1. Current Situation
Overall, the situation at the Fukushima Daiichi nuclear power plant remains very serious.
Changes to Fukushima Daiichi Nuclear Power 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). The Update Brief is based on information received by the IAEA Incident and Emergency Centre by 17:00 UTC on 2 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 stagnant water (around 120 m3) in the basement of the turbine building of Unit 6 was transferred to a temporary tank on 1 May. The transfer of stagnant water from the basement of the turbine building of Unit 6 was resumed on 2 May.
Work to block the Unit 2 trench pit was started on 1 May.
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. There was no more white smoke seen emanating 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 106 °C.
In Unit 2 the indicated temperature at the feedwater nozzle of the reactor pressure vessel is 118 °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 99 °C and at the bottom of the reactor pressure vessel is 124 °C. The reactor pressure vessel and the dry well remain at atmospheric pressure.
On 2 May an amount of 55 tonnes of fresh water was injected into the Unit 2 spent fuel pool using the fuel pool clean-up system.
There has been no change in the status in Unit 5 or in the common spent fuel storage facility.
Spraying of anti-scattering agent at the site is continuing. An area of about 1 000 m2 on the south side of the turbine building of Unit 4, and an area of about 4 400 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 1 May.
2. Radiation Monitoring
The daily monitoring of deposition of caesium and iodine radionuclides for the 47 prefectures continues. Deposition of Cs-137 and Cs-134 was detected in six prefectures on 2 May. The values reported ranged from 2.6 Bq/m2 to 19 Bq/m2. Compared with recent data, deposition of these radionuclides has been detected in fewer prefectures and in lower amounts than for previous days.
Gamma dose rates are measured daily in all 47 prefectures. A general decreasing trend has been observed in all locations since around 20 March. Gamma dose rates reported on 2 May remain at 1.7 µSv/h for Fukushima prefecture and 0.11 µSv/h for Ibaraki prefecture. The other 45 prefectures had gamma dose rates of below 0.1 µSv/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 of more than 30 km from the Fukushima Daiichi plant, ranged from 0.1 µSv/h to 19.7 µSv/h, as reported on 2 May.
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 is applicable only for one village in the Fukushima prefecture and only for infants. According to the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT), I-131 was detected in one prefecture on 29 April, with a reported value of 0.22 Bq/L; in two prefectures on 30 April, with reported levels of 0.04 Bq/L and 0.10 Bq/L respectively; and in one prefecture on 1 May, with a reported level of 0.38 Bq/L. Cs-137 was reported on 30 April in only one prefecture, with a measured level of 0.05 Bq/L. All these levels are below the limits set by the Japanese authorities for the restriction of water consumption due to the presence of radionuclides. The other samples did not show levels of radionuclides above the detection limit for I-131, Cs-134 and Cs-137.
On 1 May restrictions were lifted on the distribution of raw unprocessed milk in Fukushima prefecture from the city of Minamisouma (limited to Kashima-ku and excluding Karasuzaki, Ouchi, Kawago and Shionosaki areas) and Kawamata town (excluding Yamakiya area).
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 locations of the sampling positions have been 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.
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 to 30 April. 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 sandbags containing Zeolite® absorbers that were placed at several locations between Unit 2 and Unit 4 to reduce the concentrations of Cs-134 and Cs-137 seem to be effective.
The concentrations of the relevant radionuclides at the other TEPCO sampling positions show a general decreasing trend up to 30 April.
Monitoring performed by MEXT at off-shore sampling positions consists of:
- Measurement of ambient dose rate in air above the sea;
- Analysis of ambient dust above the sea;
- Analysis of surface samples of seawater; and
- Analysis of samples of seawater 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 were taken at stations 1 - 10 every four days after 2 April. Activity concentrations at MEXT sampling points 30 km off-shore are significantly lower than those at TEPCO sampling points 15 km off-shore. None of the activity concentrations of I-131 and Cs-137 in surface samples taken from points 1, 3, 5, 7, 9 and S-3 on 27 April and from points 2, 6 and S-4 on 25 April were above the detection limits. Samples taken from points 4, 8 and 10 showed concentrations of Cs-137 between of 10.5 Bq/L and 40 Bq/L. Only the sample from point 10 had an I-131 activity concentration, at 21.5 Bq/L, that was above the detection limit.
Samples were taken at the recently added off-shore stations at the Ibaraki prefecture on 25 April. There were no activity concentrations of I-131 and Cs-137 in the surface layer of sea water that were above the detection limits.
Radiation Monitoring in Ports
On 22 April the Japanese Ministry of Land, Infrastructure, Transport and Tourism (MLIT) issued guidelines for radiation measurements in ports in Japan in order to provide foreign port authorities with accurate data. The guidelines cover gamma dose rate measurements for export shipping containers and shipping as well as radiation monitoring of the atmosphere and of sea water in ports.
Measurements relating to export shipping containers for export and to shipping can be conducted by the port authorities, by ship operators or by other parties. The guidelines specify the measuring locations and methodology, as well as criteria for decontamination and for reporting. If measurements have been conducted in accordance with the guidelines, attestations of the measured dose rates will be issued jointly by MLIT and the port authorities.
With regard to export shipping containers, the guidelines state that decontamination is necessary if the measured dose rate exceeds three times the dose rate due to natural background radiation. Decontamination is to be carried out in an area to be specified by the port authorities. In accordance with the International Maritime Dangerous Goods Code of the International Maritime Organization, a reporting level of 5 µSv/h is set. If the dose rate exceeds this reporting level, all relevant organizations are to be informed.
With regard to shipping, the guidelines recommend that decontamination should be carried out if the measured dose rate exceeds three times the dose rate due to natural background radiation, and decontamination must be carried out if the dose rate exceeds 5 µSv/h.