Fukushima-Daiichi Accident Information note n 2 of the 8 April 2011

Transcription

1 Fukushima-Daiichi Accident Information note n 2 of the 8 April 2011 This information note, intended for French citizens in Japan, was drawn up by IRSN, the French Institute for Radiological Protection and Nuclear Safety, which currently has a radiation protection specialist among the response teams at the French Embassy in Tokyo 1. SUMMARY OF THE SITUATION REGARDING THE ACCIDENT AND ITS CONSEQUENCES This chapter summarises the main information regarding the operations carried out on the site of the Fukushima plant, the data available on radioactive discharges from the stricken reactors and the environmental consequences of the accident Situation regarding the Fukushima-Daiichi site The state of the three reactors (1, 2 and 3) is still of great concern. Fresh water is now being used to cool the reactors and pools. Even though it is reported that some progress has been made (for example, the water injection pumps are now supplied by normal external power supply rather than by mobile back-up equipment), the means used to inject water remain nevertheless precarious. Since the 6 th of April, TEPCO has been injecting nitrogen at a low rate into the containment vessel of reactor n 1 in order to limit the risk of explosion of the hydrogen present in this building. The same thing will be done afterwards in the containment vessels of reactors 2 and 3. These operations, which will last for several days for each reactor, could lead to new atmospheric discharges. The presence of contaminated water in the turbine buildings of the three units is a result of the constant water releases onto the reactors to ensure their cooling and from probable water leaks coming from the reactor vessels or containment vessels of reactors 2 and 3. Water pumping operations are currently underway, particularly in the case of reactor 1. These operations are somewhat tricky given the quantity of water that needs to be treated and its high level of contamination. A crack in the pit adjacent to the turbine building of reactor 2 led to a direct discharge of highly contaminated water into the sea. TEPCO managed to stop this discharge at around 6am local time on the 6 th of April by plugging the leak with an injection of sodium silicate. Since the 4 th of April, TEPCO has been carrying out intentional discharges into the sea of water, which it terms as slightly contaminated. This mainly involves 10,000 tonnes of liquid effluent stored in tanks, which was awaiting treatment and discharge before the accident. TEPCO justifies this operation by the need to free up storage space on the site to collect the highly contaminated water present in the buildings of the three damaged units. This discharge of liquid should be 1/6

2 finished by this weekend. Furthermore, a discharge is planned into the sea of around 1,500 tonnes of water from the buildings of units 5 and 6, probably due to the consequences of the flooding of these buildings during the tsunami. Measurements of sea water near to the plant taken over the last few days show a high level of contamination of the marine environment, a consequence of the discharge into the sea of part of the highly contaminated water present in the damaged units (see 3). Atmospheric discharges (vapour plumes) will probably continue but will be smaller than those which resulted from the containment vessel depressurisation operations that took place during the first week following the initial accident. These discharges should not modify to any notable extent, over the next few days, the environmental contamination already present History of air radioactivity measured in Tokyo since the 15 March The results of the measurements of air radioactivity (aerosols) in Tokyo, illustrated in the first graph below, show that: - A peak in radioactive pollution due to the atmospheric dispersion of discharges from the Fukushima-Daiichi accident occurred on the 15 th of March, the day on which the wind direction was towards Tokyo ; - On the 16 th of March, radioactive pollution of the air was still measurable, but at much lower levels ; - A new episode of atmospheric pollution was observed in Tokyo between the 21 and 24 March. Although the levels reached were lower than on the 15 March, the rain that fell during this period led to a radioactive deposit that resulted in a twofold increase in the ambient dose rate measured by the IRSN Téléray radiation monitors installed in the French Embassy in Tokyo (see second graph below). The rapid rise observed on March the 21st, followed by several peaks between the 21 and 23 March, indicate the influence of the radiation emitted by the deposit formed by the rain and by the radionuclides present in the air during this period; the dose rate then reached was nsv/h, a value comparable to that regularly measured in many regions due to natural radioactivity. 300,0 Evolution temporelle de l'activité volumique sur Tokyo (Bq/m3) Evolution over time of nuclear activity concentration in Tokyo 250,0 200,0 Bq/m3 150,0 100,0 I131 Cs137 50,0 0, :00-7: :00-17: :00-3: :00-13: :00-23: :00-9: :00-19: :00-7: :00-17: :00-00: :00-03: :00-13: :00-14: :00-18:00 Heures locales japonaises :00-16: :00-10: :00-24: :00-08: :00-08:00 2/6

