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Säteilyturvakeskus Strälsäkerhetscentralen Finnish Centre for Radiation and Nuclear Safety PL-P.O.BOX 268, SF-00101 Helsinki 10, Finland STUK-B-VALO 45 SECOND INTERIM REPORT RADIATION SITUATION IN FINLAND FROM 5 TO 16 MAY 1986 May 1986 Joint report Finnish Centre for Radiation and Nuclear Safety NBC Defence Office of the General Headquarters Rescue Department of the Ministry of the Interior Helsinki 1986 Government Printing Centre ISBN 951-46-9578-X ISSN 0781-2868 THE RESULTS IN THIS REPORT HAVE BEEN PREPARED BY STUK Researchers: Olli Paakkola, Hannele Aaltonen, Hannu Arvela, Leif Blomqvist, Karl-Heinz Hellmuth, Raimo Hentelä, Eero Hiukka, Erkki Hus, Juhani Lahtinen, Heikki Lemmelä, Raimo Mustonen, Marketta Puhakainen, Tua Rahola, Aino Rantavaara, Kristina Rissanen, Laina Salonen, Ritva Saxen, Kari Sinkko, Kirsti- Liisa Sjöblom, Matti Suomela, Tellervo Taipale, Markku Tapiovaara, Kaarina Tevanen, Kaj Winqvist. Assisting staff: Ritva Aalto, Marjaana Ahonen, Teuvo Ahonen, Esa Eviluoto. Anne-Maaria Hyttinen, Esko Hyttinen, Eija Hämäläinen, Sirkka Hämäläinen, Seppo Klemola, Riitta Kontro, Olli-Matti Lahtimies, Mika Mark kanen, Virpi Nuutinen, Ritva Paatelainen, Anja Pekkala, Teemu Pihkala, Sauli Pusa, Irja Putkonen, Pirkko Saira, Ritva Sarnela, Ulla Välikangas, Timo Ylhäinen. Text processing: Kristina Rautjärvi. English translation: Ari Penttilä Ministry of the Interior Janne Koivukoski, Juhani Seppä. NBC Defence Office of the General Headquarters Juhani Juutilainen 1 INTRODUCTION The accident at Chernobyl nuclear power station on 75 April 1986 started an extensive measurement and radiation monitoring program of the environment and foodstuffs, which has demanded the participa tion of most of the personnel and the full ana lysis and measurement capacity of the Surveillance Department of the Finnish Centre för Radiation and Nuclear Safety. This intensive program will still be continued for several months to come. The interim report "Fallout situation in Finland from April 26 to May 4" /l/ treating the weather conditions and radiological measurements of the first week following the accident was published about two weeks after the onset of the accident. This second interim report is a continuation of the first. It deals with the results from external radiation measurements and analysis of environ mental and foodstuff samples between 5 and 16 May, i.e. the development of the situation after the fall-out peak. As in the case of the first report, the results in this report are preliminary, and subsequent checks may cause some changes to them. Each subject matter will later be dealt with in detail in scientific papers, which will be published either as reports of the Finnish Centre for Ra diation and Nuclear Safety or in international scientific periodicals or in conference procee dings . We have considered it necessary to publish the results of the completed measurements and analyses as quickly as possible. We believe that in many sectors it is important to receive early informa tion on the spreading of the fall-out from the failed reactor, as well as an estimate of the doses received. The report also includes a list of the recommendations given in the course of the situation. 2 TAKING OF SAMPLES The first report /l/ gave an outline of the ra diation monitoring network in Finland, and de scribed briefly both STUK's nationwide radiation monitoring program and the environmental monitoring program that is applied in the surroundings of nuclear power plants. The extent of these programs under normal circumstances was explained. In the present fall-out situation, the sampling frequency and the amounts of samples are many times higher than usually. This applies especially to foodstuffs, but also the numbers of air and deposition samples, as well as certain wild plant samples, have grown considerably. The control of radioactivity in milk performed by STUK has concentrated on the daily monitoring of milk in the whole of Finland. The monitoring program encompasses 13 dairies, of which about one half are large-scale dairies. In addition, some farm-specific samples have been measured for comparing the iodine contamination in forage and in milk. Part of the sampling has been channelled through the network of local laboratories that covers the whole country. Th-jre are in total 53 local labo ratories, established by the National Board of Health, working in cooperation with STUK. The laboratories are communal food and milk inspection laboratories and they measure the radioactivity in milk, drinking water and vegetables daily in their own areas and, on request, take environmen tal and foodstuff samples for STUK. As concerns garden produce and eatable wild plants, the control has been organized in cooperation with the National Board of Trade and Consumer Interests, which takes the samples. During the growing season, all groups of vegetables and fruits are followed extensively in the whole country. The results are assessed separately in each pro duction area. Besides the sampling performed in accordance with STUK's own monitoring programs and the above- mentioned intensified sampling, the Surveillance Department has received limited amounts of samples of early vegetables produced in some private gardens, as well as samples of other foodstuffs, tap water, well water and mushrooms. Most of the place-names mentioned in the report are shown on the map in Figure 1. 