Airborne and Deposited Radioactivity in Finland in 1987
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^fi QcnmiH Siv* STUK-A75 April 1990 AIRBORNE AND DEPOSITED RADIOACTIVITY IN FINLAND IN 1987 Supplement 1 to Annual Report STUK-A74 Hannele Aaltonen, Ritva Saxen and Tarja K. Ikäheimonen STvK- h-- >C STUK-A75 April 1990 AIRBORNE AND DEPOSITED RADIOACTIVITY IN FINLAND IN 1987 Supplement 1 to Annual Report STUK-A74 Hannele Aaltonen, Ritva Sax6n and Tarja K. Ikäheimonen Finnish Centre for Radiation and Nuclear Safety P.O.Box 268, SF-00101 HELSINKI FINLAND fS3N951-47-146e-0 ISSN 0781-1705 Helsinki 1990 The Finnish Government Printing Centre 3Ai ABSTRACT The Finnish Centre for Radiation and Nuclear Safety has con tinued its nationwide monitoring of airborne and deposited radioactive substances ir, Finland. In the air surveillance programme concentrations of artificial radionuclides are monitored in the air close to the ground. In 1987, air dust samples were collected in Nurmijärvi, 40 km north of Helsinki and in Rovaniemi. When necessary, sampling was also launched in Helsinki. Several radionuclides originating from the accident in Chernobyl in 1987 could still be detected in air dust samples. The con centrations of the prevailing nuclides 134Cs and 137Cs were stable and there was no significant difference between winter and summer. Fresh fission nuclides were detected in March and August. A sharp decrease in the amounts of radioactive substances de posited was noted at all the 18 sampling stations as against 1986. The total annual amounts of deposited 137Cs varied at different stations in 1987 from 7.7 Bq/m2 (Rovaniemi) to 1500 Bq/m2 (Kauhava) and those of »° Sr in 1986 from 4.7 Eq/m2 (Tai valkoski) to 590 Bq/m2 (Kuhmo) and in 1987 from 1.4 Bq/m2 (Iva lo) to 38 Bq/m2 (Kuhmo). The total annual amounts of 236 Pu and 2 2 3 9,240Pu were 3rl and i2 mBq/m in Nurmijärvi and 3.4 and 14 mBq/m2 and Lappeenranta in 1987, respectively. In 1987, resuspension of radionuclides was characteristic cf the deposition situation. Due to this phenomenon, unexpectedly high contents of deposited radlocesium occurred occasionally in the samples, though the discharge caused by the Chernobyl accident lasted for only a short period in spring 1986. The proportion of 137Cs originating from Chernobyl in 1987 varied from 3% to about 40% of the corresponding amounts in 1986 at different stations. The corresponding percentages for 90Sr varied from 1.6% to 34%. It CONTENTS ABSTRACT 3 CONTENTS 5 1 INTRODUCTION 7 2 MATERIAL AND METHODS 8 2.1 Sampling 8 2.1.1 Airborne dust at ground level 8 2.1.2 Deposition 8 2.2 Pre-treatment of samples 10 2.2.1 Air 10 2.2.2 Deposition 10 2.3 Analysis methods 11 2.3.1 Gammaspectrometric analyses 11 2.3.2 Strontium analyses 12 2.3.3 Analyses of transuranic elements 12 2.3.4 Tritium analyses 12 3 RESULTS 12 3.1 Ground-level air 13 3.1.1 The occurrence of fresh fission products ^3 3.1.2 Chernobyl fallout 15 3.2 Deposited radioactivity 18 3.2.1 Gamma-emitting radionuclides 18 3.2.2 90Sr 21 3.2.3 137Cs/»°Sr in deposition 21 3.2.4 Transuranic elements 21 3.2.5 Tritium 24 4 DISCUSSION 25 4.1 Resuspension 25 4.2 Accumulated deposition 27 ACKNOWLEDGEMENTS 28 REFERENCES 29 TABLES 32 7 1 INTRODUCTION The Finnish Centre for Radiation and Nuclear Safety has con tinued its nationwide programme to monitor both airborne and deposited radioactivity in 1987."•• Airborne dust was collected continuously in Nurmijärvi and Rovaniemi. When necessary, airborne dust sampling was also launched in Helsinki. All samples were analysed gammaspectro- metricly. Continuous sampling at the 18 stations was carried out on a monthly basis to determine the amounts of radioactive substances deposited in different parts of the country. The samples were analysed both for gamma-emitting radionuclides and for /3-emitting 89Sr and 90Sr. Al fa-emitting transuranic elements were determined in samples from Nurmijärvi, where we have three differ*.nt sample collectors, and on temporally com bined samples from Lappeenranta. Pour stations also collected deposition for tritium determinations. This report includes results for 1987 as well as the results for 90Sr and transuranic elements in the 1986 deposition which were not included in the previous report0. 