Fukushima Daiichi Nuclear Power Plant Accident Analysis due to Atmospheric Dispersion

Language: English / Japanese

1. Summary

This study shows simulation results of ¹³⁷ Cs deposition released from Fukushima Daiichi Nuclear Power Plant Accident on March 12, 2011.

Simulation from March 11th to April 6th, 2011 was conducted and ¹³⁷Cs integrated deposition was compared to soil concentration measured within Fukushima prefecture on April 6th, 2011
(Fig 1,Fig 2).

The soil concentration distribution of the simulation reproduced the distribution in the vicinity of Iitate-mura and Koriyama well. However, some discrepancy was found, for example, simulation results showed relatively high concentration while measured data was relatively small in the vicinity of Iwaki.

Based on these simulation results and dust sample measurements, reconsideration of release rate is ongoing. Although the tendency of how radioactive plume dispersed will not drastically change, absolute value might be changed by reconsideration of source term.。

2. Conditions

¹³⁷Cs deposition calculation conditions

Items	Contents	Reference
Models	Meteorological model：RAMS Particle transport model：HYPACT	ATMET
Period	10:30 March 12th, 2011 - 0:00 April 6th
Horizontal lattice	Grid1：600×600[km]、10km mesh Grid2：114×104[km]、2km mesh
Vertical lattice	σ-z coordinate system, Grid with 20km altitude divided into 30 layers
Release point	Fukushima Daiishi Nuclear Power Plant Latitude：37.421389 deg Longitude：141.033611deg
Source nuclide	137Cs
Release rate(※1)	137Cs：1/10 of 131I
Release height	Normal：30m Explosion：30m～200m	Press release
Depositin velocity	1.0[cm/s]
Washout parameter α	1.2×10-4
洗Washout parameter β	0.8
Number of released particles	ca. 25 million particles
Meteorological data	MSM (Meso Scale Model) isobaric surface data and AMeDAS precipitation(※2)	Japan Meteorological Agency
Topography data	50m mesh numerical map	Geospatial Information Authority of Japan

※1：Based on dust sampling results at different places, reconsideration is ongoing.
※2：Precipitation map made from AMeDAS data was derived and applied because precipitation data produced by a meteorological model was not sufficient.

3. Results

3.1 Comparison to measured data

Comparison to measured data was conducted using soil concentration (publicly available) of each city in Fukushima prefecture measured by Fukushima prefecture government (See Fig.2). Measurement points are shown in Fig. 1.

Fig. 1 Measurement points for radioactive material in soil in Fukushima prefecture.

 

(a) Measured soil concentration of 137Cs as of April 6th, 2011. Rice paddy converted fields, (Bq/m2), Bulk density 1g/ml (assumption)	(b) Calculated integrated deposition (from 10:00 March 12th to 0:00 April 6th, 2011)
Fig. 2 Comparison of 137Cs integrated deposition [Bq/m2] between measurement and calculation in Fukushima prefecture.

Fig. 3 Calculated ¹³⁷Cs integrated deposition [Bq/m²] in the greater Tokyo area (from 10:00 March 12th to 0:00 April 6th, 2011)

 

3.2 ¹³⁷Cs that reached the greater Tokyo area

Fig. 4 shows the time series of calculated integrated deposition. This figure evidently shows ¹³⁷Cs deposition occurred from March 15th to 16th and March 21st in the greater Tokyo area.

It is estimated that radioactive plume released from Fukushima Daiichi Nuclear Power Plant passed the greater Tokyo area by north wind and east wind from the Pacific Ocean on March 15th (Refer to the concentration distribution).

Similar wind directions were observed on March 21st, with the addition of rain in the greater Tokyo area.

Calculated time series of ¹³⁷Cs ground level air concentration at Kashiwa City Hall overlaid on measured precipitation at nearest AMeDAS in Abiko is shown in Fig.5.
The peaks of air concentration and precipitation were in good agreement on March 21st in Fig. 5, therefore it suggests that most of total amount of ¹³⁷Cs deposited in Kashiwa was caused by precipitation which occurred from 10:00 to 12:00, March 21.

Time series of ¹³⁷Cs deposition calculated at various locations in Fukushima and the greater Tokyo area.

Fig. 5 Calculated ¹³⁷Cs ground level air concentration at Kashiwa City Hall and measured precipitation at nearest AMeDAS in Abiko.

Revisions

May 30th, 2011	Misspelling corrected for Abiko in Japanese Kanji letters.