The effects of the fuel cleaning incident at the Paks nuclear power plant on the radionuclid concentrations of the environmental samples

Transcription

1 AARMS Vol. 3, No. 2 (2004) RADIATION TECHNOLOGY The effects of the fuel cleaning incident at the Paks nuclear power plant on the radionuclid concentrations of the environmental samples RÓBERT BODNÁR, ENDRE GERMÁN Nuclear Power Plant, Paks, Hungary On the eve of 10th April, 2003 there was a serious incident in a cleaning tank used for the chemical cleaning of the fuel assemblies located in the service pit of Unit 2 at Paks Nuclear Power Plant. Radioactive emission was detected by the alarm systems, but the operators could not remove the cover of the tank for inspection. The event resulted low, but detectable emission, which was below the emission limit values. The incident was ranged into level 2 on INES scale and the Emergency Preparedness organisation was set up. On 16th April the cover was successfully lifted off revealing that all the 30 of the fuel rods in the cleaning tank had been damaged. Then the incident was ranged into level 3. In order to reveal the nuclear environmental effects of the 11 April fuel cleaning incident, the Environmental Monitoring Laboratory carried out a concentrated control sampling and measuring for several weeks. As the emission of radioisotopes took a longer period but in falling degree the basis of environmental survey was the checking of samples from the stations which were in the area of the actual air-current. In connection with the incident the laboratory determined the concentration of radioisotopes with gamma-spectrometry of 200 different samples (aerosol, iodine filter, falling-out, soil, grass, grain, milk, fish etc.) In addition we carried out nearly 30 insitu gamma-spectrometrical analysis with the radiation protection mobile vehicle and 4 onboard gamma-doserate monitoring around the main buildings of the power station and in some selected areas. The fall-out of I-131 was determined with in-situ investigation and gammaspectrometrical analysis methods in all stations by the laboratory. The largest fall-out was experienced around station A9, where the soil surface results of the different analysis were between Bq/m 2. In the vegetation (corn, grass) and soil samples gathered from the surrounding points of the power station radio-isotope I-131 was traceable only in the samples which were coming from the territories of the prevailing wind, the maximum value of I-131 was 140 Bq/kg and 50 Bq/m 2. Radioisotope I-131 wasn t traceable in the examined milk samples nor in water and fish samples from the nearby fish-pond (MDA: 0.5 Bq/kg). Received: February 26, 2004 Address for correspondence: RÓBERT BODNÁR Nuclear Power Plant, Paks, Hungary bodnarr@hpp.hu

2 Introduction Figure 1 shows distribution of sampling and monitoring stations around the Paks Nuclear Power Plant site, and the positions of the in-situ measurements and sampling points after the incident. In the morning of 11 April the BITT probe of the A1 station of the monitoring network indicated an increasing gamma-doserate of the environmental radiation. Searching the searching the reason we learnt about the incident which occurred during the cleaning of the fuel elements in block II. After that we compiled the data of the last hours wind direction and with this knowledge first from the exposed areas our team started to gather environmental samples. During the gathering, processing and measuring the samples we paid attention not to contaminate the measuring instruments nor the different samples. On the first day of the incident the wind first was blowing towards the north (station A1) and then towards the east and south-east (stations A8, A4). Later on the direction of the wind was nearly always southern, south-western and western (stations A9, A5, A6). We understood under the direction of wind the movement of the wind according to the appropriate corner. The emitted radioactive material was spreading, became diluted and approached the surface of the ground and settled outwards with the direction of the wind. (Notable precipitation significantly influential to the imission leaching out did not occur). Beside using air-sampling filters (type FPP polymeric ultrafine fibres with average diameter 1.5 µm; and) with big capacity, which is established to determine the characteristics of the imission near the ground surface, we examined the degree of fallout with alternative methods. In the case of fall-out samples we put a liquid with radioiodine fixing reagents (NaI, NaHCO 3 ) into a tray with known surface (0.14 m 2 ) and after a few day exposition we measured the sample coming from the fall-out in the laboratory. In an other method we gathered samples from the station s environmental area from the soil s upper 1 3 cm part with known surface and without any drying we determined the isotope content with the help of gamma-spectrometry. During our survey-work we carried out an in-situ gamma-spectrometrical measuring in a given area, with which we could determine the activity reached the ground surface. We tried to examine the environmental effects of the discharge emission in various ways and in the wider surroundings of the power station as well. For this reason we gathered environmental samples from different places and in different forms (grass, lucerne, corn leaf, cow milk, goat milk, honey etc.) and examined them with gammaspectrometrical method. We analysed water and fish samples of the artificial lakes near the power station with the same method. 210 AARMS 3(2) (2004)

3 Figure 1. Distribution of sampling and monitoring stations around the Paks Nuclear Power Plant site, and the positions of the in-situ measurements and sampling points after the incident. AARMS 3(2) (2004) 211

4 It is typical of the emission during incidents that they mostly consist of fission products and that 99% of them arise during the first 24 hours. As in connection with the incident I-131 radioisotopes had been detected in the environmental samples on some occasions, the daily development of the I-131 radio-iodine emission equivalent was selected from the emission data and is displayed in Figure 2. The I-131 radio-isotope emission equivalent was defined by having considered all the radio-iodine forms (aerosol, elemental and organic) taken from the so called daily operational measurement data of the sampling line of the highest activity, which was sampled at shorter intervals during the incident. Figure 2. The daily releases of radioiodines transformed to 131 I equivalent 212 AARMS 3(2) (2004)

