1. Current situation
Overall, the situation at the Fukushima Daiichi nuclear power plant remains very serious.
Changes to Fukushima Daiichi plant status
The IAEA receives information from various official sources in Japan through the Japanese national competent authority, the Nuclear and Industrial Safety Agency (NISA). Additional detail is provided in the IAEA Incident and Emergency Centre (IEC) status summary with information received by 17:00 UTC on 1 May 2011.
Management of on-site contaminated water
According to the 25 April evaluation by NISA of the report submitted by the Tokyo Electric Power Company (TEPCO), there is a little less than 70,000 tonnes of stagnant water with high level radioactivity in the basement of the turbine buildings of Units 1, 2 and 3. The transfer of stagnant water from the turbine building of Unit 2 to the radioactive waste treatment facilities was resumed on 30 April.
On 27 April TEPCO provided an update of the estimated percentage of core damage for Units 1, 2 and 3 following an assessment (the values assessed previously which TEPCO had provided on 15 March are given in parentheses): Unit 1: 55% core damage (70%); Unit 2: 35% core damage (30%); Unit 3: 30% core damage (25%). This reflects a revised assessment rather than any recent changes in conditions in the reactor cores.
White smoke continues to be emitted from Unit 2 and Unit 3. No more white smoke was seen coming from Unit 4 as of 21:30 UTC on 25 April or from Unit 1 as of 21:30 UTC on 30 April.
In Unit 1 fresh water is being continuously injected into the reactor pressure vessel through the feedwater line at an indicated flow rate of 6 m3/h using a temporary electric pump with off-site power.
In Unit 2 and Unit 3 fresh water is being continuously injected into the reactor pressure vessel through the fire extinguisher line at an indicated rate of 7 m3/h using temporary electric pumps with off-site power.
On 29 April, TEPCO checked the status inside the reactor building of Unit 1 using a remotely controlled robot and confirmed that there was no significant leakage of water from the primary containment vessel. Nitrogen gas is still being injected into the containment vessel in Unit 1 to reduce the possibility of hydrogen combustion inside the containment vessel. The indicated pressure in the reactor pressure vessel is still increasing.
In Unit 1, the indicated temperature at the feedwater nozzle of the reactor pressure vessel is 142 °C and at the bottom of reactor pressure vessel is 105 °C.
In Unit 2 the indicated temperature at the feedwater nozzle of the reactor pressure vessel is 119 °C. The reactor pressure vessel and the dry well remain at atmospheric pressure. On 28 April an amount of 43 tonnes of fresh water was injected into the spent fuel pool using the spent fuel pool clean-up system.
In Unit 3 the indicated temperature at the feed water nozzle of the reactor pressure vessel is 91 °C and at the bottom of the reactor pressure vessel is 118 °C. The reactor pressure vessel and the dry well remain at atmospheric pressure.
Radionuclide analysis of a water sample taken from the Unit 4 spent fuel pool on 28 April detected levels of Cs-134of 49 Bq/cm3; levels of Cs-137 of 55 Bq/cm3; and levels of I-131 of 27 Bq/cm3.
There has been no change in the status in Unit 5 or Unit 6 or in the common spent fuel storage facility.
Spraying of anti-scattering agent at the site is continuing. An area of about 2000 m2 on the south side of the turbine building of Unit 4, and an area of about 5400 m2 of the surface on the slope around the former main office building, near the on-site gymnasium and on the west side of the shallow draft quay, were sprayed on 30 April.
2. Radiation monitoring
Deposition of Cs-137 was detected in ten prefectures in the period 28 April to 1 May. The values reported ranged from 1.3 Bq/m2 to 90 Bq/m2. I-131 deposition was reported for two prefectures on 28 April and for one on 1 May, with values of 45 Bq/m2, 89 Bq/m2 and 1.8 Bq/m2.
Gamma dose rates are measured daily in all 47 prefectures. A general decreasing trend has been observed in all locations since around 20 March. For the Fukushima prefecture gamma dose rates were 1.7 ?Sv/h on 1 May. In Ibaraki prefecture gamma dose rates were 0.11 ?Sv/h. The other 45 prefectures had gamma dose rates of below 0.1 ?Sv/h, falling within the range of local natural background radiation levels.
