Fukushima TEPCO Nuclear Power Plant Accident (2011)

Accident scenario

After the 11 March 2011 Tohoku Great earthquake and associated Tsunami, TEPCO Fukushima Daiichi nuclear power station (NPS) was severely damaged. The damage resulted in loss of cooling to the three reactor units, and eventually led to overheating, hydrogen explosion and partial melting of the core of reactors, Units 1, 2 and 3. As a consequence, significant amount of radioactive material (¹³¹I, ¹³⁷Cs and ¹³⁴Cs, a total of about 10¹⁷ Bq in iodine equivalent) was released into the environment during 12-15 March. In particular, large amount of radioactive materials were discharged from Unit 2 during the morning of 15 March and diffused to the northwest of the NPS with wind and precipitated onto ground with heavy snow and rain.

Upon notification of the TEPCO on nuclear disaster, the goverment decleared a nuclear emergency at 19:03 on 11 March, and at 21:23 on 11 March ordered the residents within 3 km radius to evacuate (evacuation zone) and poeple withing 3-10 km radius to stay within their houses (sheltering zone). At 18:25 on March 12, the evacuation zone was expandend to 20 km radius, and furthermore at 11:00 on March 15, the sheltering zone was expanded to a 30 km radius. On 22 April, a northwest area out of 20 km limit was established as the delibrate evacuation zone because this area was heavily contaminated and predictive dose to the residents could exceed 20 mSv per year. (See Review by Tanaka, S. Proc. Japan Acad., Ser. B, 88:2012; Pollution map in right panel: MEXT; radioactivity.nsr.go.jp)

More than 90,000 residents evacuated from areas near the NPS. As of 1 December 2011, more than 230,000 people have been screened for radioactive contamination. Some NPS workers involved in the restoration have been exposed. The number of workers and radiation dose accumulated from 11 Mach 2011 to 31 December 2011 are as follow. Chromosome aberration analysis has been made in National Institute of Radiological Science (NIRS) for several of these NPS workers in blood samples obtained from 21 March to 1 July 2011.

.	Radiation exposure (mSv) from 11 March to 31 December, 2011	Number of NPS workers
	100-150	135
	150-200	23
	200-250	3
	250-678	6
	*) Japan Atomic Industrial Forum (http://www.jaif.or.jp/english/news_images/pdf		.

Reference

Suto, Y., Hirai, M., Akiyama, M., Kobashi, G., Itokawa, M., Akashi, M. and Sugiura, N. (2013). Biodosimetry of restoration workers for the Tokyo electric power company (TEPCO) Fukushima Daiichi nuclear power station accident. Health Phys., 105:366-373.
Suto, Y. (2016). Review of cytogemnetic analysis of restoration workers for Fukushima Daiichi nuclear power station accident. Radiat. Prot. Dosimet., 171:61-63.

Chromosome aberration analysis in NPS restoration workers

.
	Subject	Sex	Age	Dosimeter	Acute	No. of	No. of	Dose stimated by dicentric frequency, mSv
	ID	Sex	(yrs)	(mSv)	symptom	cells	dics	mean (95% C.I.)
	Fu-3	M	27	179	-	1,003	7	120 (77, 298)
	Fu-4	M	34	180	-	1,000	7	171 (77, 299)
	Fu-5	M	32	173	-	1,000	5	129 (45, 235)
	Fu-6	M	38	87	-	1,036	1	26 (0, 137)
	Fu-7	M	43	38	-	1,005	4	105 (29, 230)
	Fu-8	M	52	102	-	1,013	4	105 (29, 229)
	Fu-9	M	28	unknown	-	1,035	6	146 (59, 271)
	Fu-10	M	24	17	-	1,037	3	79 (14, 199)
	Fu-11	M	20	4	-	1,042	1	26 (0, 136)
	Fu-12	M	34	unknown	-	1,004	2	55 (3, 174)
									.

The dose estimation by chromosome aberration frequencies has been made by direct inference to the dose-response relationship of dicentric chromosomes by acute Co-60 gamma-ray irradiation in vitro.
It was: Y(Dic)=(0.00015±0.00017)+(0.0302±0.0044)×D+(0.0588±0.0028)×D²……….Y is dicentrics per cell and D is dose in Gy.