The Release of Low-Level Contaminated Water into the Ocean from the Fukushima Daiichi Nuclear Power Plant

April 9, 2011

1. Why the release ?

(1) Some 20,000 tons of water contaminated with radioactive materials had collected in the basement of each turbine building of Units 1-3 at the Fukushima Daiichi Nuclear Power Plant. The water in the Unit 2 turbine building basement was the highest in radioactive content and was leaking into the ocean. In order to avert the outflow of this highly radioactive water from Unit 2 into the external environment, it was necessary to transfer the water promptly to the Central Radioactive Waste Disposal Facility (CRWDF). This in turn required discharging the low-level radioactive water that had been accumulated in the facility. This discharge was an unavoidable step. Had more contaminated water escaped into the ocean, marine pollution would have been much greater.

(2) The concentration of radioactive materials in the water in the CRWDF was relatively low, with the density of iodine 131, for example, measuring 6.3 becquerels per cubic centimeter (Bq/cc). This contrasted sharply with the 5.2 to 5.4 million Bq/cc contained in the water in a pit near Unit 2 or the water being leaked into the ocean.

(3) Units 5 and 6 have remained in cold shutdown (at a low-temperature and therefore stable condition). It was feared, however, that water, probably groundwater in origin, would collect in the basement of these two units and result in the failure of the equipment essential for securing the safety of the units, thereby aggravating the situation. To avoid this scenario and maintain the stability of Units 5 and 6, Tokyo Electric Power Company (TEPCO) decided to take the inevitable step of releasing the low-level contaminated water that had collected around these two units – the water containing radioactive materials that were thought to have leaked out onto the site as a result of the accident.

2. Possible health hazards of the contaminated water discharge

(1) Of the discharged water, the water from Unit 6 was most highly contaminated, with iodine 131 standing at 20 Bq/cc, cesium 134 at 4.7 Bq/cc, and cesium 137 at 4.9 Bq/cc.

(2) An assessment of the health effects of the contaminated water that was released into the ocean based on these figures shows that the daily consumption of 200 grams of fish, 20 grams of marine invertebrates, and 40 grams of seaweed for a period of one year would amount to an annual dose of 0.6 mSv, below the whole body dose limit of 1 mSv for the public. (A single X-ray examination of the stomach exposes a person to about 0.6 mSv.)

3. Radiation monitoring

The government authorities and TEPCO have been stepping up their seawater monitoring. The Nuclear and Industrial Safety Agency has instructed TEPCO to maintain and strengthen ongoing ocean monitoring, investigate and determine the impact of the dispersion of radioactive materials, make every effort to publicize the findings, and study ways to minimize discharges into the ocean. TEPCO has been conducting regular monitoring at a number of locations, including the north and south discharge canals at Fukushima Daiichi, the north discharge canal at Fukushima Daini, and some points 15 kilometers offshore. The waters some 30 kilometers offshore from the nuclear power plants have been monitored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT). These monitoring activities will be further intensified and maintained. These monitoring data are available on the websites of TEPCO and MEXT.

4. The release of low-level contaminated water into the ocean was inevitable. Japan will cooperate more closely in sharing updated information on the status of the nuclear power plants with neighboring countries and the international community as a whole.