Gamma dose rates reported specifically for the eastern part of Fukushima prefecture, for distances beyond 30 km from the Fukushima Daiichi plant, showed a similar general decreasing trend, ranging from 0.1 to 18.1 ?Sv/h, as reported on 30 April.
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 approx. 1 km). The values reported for the period 27 April-1 May ranged from 0.4x 10-4 Bq/cm3 to 2.2 x 10-4 Bq/cm3 for total I-131 and 0.1 x 10-4 Bq/cm3 to 0.3x 10-4 Bq/cm3 for particulate Cs-137.
Since 1 April there has been one remaining restriction on the consumption of drinking water relating to I-131 (with a limit of 100 Bq/L), which applies to one village in the Fukushima prefecture and only for infants. According to the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT), only three samples of drinking water reported on 28 April from the 47 prefectures had levels above the detection limit for I-131. These three levels, which measured 0.14 Bq/L, 0.16 Bq/L and 0.54 Bq/L, were all below the regulation value set by the Japanese authorities. No samples were reported with levels above the detection limit for Cs-134 and Cs-137.
Food monitoring data were reported by the Japanese Ministry of Health, Labour and Welfare on 28, 29, 30 April and 1 May for a total of 190 samples taken on 25-30 April and 1 May from 12 prefectures (Chiba, Fukushima, Gunma, Hyogo, Ibaraki, Kanagawa, Nagano, Niigata, Saitama, Tochigi, Tokyo and Yamagata). Analytical results for 180 of the 190 samples for various vegetables, mushrooms, fruit (strawberry), beef, pork, poultry eggs, seafood, fresh milk, raw unprocessed milk and yoghurt indicated that I-131, Cs-134 and Cs-137 were either not detected or were below the regulation values set by the Japanese authorities. In Fukushima prefecture, one sample of bamboo shoot from 27 April and one sample of ostrich fern and five samples of shiitake mushrooms from 28 April were above the regulation values set by the Japanese authorities for Cs-134/Cs-137. In Ibaraki prefecture, three samples of seafood (sand lance) from 28 April (one sample) and 29 April (two samples) were above the regulation values set by the Japanese authorities for Cs-134/Cs-137.
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 the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT). The locations of the sampling positions were provided in previous briefings. Increased radioactivity in the marine environment occurred by aerial deposition and by discharges and outflow of contaminated water with a high radioactivity level.
In a news release issued on 25 April, NISA communicated its evaluation of a report submitted by TEPCO on 21 April in relation to contaminated water with a high radioactivity level that flowed out from Unit 2 of the Fukushima Daiichi plant. The outflow rate is estimated to have been approximately 4.3 m3/h. The concentrations of the relevant radionuclides, estimated from measurements, were 5400 MBq/L of I-131, 1800 MBq/L of Cs-134 and 1800 MBq/L of Cs-137.
Sea water monitoring
The activity concentrations of I-131, Cs-134 and Cs-137 in sea water at the screen of Unit 2 were measured every day from 2 April 2011 to 30 April 2011. The concentrations fell by several orders of magnitude from initial values of more than 100 MBq/L at the beginning of April to less than 10 kBq/L for Cs-134 and Cs-137 on 30 April, with a continuing decreasing trend. However, levels of I-131 remained at around 100 kBq/L from 26 April to 30 April at this sampling position.
The concentrations of the relevant radionuclides at the other TEPCO sampling positions show a general decreasing trend up to 30 April.
Monitoring 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 sea water;
4. Analysis of samples of sea water collected at 10 m above the sea bottom.
The analysis for almost all sampling positions has shown a general decreasing trend in concentrations of the relevant radionuclides over time. Samples from coastal positions still show higher concentrations of such radionuclides than samples from off-shore positions. The radionuclides I-131, Cs-134 and Cs-137 are still detected in most sea water samples, but no longer for some of the off-shore positions.
You can view the full IAEA update here.