IAEA Briefing on Fukushima Nuclear Accident (29 March 2011, 16:30 UTC)
IAEA Briefing on Fukushima Nuclear Accident (29 March 2011, 16:30 UTC)1. Current SituationThe situation at the Fukushima Daiichi plant remains very serious.Accumulated contaminated water was found in trenches located close to the turbine buildings of Units1 to 3. Dose rates at the surface of this water were 0.4 millisieverts/hour for Unit 1 and over 1000millisieverts/hour for Unit 2 as of 18:30 UTC on 26 March. The Nuclear Safety Commission of Japansuggests that higher activity in the water discovered in the Unit 2 turbine building is supposed to becaused by the water, which has been in contact with molten fuel rods for a time and directly releasedinto the turbine building via some, as yet unidentified path. An investigation is underway as to how thewater accumulated in the trenches. Measurements could not be carried out at Unit 3 because of thepresence of debris.Fresh water has been continuously injected into the Reactor Pressure Vessels (RPVs) of Units 1, 2and 3. From today at Unit 1, the pumping of fresh water through the feed-water line will no longer beperformed by fire trucks but by electrical pumps with a diesel generator. The switch to the use of suchpumps has already been made in Units 2 and 3. At Unit 3, the fresh water is being injected throughthe fire extinguisher line.At Unit 1, there has been an increase in temperature at the feed-water nozzle of the RPV from 273.8Cto 299C. The temperature at the bottom of the RPV remained stable at 135. Temperatures at Unit 2appear relatively stable at the same measurement points. At Unit 3, the temperature at the feed-waternozzle of the RPV is about 61.5C and 120.9C at the bottom of the RPV. The validity of the RPVtemperature measurement at the feed water nozzle is still under investigation.With the increase in temperature at Unit 1, there has been a corresponding increase in Drywellpressure. In the Drywell of Unit 2, the indicated pressure dropped slightly and is just aboveatmospheric.It is planned to begin pumping fresh water into the spent fuel pool of Unit 4 today, on 29 March.Units 5 and 6 remain in cold shutdown.2. Radiation MonitoringOn 28 March, deposition of iodine-131 was detected in 12 prefectures, and deposition of cesium-137in 9 prefectures. The highest values were observed in the prefecture of Fukushima with 23000becquerel per square metre for iodine-131 and 790 becquerel per square metre for caesium-137. In theother prefectures where deposition of iodine-131 was reported, the range was from 1.8 to 280becquerel per square metre. For caesium-137, the range was from 5.5 to 52 becquerel per squaremetre. In the Shinjyuku district of Tokyo, the daily deposition of both iodine-131 and cesium-137 wasbelow 50 becquerel per square metre. No significant changes were reported in the 45 prefectures ingamma dose rates compared to yesterday.As of 28th March information on radioactivity in drinking water collected mainly from the JapaneseMinistry of Health, Labour and Welfare indicates that recommendations for restrictions based on I-131concentration remain in place only in four locations in the prefecture of Fukushima. To date, norecommendations for restrictions have been made based on Cs-137.The Japanese limits for theingestion of drinking water by infants is 100 becquerel per litre.