3 Evolution of ambient dose rates measured at the French Embassy in Tokyo Since the 23 March, no significant presence of artificial radionuclides has been detected in the air in Tokyo. Only the radiation emitted by radioactive deposits contributes to the dose rate measured by the Téléray radiation monitor. This dose rate drops regularly due to the radioactive decay of the short half-life radionuclides present in the deposits. For instance, on the 8 April, it was 84 nsv/h Earthquake of the 7 April 2011 At 23.23pm JST on the 7 April, there was an earthquake at a depth of a 50 km. The epicentre was located several kilometres from the east coast of Honshu Island, around 20 km from the Onagawa nuclear power plant. There was a Tsunami warning but the alert was later lifted. The Fukushima Dai-chi plant was not affected. Three other nuclear facilities were however affected by this earthquake, namely reactors 1 to 3 of the Onagawa plant, reactor 1 of the Higashidoori plant and a fuel storage pool in Rokkasho. Temporary external power supply cuts were reported, but back-up diesel systems ensured the facilities were kept supplied with electricity. These power cuts led to a temporary rise in the temperature of the nuclear storage pools due to loss of cooling. In addition, there were small overflows of slightly contaminated water from the fuel storage pools in the buildings of reactors 1 to 3 in Onagawa. Finally, the operator (Tohoku) of the Onagawa reactors reported that other leaks had taken place in the buildings and that equipment had been damaged. There have been no discharges recorded to date linked to this earthquake. 2. GENERAL RECOMMENDATIONS FOR FRENCH RESIDENTS IN JAPAN Radioactive discharges from the site of the damaged plant have led to radiological pollution in some areas of the country, essentially in the four prefectures neighbouring the site. The purpose of the following recommendations is to limit as far as possible exposure to radiation induced by these discharges. Direct exposure to radioactive discharges dispersed in the air (external exposure to the radiation emitted by the radioactive plume and inhalation of radioactive particles) is essentially no longer a risk, since the discharges are now of little significance. Today, the risk of exposure is mainly linked to the consumption of foodstuffs contaminated by atmospheric fallout. The most sensitive foodstuffs to this radioactive pollution are leafy vegetables and milk from animals that consume contaminated grass or fodder. In certain areas of Fukushima prefecture and also outside of the 30 km zone around the nuclear site, important deposits have been identified and may lead to a significant dose through irradiation in the case of prolonged exposure. Naturally, the following recommendations should not stand in the way of the application of measures decreed by the Japanese authorities. 3/6

4 2.1. Eating and drinking recommendations for all French residents in Japan IRSN recommends: - avoiding eating vegetables (spinach, hana wasaki, kakina, komatsuna, lettuce, chrysanthemum, cabbage, white cabbage, celery, broccoli, bok choy, parsley) and fresh milk produced since the 11 March in the prefectures of Fukushima, Tochigi, Ibaraki and Miyagi, Gunma. - ensuring that the fresh foodstuffs listed above from prefectures where levels for these products have been observed higher than the norms authorising consumption (Saitama, Tokyo, Kanagawa, Chiba) comply with the Japanese regulations in force; - in the absence of information regarding the source and the radiological quality of fresh foodstuffs, varying one s diet and avoiding, as far as possible, the prolonged consumption of leafy vegetables (spinach, hana wasaki, kakina, komatsuna, lettuce, chrysanthemum, cabbage, white cabbage, celery, broccoli, bok choy, parsley). No restrictions regarding the use of tap water for preparing and cooking food need to be envisaged. Products stored in hermetic packaging (tinned food, dried products, long-life milk or bottled mineral water) may be consumed without any risk. It is important to note that occasionally eating foodstuffs contaminated at levels slightly above the authorised norms presents no significant risk to health Recommendations for French residents in the areas the most affected by radioactive deposits Generally speaking, it is recommended to avoid going to the prefectures of Miyagi, Fukushima, Ibaraki and Tochigi unless absolutely necessary, in order to avoid needlessly receiving external radiation doses due to the radioactive deposits, which can be considerable in certain spots, especially in the north west of Fukushima prefecture (see below). French nationals residing in these prefectures are advised to follow the instructions given out by the Japanese authorities. Whatever the case, IRSN recommends: - preparing the food of babies and young children with bottled mineral water, - limiting as far as possible the consumption of foodstuffs from vegetable plots or kitchen gardens, - thoroughly washing fruit and vegetables. IRSN also recommends basic hygiene practices in the home in order to limit the transfer of contamination inside buildings: - leaving shoes outside, especially with rainy weather, - regularly washing floors with a damp cloth, - cleaning air vents and ventilation systems, - regularly vacuuming the surfaces of furniture, carpets and rugs (regularly change the vacuum cleaner bags). It is also recommended to wash one s hands regularly with liquid soap from a dispenser so as to limit the risks of involuntary contamination by hand to mouth contact. 4/6

