1. Current Situation

Overall, the situation at the Fukushima Daiichi nuclear power plant remains very serious.

The IAEA receives information from various official sources in Japan through the Japanese national competent authority, the Nuclear and Industrial Safety Agency (NISA). This Update Brief is based on information issued by the IAEA Incident and Emergency Centre up to 17:00 UTC on 11 May 2011.

Fukushima Daiichi Nuclear Power Plant Status

The three attached charts, one for each of the Units 1 - 3, track progress made towards fulfilling the three basic safety functions of the IAEA safety standards: prevention of criticality, removal of decay heat and mitigation of radioactive releases. The chart replaces the three-colour status chart that was used previously. The charts are cross-referenced to the Tokyo Electric Power Company (TEPCO) Roadmap plan to bring the nuclear reactors and the spent fuel pools at the Fukushima Daiichi plant to a stable cooling condition and to mitigate radioactive releases.

Nitrogen gas is still being injected into the containment vessel in Unit 1 to reduce the possibility of hydrogen combustion inside the containment vessel.

TEPCO has elaborated a measure to fill the primary containment vessel of Unit 1 with water up to a level above the reactor fuel rods. This measure is intended to provide stable cooling of the reactor and reactor pressure vessel. The planned steps are:

 1. Reduce radiation levels in the reactor building by installing a filtered air circulation system (completed), remove rubble, decontaminate and install shielding;

 2. Recalibrate existing reactor pressure vessel water level and pressure instruments and install additional reactor pressure vessel water level gauges to improve monitoring of conditions inside the reactor pressure vessel;

 3. Install primary and secondary closed-loop cooling systems;

 4. Flood the containment to provide a water supply for the primary system.

In Units 1, 2 and 3 fresh water is being continuously injected into the reactor pressure vessel; temperatures and pressures remain stable.

To protect against potential damage as a result of future earthquakes, TEPCO started work on 9 May to install a supporting structure for the floor of the spent fuel pool of Unit 4.

Fresh water is being injected as necessary into the spent fuel pools of Units 1 - 4.

Stagnant water with high-level radioactivity in the basement of the turbine buildings of Unit 1, Unit 2 and Unit 3 is being transferred to the condensers, the radioactive waste treatment facility and temporary storage tanks. Stagnant water in the basement of the turbine building of Unit 6 is being transferred to a temporary tank. Countermeasures against the outflow of water to the sea and to prevent and minimize the dispersion of radionuclides in water have been put in place.

Full-scale spraying of anti-scattering agent is continuing at the site with the use of both conventional and remote controlled equipment.

2. Radiation Monitoring

Deposition in 47 Prefectures

The daily monitoring of the deposition of caesium and iodine radionuclides for 47 prefectures is continuing. For the period 5 - 10 May, deposition of I-131 was detected in three prefectures, with values ranging from 1.5 Bq/m2 to 4.5 Bq/m2. Deposition of Cs-137 was detected in eight prefectures in the same period, the values reported ranging from 3 Bq/m2 to 44 Bq/m2. The reported values show that variable but low level deposition of radionuclides was still occurring in some prefectures.

Gamma Dose Rates in 47 Prefectures

Gamma dose rates are measured daily in all 47 prefectures. On 10 May the value of gamma dose rate reported for Fukushima prefecture was 1.7 µSv/h. In all other prefectures, reported gamma dose rates were below 0.1 µSv/h with a general decreasing trend.

Gamma Dose Rates in Areas More Than 30 km from Fukushima Daiichi Plant

Gamma dose rates reported specifically for the monitoring points in the eastern part of Fukushima prefecture, for distances of more than 30 km from the Fukushima Daiichi plant, showed a general decreasing trend, ranging from 0.1 µSv/h to 20.3 µSv/h, as reported for 10 May.

Maps of gamma dose rates, deposition of Cs-134 and deposition of Cs-137 within the 80 km zone around the Fukushima Daiichi plant were produced by means of aerial gamma ray monitoring by the Nuclear Safety Technology Centre of MEXT and the United States Department of Energy (DOE). The maps show that the results obtained are consistent with all previous measurements of deposition in soil and of dose rates.