3 RESULTS External exposure The first report on the Chernobyl accident /l/ described the radiation situation up to the 5th of May. Thereafter the external radiation has been diminishing, as expected. In mid May there were rainy spells in the whole of Finland, but they brought only slight amounts of radioactive substances to the ground. By that time, the air masses had already become almost totally clean. Table 1 shows ~:he daily exposure rates measured at 12 radiation monitoring stations from 5 to 16 May 1986. Figures 2 and 3 illustrate the deve lopment of the exposure rate in five localities. Figure 4 shows a map of the radiation situation on 15 May. To supplement the results of the measurements performed by the stationary radiation monitoring stations of the Ministry of the Interior and the Defence Forces, the Finnish Centre for Radiation and Nuclear Safety has been mapping the exposure rates by means of sensitive Geiger-counters placed in cars. Figures 5 to 7 show the routes driven and a conclusion of the results. The gammaspectrometric measurements of the radia tion field were continued by conducting a mapping trip to Central Finland on 6 and 7 May (Figure 7). A spectrometer was measuring all the time inside the car. Additional measurements were made outside the car. On the basis of The measurements it is possible to estimate the contributions of various nuclides to the total exposure rate of the fall-out (Table 2). The table also includes a rough estimate of the activities of various nuclides in the ground. The differences detected in the relative amounts of the nuclides were no longer as great as in the corresponding measure ments performed on 29 April (see /l/, Table 3). Radiation monitoring flights The daily radiation monitoring flights were con tinued till 11 May with the purpose of surveying radioactivity in the Finnish air space. Special attention was paid to air masses coming from the direction of the accident area and containing the possibility of rain. Air measurements showed that some radioactive substances still came to Finland in the report period but the amounts were no longer significant. 4 Table 3 shows the concentrations of the most important nuclides in the air samples taken during the flights. Air The national radiation monitoring station of air is situated in Nurmijärvi, where samples are taken with a high-capacity air dust sample collector. In the present fall-out situation, supplementary samples are also taken in Konala, Helsinki, and in Rovaniemi. The samples have been analysed daily. In Nurmijärvi, the air sample is collected on a glass-fibre filter. Another sample is taken after the air has gone through the filter into an acti vated charcoal cartridge. Since Monday, 28 April, the glass-fibre and charcoal filters have been replaced more often than normally; first twice a day and then once a day. On Monday, 28 April, the taking of air samples was also begun with a smaller collector including a glas-fibre filter, in Konala, Helsinki. Air samples have been taken in Rovaniemi since 6 May. Table 4 shows the concentrations of radionuclides detected in the air sample collected in Nurmijärvi on 28 April 1986 from 3.00 to 9.00 p.m. During this period, the concentrations of radionuclides reached their maximum value in air. Table 5 gives the concentrations of iodine 131 in air, as de termined by means of the glass-fibre and charcoal filters. The results indicate that, on an average, 85 % of the iodine isotopes have penetrated the glass-fibre filter. The concentrations of iodine given in the tables have been corrected with the measured coefficients of penetration. Table 6 shows the concentrations of various nuclides in ground level air in Nurmijärvi from 28 April to 16 May 1986. Figures 8 and 9 illustrate the deve lopment of the concentrations of iodine 131 and cesium 137 in the same period of time. Table 7 gives the concentrations of ten radionuc lides in the air in Konala from 5 to 16 May (con tinuation to Table 5 in the previous report /l/). Also in this table, the concentrations of the iodine isotopes have been corrected by assuming that 85 % of the iodine has penetrated the glass- fibre filter. During the last days of April and the first days of May, small amounts of carbon 14 and tritium were detected in the air samples taken in Konala (Table 8). Thereafter the concentrations have remained below the detection limit. b Deposition The radionuclide composition of the deposition was followed daily by means of rain water and dry deposition samples taken in Nurmijärvi and in Konala, Helsinki.