5 3. GENERAL INFORMATION ON ENVIRONMENTAL RADIOLOGICAL POLLUTION AND ITS CONSEQUENCES The fate of radionuclides discharged into the environment (atmosphere, soil, water) following an accident obeys complex physical laws that IRSN has been studying for many years, particularly following the Chernobyl accident. The aim of this chapter is to share some of the results of this research, which will make it possible to better understand and predict the risks for ecosystems and for humans resulting from radiological pollution of the environment. As long as the atmospheric discharge continues and up to the complete dissipation of the radioactive plume caused by the accident, part of the radionuclides (in the form of aerosols or water soluble gases) are deposited on the ground, according to two complementary processes (see figure below): - Dry deposits, which form on all surfaces of the ground in contact with the radioactive particles in the air. The higher the concentration of radionuclides in the air and the longer the air pollution lasts, the greater the amount of dry deposits ; - Wet deposits, which only form if it rains or snows. This type of deposit can be much greater than dry deposits formed at the same spot, because the drops of rain or flakes of snow concentrate the radioactive particles in the air and convey them to the ground. Some of these wet deposits remain where they are formed, but some can run off the surface into water courses. Dépôts secs Dépôts humides The geographic distribution of deposits around the Fukushima-Daiichi plant is both non-uniform and complex. It depends both on the successive paths of the radioactive plumes formed by discharges from the plant, which took place over several days, and the location and the extent of precipitation over the same period. Consequently, the amount of radioactive materials deposited not only depends on the distance from the nuclear site but also on the terrain and the actual spot where such precipitation occurred during the discharges. Spots of radioactive deposits can also form at some distance from the site. On the basis of the results of dose rate measurements conducted several days ago by an American airplane (DOE/NNSA), IRSN has estimated the dose likely to be received over the first year by the local population on account of external radiation due to exposure to radionuclides already deposited on the ground. The estimated values are shown in the map below. These estimations do not take account of doses likely to be received by internal contamination resulting from the consumption of locally produced foodstuffs, which are over and above the former doses. These initial dose estimations, provided here by way of indication, still need confirmed on the basis of new measurement results as and when they become available. 5/6

6 Estimation of doses received over the first year by external radiation, based on US DOE/NNSE airborne measurements conducted between 30 March and 3 April 2011 This map shows a NW-SE strip of land, several tens of kilometres long, where the deposits seem to have been much more important than elsewhere, doubtless because of the rain or snow that fell in this area when the radioactive plume was being dispersed. By way of comparison, the whole body dose received annually by people living in France is on average 3.7 msv, mainly resulting from exposure to natural radioactivity or from medical applications. 4. CONCLUSIONS Living in Tokyo or in the Tokyo region does not constitute a real risk to health, but particular attention should be paid over the next few weeks to the origin of fresh foodstuffs consumed and to adopting a general attitude of vigilance, for as long as the situation of the damaged nuclear reactors is not totally stabilised. 6/6