The IAEA provided the following information on the status of nuclear safety in Japan:
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 16:00 UTC on 31 May 2011.
1. Fukushima Daiichi Nuclear Power Plant Status
Tables 1 - 4 track progress for Units 1 - 4 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 tables replace the three-colour table 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.
On 17 May 2011, TEPCO provided a status report against the TEPCO Roadmap showing progress since the Roadmap was issued on 17 April 2011. While the basic policy and targets defined in the Roadmap remain, several changes were made to account for new information obtained and progress made to date.
On 13 May TEPCO commenced the preparatory work for the installation of a cover for the reactor building of Unit 1. The reactor building cover will be installed as an emergency measure to prevent the dispersion of radioactive substances until mid- to long term measures, including radiation shielding, are implemented.
TEPCO has reported that information obtained after calibration of the reactor water level gauges of Unit 1 shows that the actual water level in the Unit 1 reactor pressure vessel was lower than was indicated, showing that the fuel was completely uncovered. The results of provisional analysis show that fuel pellets melted and fell to the bottom of reactor pressure vessel at a relatively early stage in the accident.
TEPCO reported that most part of the fuel is considered to be submerged in the bottom of reactor pressure vessel and some part exposed. TEPCO also reported that leakage of cooling water from the reactor pressure vessel is likely to have occurred. However, TEPCO considers that the actual damage to the reactor pressure vessel is limited, on the basis of the temperatures now being measured around the reactor pressure vessel.
The results of the analysis are provisional; TEPCO will continue to conduct investigations. Similar analyses will be conducted for Units 2 and 3 when radiation levels allow calibration of the instrumentation.
Nitrogen gas is still being injected into the containment vessel in Unit 1 to reduce the possibility of hydrogen combustion inside the containment vessel.
In Units 1, 2 and 3 fresh water is being continuously injected both via the feed water system lines and the fire extinguishers lines 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. TEPCO has formulated the hypothesis that the damage to the Unit 4 building could have been caused by hydrogen generated at Unit 3 that flowed into Unit 4.
Fresh water is being injected as necessary into the spent fuel pools of Units 1 - 4. Water supply from concrete pump trucks is being gradually replaced by the Fuel Pool Cooling and Clean-up system in Units 1 to 3. However, closed loop cooling has not been yet established.
Stagnant water with high levels of radioactivity in the basement of the turbine buildings of Units 1 and 3 is being transferred to the condensers, the radioactive waste treatment facility, the high-temperature incinerator building 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
The daily monitoring of the deposition of caesium and iodine radionuclides for 47 prefectures is continuing. Since 17 May, deposition of I-131 has not been observed. Low levels of Cs-137 deposition were reported in a few prefectures on a few days since 18 May; the reported values range of from 2.2 to 91 Bq/ m2 for Cs-137.
Gamma dose rates values for all 47 prefectures are reported daily by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. On 31 May the gamma dose rate reported for Fukushima prefecture was 1.5 µSv/h. In all other prefectures, reported gamma dose rates were below 0.1 µSv/h; with a general decreasing trend. Meanwhile, the decrease of the gamma dose rate has slowed down, since the short-lived radionuclides have decayed away.
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 17 µSv/h, as reported for 31 May.
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 29 May were about 3 Bq/m3 for I-131 and about 9 Bq/m3 for Cs-137. The values observed in the previous days show daily fluctuations with an overall decreasing tendency.
In April, the Government of Japan announced protective actions to reduce the external exposure to the population beyond a distance of 30 km from the Fukushima-Daiichi Nuclear Plant. NISA has reported that the evacuation of the Planned Evacuation Zones within Iitate village and Kawamata town commenced on 15 May. Confirmation of completion of the evacuation is awaited.
Food Monitoring and Food Restrictions
Food Monitoring (Reported from 19 to 31 May)
Food monitoring data were reported from 19 to 31 May by the Ministry of Health, Labour and Welfare for a total of 818 samples collected in 18 different prefectures. Most of the monitoring continues to be concentrated in Fukushima prefecture, where 328 out of the 818 samples (over 40%) were collected.
Analytical results for 766 samples (over 93%) of the 818 samples indicated that Cs-134 and Cs-137 or I-131 were either not detected or were below the regulation values set by the Japanese authorities. However, 52 samples were above the regulation values for radioactive caesium and/or iodine.
In Fukushima prefecture, five samples of fishery products collected on 16 and 17 May; one sample of unprocessed tea leaves collected on 17 May; three samples of shiitake mushrooms and nine samples of bamboo shoots collected on 19 May; five samples of seafood collected on 20, 21 and 23 May, and; one sample of Japanese apricot, two samples of shiitake mushrooms and seven samples of bamboo shoots collected on 26 May were above the regulation values for Cs-134/Cs-137. One sample of algae collected on 21 May was also above the regulation values for Cs-134/Cs-137 and I-131.
In Chiba, Gunma, Ibaraki and Tochigi prefectures, eighteen samples of unprocessed raw tea leaves collected on 17, 19, 24 and 26 May were above the regulation values for Cs-134/Cs-137.
Consolidated and updated information on food restrictions in Fukushima prefecture were reported on 30 May by the Ministry of Health Labour and Welfare indicating that restrictions on the distribution of bamboo shoots were lifted in the Hirata-Mura area. However, restrictions remain in effect on the distribution of raw unprocessed milk, turnips, bamboo shoots and ostrich fern in specific areas of the prefecture. Restrictions on the distribution and consumption of sand lance fish (the whole prefecture) and specific non-head type (e.g. spinach) and head-type leafy vegetables (e.g. cabbage), flower head brassicas (e.g. broccoli, cauliflower) and shiitake mushrooms (specific areas of the prefecture) also remain in effect.
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 at 22 locations and at off-shore stations by MEXT on 16 stations. The radioactive contamination of the marine environment had occurred by aerial deposition and by continuing discharges and outflow of water with various level of radioactivity from the four damaged reactors at Fukushima Daiichi.
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 of more than 100 MBq/L initially to less than 5 kBq/L on 7 May but increased again to levels of around 20 kBq/L at the 16 May and to about 10 kBq/L on the 17 May. Since then the concentrations dropped slowly to less than 2 kBq/L but increased to about 5 kBq/L on 29 May. The levels of I-131 are varying significantly and the activity ratio to radio-caesium is not constant. On 28 and 29 May the concentrations were around 20 kBq/L. The variability of I-131 relatively to the radio-caesium concentrations could be an indication of retention of caesium by the zeolite sandbags in place, which would have almost no effect on iodine or further production of decay products in the reactor.
Monitoring of the marine environment is performed by TEPCO on the near field area and by MEXT at off-shore sampling positions. The monitoring of MEXT includes also measurement of ambient dose rate in air above the sea, analysis of ambient dust above the sea, analysis of surface samples of sea water and analysis of samples of sea water collected at 10 m above the sea bottom and in a mid-layer as well at a few locations for sediments. On most of the offshore stations I-131, Cs-134 and Cs-137 reached levels below the applied detection limit of 10 Bq/L. There will be a further decrease of the concentration during the propagation of contaminated waters in the sea. The activity found in surface sediments at the near shore stations close to the reactors was between 24 and 320 Bq/kg for Cs-137 in the middle of May. The activity in sediments decreases with distance, but is also highly dependent upon the sediment type. The contamination of marine sediments indicates the enrichment of radio-caesium on particulate matter and its removal from the water column into the sea floor.