Five soil samples, collected at distances between 500 and 1000 metres from the exhaust stack of Unit1 and 2 of the Fukushima Nuclear Power Plant on 21 and 22 March, were analysed for plutonium-238and for the sum of plutonium-239 and plutonium-240. (Due to analytical reasons, the isotopesplutonium-239 and plutonium-240 cannot be measured separately). Plutonium-238 was detected in 2of the 5 samples, while plutonium-239/240 was detected in all samples as expected.Concentrations reported for both, plutonium-238 and plutonium-239/240 are similar to those depositedin Japan as a result of the testing of nuclear weapons. The ratio of the concentrations of plutonium-238and plutonium-239/240 in two of the samples indicate that very small amounts of plutonium mighthave been released during the Fukushima accident, but this requires to be further clarifiedAs far as food contamination is concerned, 63 samples taken from 24-29 March, and reported on from27- 29 March, for various vegetables, fruit (strawberry), mushrooms, eggs, seafood and pasteurizedmilk in eight prefectures (Chiba, Fukushima, Gunma, Ibaraki, Miyagi, Niigata, Tochigi andYamagata), stated that results for iodine-131, caesium-134 and caesium-137 were either not detectedor were below the regulation values set by the Japanese authorities.The Joint FAO/IAEA Food Safety Assessment Team met with local government authorities in Ibarakiprefecture on Monday and provided advice related to contamination of food and the environment,including the mechanisms and persistence of such contamination, examples of remediation strategies,international standards and sampling plan designs, and radionuclide transfer from soil to plants,particularly as related to rice production in the area.Local government authorities briefed the FAO/IAEA Team on the extent of contamination in Ibaraki,the principle agricultural products affected, the main production areas and production methods(greenhouse, open-air) and levels of contamination found.The FAO/IAEA team is also meeting with the local authorities in Tochigi prefecture today, and willmeet with local government officials in Gunma tomorrow.Sea water samples:No new results from the marine monitoring stations 30 km off-shore were reported for 27 or 28March. However, new analyses in seawater 330 m east to the discharges point of NPP Units 1 - 4 weremade available for 27 March. These concentrations show a significant decrease from 74000 Becquerelper litre of iodine-131, 12000 Becquerel per litre of cesium-137, and 12000 Becquerel per litre ofcesium-134 on 26 March to 11000 Becquerel per litre of iodine-131 and 1900 Becquerel per litre ofcesium-137 on 27 March.Sea water samples were also collected daily at a location 30 m from the common discharge point forUnits 5-6. These results also show an increase in the radionuclide concentrations on March 26. The seawater samples collected on March 27 show as well a decrease of the radionuclide concentration.
Fig. 1: Temporal trend of the seawater concentration near the Fig. 2: Temporal trend for radionuclide concentration indischarge point of units 1 to 4 of I-131, Cs-134 and Cs-137. seawater near the discharge point of Units 5 and 6.Limits set by Japanese authorities are also indicated with 40Bq/L for I-131 and 90 Bq/L for Cs-137. Other reportedradionuclides at this location were I-132, Cs-134 and Cs-136. (NB: Data are reported in Becquerel per cubic-centi-meter. For conversion into Bq/L the values have to bemultiplied by a factor of 1000.)It can be expected that the data will be quite variable in the near future depending on the dischargelevels. In general, dilutions by ocean currents and into deeper waters as well decay of short livedradionuclides e.g. I-131 or I-132 will soon lead to lower values.Marine Organisms:First analyses were reported in Fish carried out by the National Research Institute of Fishery Research.5 samples of fish were collected from the port of Choshi (Chiba prefecture) and 4 of 5 samples showedCs-137 concentrations below limit of detection. In one sample Cs-137 was found with 3 Bq/kg (freshweight) and it was reported that it was slightly above the limit of detection. This concentration is farbelow any concern for fish consumption.It is still too early to draw conclusions for expected concentrations on marine food, because thesituation may change rapidly, however, it is expected that the detected initial concentrations ofseawater will soon drop to lower values by dilution and the levels in marine food will most likely notreach levels above given limits for consumption, (presuming that discharges of contaminated seawaterfrom the reactor will not continue). It is not expected that fish or other marine food will be collected ina close area to the NPP Fukushima at the present situation. Some marine algae are known toaccumulate in particular I-131 and Tc-99m. However, these values will soon be of no concern due tothe short half-lives of the radionuclides mentioned.Modelling the marine dispersionThe Group SIROCCO of the Observatoire Midi-Pyrenées of the University of Toulouse, CNRS, iscontinuing to carry out model calculations. The model is based on an ocean circulation and currentweather conditions and they results showed an initial north-eastern transport of liquid releases fromthe damaged reactors and the contaminated water would reach the northern monitored stationsbetween 1 and 2 weeks later.A model with tracer release directly in the sea show an along shore propagation in the southerndirection and a northeast propagation moving away from the coast.With tracer release from atmospheric deposition, the propagation stretch offshore entering the Kuro-Shivo current in few days. The first results are shown in Fig. 3 and 4. The data are converted into Bq/Lby assuming arbitrary discharge or aerial release activities, respectively. The results should just betaken as indication of the dilution capacity and transport route of sea water.
Fig. 3: Results of the dispersion model of an intial discharge Fig. 4: Results of model calculation for the aerial depositionfrom the Fukushima NPPs into sea water and the expected of radionuclides on sea surface and the calculateddistribution of surface concentration for 25 March. concentration field on 25 March in surface seawater.Discharge of the model started on 11 March.