Air Concentrations of Radionuclides On-site at Fukushima Daiichi Plant

On-site measurements at the west gate of the Fukushima Daiichi plant indicate the presence of I-131 and Cs-137 in the air in the close vicinity of the plant (within approximately 1 km). The concentrations in air reported for 10 May were 4 Bq/m3 for total I-131 and 16 Bq/m3 for total Cs-137. The values observed in the previous days show daily fluctuations with an overall decreasing tendency.

Concentrations of Radionuclides in Drinking Water

As of 10 May, the restriction on the consumption of drinking water relating to I-131 - which had been applied since 1 April as a precautionary measure for one remaining location (the village of Iitate in Fukushima prefecture), and only for infants - was lifted.

Food Monitoring and Food Restrictions

Food Monitoring

The Ministry of Health, Labour and Welfare reported on the radionuclide test results for 4 - 11 May for 436 food samples from 14 different prefectures. The prefectures of Chiba, Fukushima, Gunma, Ibaraki, Miyagi, Niigata and Tochigi accounted for more than 90% of the reported food analysis results, with most food monitoring concentrated in Fukushima prefecture (52% of samples analysed and reported until 11 May). In two prefectures (Fukushima and Kanagawa), 17 out of 436 (3.9%) samples were found to have radioactivity above the Japanese regulation values.

In Fukushima prefecture, levels in 212 (93%) of the 228 samples reported were below the regulation values for I-131 and radioactive caesium. However, 16 of the 228 samples (7%) exceeded the regulation values set by the Japanese authorities for Cs-134/Cs-137, including bamboo shoots (eight samples), shiitake mushrooms (four samples), ostrich fern (two samples), turnip (one sample) and sand lance fish (one sample).

In Kanagawa prefecture, unprocessed tea leaves were the only food that exceeded the regulation values set by the Japanese authorities for Cs-134/Cs-137 (one of thirteen samples, i.e. 7.7%).

Food Restrictions

As of 11 May, the only food restrictions remaining are in Fukushima prefecture and for the cities of Kitaibaraki and Takahagi in Ibaraki prefecture.

In Fukushima prefecture there are restrictions on the distribution and consumption of sand lance fish. In specified areas of Fukushima prefecture there are also restrictions on the distribution of raw unprocessed milk, turnips, bamboo shoots, ostrich ferns and shiitake mushrooms, and restrictions on the distribution and consumption of specific non-head type and head-type vegetables (e.g. spinach and cabbage), flowerhead brassicas (e.g. cauliflower) and shiitake mushrooms.

In Ibaraki prefecture there is a continuing restriction on the distribution of spinach produced in the cities of Kitaibaraki and Takahagi.

3. Marine Monitoring

The marine monitoring programme is carried out both near the discharge areas of the Fukushima Daiichi plant by TEPCO and at off-shore stations by MEXT. The increase in the radioactivity in the marine environment had occurred by aerial deposition and by discharges and outflow of water with high level radioactivity.

Seawater Monitoring

The activity concentrations of I-131, Cs-134 and Cs-137 in seawater close to the Fukushima Daiichi plant at the screen of Unit 2 have been measured every day since 2 April. Concentrations of Cs-134 and Cs-137 decreased from initial values of more than 100 MBq/L to less than 10 kBq/L on 30 April and have remained constant at this level to the present.

Levels of I-131 on 7 May remained at around 200 Bq/L.

As of 7 May, no relevant changes in the radionuclides concentrations at the other TEPCO sampling positions have been reported.

Monitoring performed by MEXT at off-shore sampling positions consists of:

   1. Measurement of ambient dose rate in air above the sea;

   2. Analysis of ambient dust above the sea;

   3. Analysis of surface samples of seawater;

   4. Analysis of samples of seawater collected at 10 m above the sea bottom and in a mid-layer.