5 Measuring methods Analysis methods of the gathered samples after the incident were chosen on the basis of sample characteristics, the goal of the analysis, the ecomplexity of the method and the range of traceable isotopes and concentration. On this account firstly we used the gamma-spectroscopy method with the analysis time of sec. Table 1 shows the parameters of the gamma-spectroscopic system which is used in our laboratory. Table 1.The specifications of the laboratory s gamma-spectroscopical measuring system HPGe detector Name Producer Type Relatice efficiency [%] Spectroscopic Amplifier High Voltage Milti Channel Analyser PGT EG & G Ortec Canberra Merion Ltd. (EMG) DL-466 GEM GC 3018 NU 8210 Model 2026 Model 2020 Model 3105 Model 3106D Series 35 Series FWHM at 1333 [kev] During the preparation of the samples we paid attention on keeping the original parameters of the sampling site (relative humidity, degree of fall-out, etc.) unchanged. In the case of unknown samples to determine the concentration of isotopes we used the closest matching certified samples of the OMH. To analyze the spectrums we used the Genie 2000 software (Canberra). Detection limit values of certain isotopes were determined by the method of CURRIE. 1 We carried out our in-situ measurements at a height of 1 metre over the ground. The ground was covered with some centimetres wide undergrowth (mainly grass) and the area was an uncultivated one. We indicated the results of the measurements separately where the ground was not covered by grass. In the case of in situ analysis, GC4020 portable detector (rel. eff.: 40%, FWHM at kev 2.0) was used equipped with Inspector Due to the lack of information about the isotope distribution and uncertainties of the incident, even superficial distribution was assumed for the imitted activities. AARMS 3(2) (2004) 213

6 Results and conclusions In connection with the incident we carried out the analysis of different environmental samples in our laboratory, but only the short half-life (8.02 day) radio-isotope I-131 could be detected. Representative data of our analysis results of the samples from the sampling and monitoring stations (type A) stations and their surroundings and our fieldworks can be seen in Tables 2 to 6. In Table 7 the I-131 concentrations of the samples gathered from around the power station are collected. The activity of I-131 is given relating to 11 April am. Sign < is used for the detection limit of the given method. We missed out the results of the measurements of the aerosol and iodine samples as well as other measurement figures because of the limited size of the article. Table 2. Activity concentration data of grass, soil and fall-out samples from Station A1 and superficial soil concentrations determined by in situ gamma-spectroscopy Sampling date [day, month; hour, min.] Grass (fresh) [Bq/kg] Soil Fall-out In situ : : : :00 < : : :00 <3 NOTE: Doserate at were 80, 81 and 75 nsv/h measured with FHZ621 B, AUTOMESS and BITT probe of the station, respectively. Table 3. Activity concentration data of grass, soil and fall-out samples from Station A4 and superficial soil concentrations determined by in situ gamma-spectroscopy Sampling date [day, month; hour, min.] Grass (fresh) [Bq/kg] Soil Fall-out In situ : : : : : :10 <2 NOTE: Doserate in apryl was 98 nsv/h measured with BITT probe of the station. 214 AARMS 3(2) (2004)

7 Table 4. Activity concentration data of grass, soil and fall-out samples from Station A5 and superficial soil concentrations determined by in situ gamma-spectroscopy Sampling date Grass (fresh) Soil Fall-out In situ Note [day, month; hour, min.] [Bq/kg] :40 < :40 < cm layer : :00 < :00 MDA : :00 < : :00 < : :00 < : : : :00 <2 NOTE: Doserate in apryl was 74 nsv/h measured with BITT probe of the station. Table 5. Activity concentration data of grass, soil and fall-out samples from Station A9 and superficial soil concentrations determined by in situ gamma-spectroscopy Sampling date [day, month; hour, min.] Grass (fresh) [Bq/kg] Soil Fall-out In situ : : : : : : : : : : : : : : : : : NOTE: Doserate in apryl was 75 nsv/h measured with BITT probe of the station. AARMS 3(2) (2004) 215

8 Table 6. Activity concentration data of grass, soil and fall-out samples from Station B24 and superficial soil concentrations determined by in situ gamma-spectroscopy Sampling date Grass (fresh) Soil Fall-out In situ Note [day, month; hour, min] [Bq/kg] :00 <0.9 <2 (0 1 cm) MDA 46 o // 18 o :00 MDA** 46 o // 18 o :00 <1.3* <4 (0 1 cm) MDA** 46 o // 18 o :00 <0.6* <2.0 MDA** 46 o // 18 o : :20 < : :50 <3 NOTE: *: corn leaf **: The in-situ measurements were carried out at an agricultural area, which was putting under corn leaf. Table 7. I-131 concentrations of the samples gathered from around the power station are collected Name of sample Sample site Date of sampling Amount of sample Activity concentration Chopped fish Anglers lake kg <2 Bq/kg Surface water Füzes II. lake l <0.3 Bq/l Surface water Füzes IV. lake l <1.0 Bq/l Alfalfa K7 meas. point g <3 Bq/kg (fresh) Goat milk Dunaszentgy ml <10 Bq/l (fresh) Goat milk Dunaszentgy ml <0.3 Bq/l Cow milk Dunaszentgy l <0.3 Bq/l Cow milk Gerjen l <0.1 Bq/l Soil from creche Paks, Ifj. str kg <3 Bq/kg Sand from kindergarden Paks, Ifj. str kg <4 Bq/kg Note: We carried out in-situ measurements at K7 control point as well, where the land was planted with lucerne ( 131 I=MDA). We tried to make an effort to carry out our examination the widest possible variety of samples and the territories. Based on the results we can make the conclusion that neither the population nor the environment got measurable surplus radiation in connection with the incident happened at the power station. References 1. CURRIE, L. A.: Limits for qualitative detection and quantitative determination: Application for radiochemistry, Analytical Chemistry, 40 (1968) AARMS 3(2) (2004)