/m after the first substantial rain on March  Ions (i.e., source of the ionizing radiation) at higher altitude are removed by the rain (cf. Fig 6). Slightly before the rain, large radioactive plume arrived (light-red arrow) carried by strong wing blew from the northeast (the direction of the FNPP-1). The plume simply passed above, but the rain deposited substantial mount to the surface, resulting a jump of the radiation dose rate from 0.2 µGy/h to 0.4 µGy/h. Such a jump is also seen at 2 UT on 20 March, but the amount of this dry deposition is very little. (a) (b) Figure 8:

More about plumes exponential decay = 8-day as expected initial I/Cs ratio soil sampling Figure 9: soil sample data are classified into different regions.

Faster decay at Takahagi is due to higher I/Cs ratio. Combining with the soil sampling data (Fig. 9) which shows the same I/Cs ratio between Hirono and Iwaki (Iwaki has the same decay rate as Kitaibaraki), the plume on 20 March had different I/Cs ratio between it east part (red route in Fig. 11) and west part (orange route in Fig. 11). Thus the data can even show the internal structure of the plume. Figure 11 Figure 10

Other features in Fig ,132 I 134,137 Cs Daily variation of PG in sunny days (purple arrows in Figs. 3 and 12) disappear and recover, and disappear again (yellow allows in Fig. 3). The peak at local noon (Fig.12) means that the driver is daily convection. The disappearance is simultaneous with the reset of baseline PG  new deposition, either from trees or from the FNPP-1. Resetting ceased end of April There are minor plumes from FNPP-1 until summer 2011, but they are not visible in the PG data al. Increase of PG during April  ? Figure 13: Low PG even after 1 year because of radiocesium (half-life is 134 Cs = 2 yr, 137 Cs = 30 yr). Figure 12

Summary: Combination of Atmospheric Electric field (PG) data and radiation dose rate data showed that the major southward depositions took place twice, dry one on 14 March, and wet one on March. Even the latter one did not settle the radioactive materials as firmly as the Chernobyl case, causing re-suspension afterward. Figure 14

Summary Kakioka's PG & Radiation dose rate data at different places are compared to study the radioactive deposition processes. PG data give independent information from the radiation dose rate data. Dry deposition on 14 March was driven by the surface wind, leaving radioactive materials suspended above the surface (= can easily be lifted up). Wet deposition on 20 March washed the suspended radioactive down to the surface. Wet deposition does not mean firm settlement (= can be re- suspended by wind). It is recommended that all nuclear power plant to have a network of PG observation surrounding the plant. Combination of soil sampling and radiation dose rate revealed I/Cs ratio gradient in a single plume

MainSub ElectrometerElectrostatic sensor type Field mill type Collector TypeWater-dropperMechanical Height2.55 m1.00m Separation from the wall 1.17 m Sampling1 sec LatitudeLongitude 36  13'56"N140  11'11"E PG measurement at Kakioka

We derive spread & deposit of Radioactive materials from available data Strong public demand to scientists on spread & deposit of the radioactive materials to protect their health (internal dose problem). Accident took place in a dense network of measurements, producing a vast data first time in history (new research field). Estimating particle motion from multi-instrument / multi-point measurements is a geoscience problem (our duty as geoscientist). Urgently needed task.

After May 2011 Minor re-suspension (release) from the FNPP-1 continued even June 2011 CTBT Takasaki-station

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