8 2 MATERIAL AND METHODS 2.1 Sampling 2.1.1 Airborne dust at ground level In Nurmijärvi, 40 km north of Helsinki, airborne dust is col lected on a glass fibre filter. The air flow rate through the glass fibre filter in Nurmijärvi is about 750 m3/h which corre sponds to a face velocity of 0.9 m/s. In Rovaniemi, northern Finland, airborne dust was collected up to the beginning of October using a portable sampler (STAPLEX*) the capacity of which is about 100 m3/h. Since October, a transportable sampler with a capacity of 150 m3/h has been used. In Helsinki, an air sampling system was launched for special reasons, using a sampler with a capacity of about 150 m3/h. The locations of the sampling stations are shown in Figure 1. Airborne dust is collected on a glass fibre filter, type Whatman GF/A, and normally the filters are changed twice a week to avoid overloading and to ensure a high retention of particulate nu clides. When necessai/, the filters are changed more often. The Nurmijärvi sampler is also equipped to collect gaseous fractions of iodine. Some of the air that has passed the glass fibre filter is sucked through activated carbon impregnated with potassium iodide, type SS 207B-1.5 KI 8-12 mesh. The air flow rate through the carbon bed is adjusted to a maximum of about 12 m3/h, corresponding to the residence time of 0.2 s in the carbon bed, to ensure a very high retention of iodine, even in the form of methyl iodide. If nothing unusual is detected in analyses of the glass fibre filters, the charcoal filter is changed once a month. 2.1.2 Deposition Wet and dry deposition was monitored continuously at 18 sta tions. The surface areas of the samplers are 0.05, 0.07 or 1 m2. The sample collectors are made of stainless steel or 9 Fig. 1. Sampling stations collecting continuously airborne radioactivity (•). Samples were analysed gammaspectro- metricly. Sampling stations for deposition samples and deter minations made on the samples from different stations: • gamma-emitting radionuclides and radiostrontiurn A tritijm • transuranic elements 1 Nurmijärvi 10 Kauhava 2 Mariehamn 11 Kuopio 3 Jokioinen 12 Kuhmo 4 Niinisalo 13 Kajaani 5 Lappeenranta 14 Taivalkoski 6 Savonlinna 15 Rovaniemi 7 Jyväskylä 16 Sodankylä 8 Joensuu 17 Ivalo 9 Vaasa 18 Inari 10 brass. Samples from the steel collectors are used to analyse gamma-emitting radionuclides, strontium and transuranic iso topes, and those from the brass collectors for tritium determi nations. At two stations. Nurmijärvi and Lappeenranta, there are two stainless steel samplers, with surface areas of 0.05 and 1 m2 . Samples from the big collector in Nurmijärvi and Lappeenranta were used for transuranic element determinations. The collectors were usually emptied at the end of each month. The stainless steel collectors were rinsed with dilute nitric acid 2nd distilled water after every emptying to prevent adsorp tion of radionuclides onto the walls of the samplers. The rins ing solutions were added to the sample. The brass samplers were not rinsed with th<3 acid solution. All the sampling sta tions are shown in Fig. 1. 2.2 Pre-treatment of samples 2.2.1 Air dust samples After sampling, the glass fibre filters changed on Monday and Thursday were combined to form a week sample. The filters were pulverized by compressing, and the powder was homogenized and re-pressed onto discs to provide a suitable counting geometry for gammaspectrometric analysis. The activated charcoal was measured without any pre-treatment in a Marinelli-beaker (0.5 litre). 2.2.2 Deposition samples Known amounts (10-20 mg) of Sr, Cs, Ba and Ce carriers were added to the deposition samples used to analyse gamma-emitting radionuclides and radiostrontium, and the samples were acidified with nitric acid on arrival at the laboratory. The samples were concentrated by evaporating them and ashing the dried residue at 450 °C. Radiochemical " Sr and 90 Sr analyses were performed on the ashed samples after the gammaspectrometric analyses. The monthly samples were not analysed separately for strontium isotopes, but those from certain stations were combined quaiter- 11 ly before the radiostrontium analyses. Samples from the stations with the lowest deposition were combined every half year to get the total amounts of ,0 Sr deposited at these stations. Samples from the 1 m2 collectors, used to analyse transuranic elements were also evaporated and ashed. If the monthly samples vrere combined temporally, this was done after ashing. The sam ples were then dissolved in 8 M nitric acid. 242Pu and 243Am tracers were added to tha samples and they were mixed overnight. After filtration with glass fibre filters the remaining residues were wet ashed with nitric and hydrochloric acids. After filtra tion these acid-leached parts of the samples were evaporated, dissolved in 8 M nitric acid and combined with the rest of the sample. 2.3 Analysis methods 2.3.1 Gammaspectrometric analyses Three spectrometry systems were used for airdust and deposition samples. The relative efficiency of the detectors varies between 30% and 39%. The smallest detector is lithium drifted germanium crystal, the others being high purity germanium crystals. The detectors are placed in background shields made of 12-14 cm thick lead rings which are gradually linod with cadmium (1 mm) end copper (0.5 mm) to reduce the effect of X-rays.