JAPAN | Situation clearer, but problems remain after 4 months of nuclear crisis

Posted on July 12, 2011


JAPAN | ASAHI SHIMBUN | 12 July 2011

The situation seems to have stabilized somewhat at the Fukushima No. 1 nuclear power plant after four months of fears and uncertainties–and an early spewing of radioactive materials that spread to 12 prefectures.

But Tokyo Electric Power Co., operator of the plant, and government officials still face a host of problems. Early damage to the plant continues to present a risk of further radioactive leaks. And questions remain over the actual extent of damage to human health and the environment from the estimated 770,000 terabecquerels of radioactive materials released into the air so far from the damaged reactors.

The amount from the Fukushima plant is more than 10 percent of the 5.2 million terabecquerels discharged during the Chernobyl nuclear disaster in 1986.

Officials said radioactive materials escaped into the atmosphere when TEPCO workers vented reactor containers to reduce the pressure inside.

But most of the radioactive materials were emitted on March 15, when a hydrogen explosion in the suppression pool, which is linked to the containment vessel, took place in the No. 2 reactor building around 6 a.m. Another explosion hit the No. 4 reactor building around the same time.

Earlier, on March 12, a hydrogen explosion occurred at the No. 1 reactor building.

After the explosions on March 15, readings of 11,930 microsieverts of radiation per hour were recorded around 9 a.m. near the front gate of the plant, the highest measurements so far on the edge of the plant’s compound.

To estimate the spread of radioactive materials from March 15 through March 16, the Japan Atomic Energy Agency (JAEA) used a simulation that took into account weather conditions and geographical data.

The study covered a 190-kilometer-by-190-km area that was split into 1-square-km units.

The results of the simulation were nearly identical to the high levels of iodine and cesium measured in Shirakawa and Koriyama cities in the Nakadori area, as well as Iitate village located northwest of the Fukushima plant.

Monitoring by the Ministry of Education, Culture, Sports, Science and Technology also found high radiation levels in 12 prefectures, including those in the Kanto region.

Radioactive materials currently in the soil fell to the ground with the rain around those days, officials said.

To determine the amount of radiation exposure to humans in the two months following the explosions, JAEA researchers used the System for Prediction of Environmental Emergency Dose Information (SPEEDI) to cover all of Japan, split into 10-km-by-10-km areas.

According to JAEA, only people in parts of eastern Fukushima Prefecture were exposed to radioactive iodine and cesium levels exceeding the annual limit of 1 millisievert.

Areas where people were exposed to more than 0.01 millisievert of radiation through intake of food, water and other materials included the central and eastern Kanto areas and the southern Tohoku region.

However, the SPEEDI simulation was based on the unlikely scenario of people remaining outdoors for 24 hours a day.

To determine actual radiation exposure levels from 9 a.m. on March 12 to midnight on March 17, the government plans to conduct long-term health studies on people in Fukushima Prefecture using data kept by the prefectural government.

In health checks on 2 million people, the prefectural government has gathered data on what each person was doing during every hour of that period.

That data, combined with actual hourly radiation readings, will make it easier for researchers to accurately estimate the level of radiation exposure for each person.

For areas where radiation measurements were not conducted, the researchers will use figures calculated by SPEEDI.

The radiation level near the front gate of the Fukushima plant is currently about 30 microsieverts per hour, or one-400th of the peak on March 15.

Under the road map to bring the reactors at the Fukushima plant to a cold shutdown, TEPCO will steadily decrease the amount of radioactive materials released by mid-July.

It will also estimate the amount of radioactive materials discharged from each reactor and make public the results.

Radioactive materials can still easily leak into the atmosphere from the plant because the roofs of the No. 1, No. 3 and No. 4 reactors were destroyed in the hydrogen explosions, leaving pools of spent fuel rods exposed to the air.

TEPCO is working to cover the damaged reactor buildings with specially coated polyester fiber sheets to prevent radioactive materials from escaping. The work will be completed on the No. 1 reactor building by the end of September, according to TEPCO’s road map, followed by similar work at the No. 3 and No. 4 reactor buildings.

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