Sources of contamination

Transcription

1 RADIOACTIVE CONTAMINATION OF WOOD IN LITHUANIA L. Pilkytė, R. Ladygienė, A. Orentienė, A. Skripkienė Radiation Protection Centre Lithuania

2 CONTENT OF PRESENTATION Sources of radioactive contamination Methods of measurements Radioactive contamination of Lithuanian wood Radioactive contamination of wood imported to Lithuania Radioactive contamination of wood fuel and ash Evaluation of effective dose to population due to different use of wood

3 Sources of contamination Global radioactive fallout after nuclear weapons test in atmosphere and fallout after Chernobyl nuclear accident contaminated Lithuanian environment with mixture of radionuclides Recently short lived radionuclides already decayed. Contamination with long lived fission products Cs-137 and Sr-90 declines slowly After Chernobyl accident radioactive pollution fell down in dust form and not heavily contaminated territory of Lithuania Fallout resulted contamination with Cs-137 up to 18.5 kbq/m 2 and the highest contaminated areas were in some places in the west and south part of Lithuania where contamination was approximately 19.7 kbq/m 2 (Institute of Physics) Radiation Protection Centre started multipurpose survey including in situ gamma spectrometry measurements of Cs-137 in environment in 2008 with grid km 2 Cs-137 activity is in range kbq/m 2

4 Methods Sampling Gamma spectrometry method IEC 1452:1995, high-purity germanium detectors, relative efficiencies 27% the spectra analysis software Genie2000. Efficiency calibrated using mathematical calibration with LabSOCS in energy range kev for particular measurement geometries and densities of wood and ash Sr-90 activity concentration in the wood and ash was determined after radiochemical separation by counting the Cherenkov radiation with liquid scintillation counter Quantulus Methods accredited according requirements of ISO standard 17025:2005 Calculations of gamma dose rate due to Cs-137 in wood building materials performed by Visiplan 3D ALARA planning tool which allows to describe sources of any geometry and with any gamma radionuclide composition and to perform calculations of dose rate distribution around these sources. For estimation of effective dose to the population due different use of wood were used models from IAEA-TECDOC-1376 Assessing radiation doses to the public from radionuclides in timber and wood products.

5 Contamination of Lithuanian wood Wood samples investigated by gamma spectrometry during were: wooden board, chipboard, parquet, wood fiber, plywood, wooden door, timber. Types of investigated wood were: fir, pine, birch, oak, alder and asp During production process of wood items Cs-137 activity concentration in wood remains the same. Cs-137 activity concentration in different kinds of wood products was in range from MDA (less than minimum detectable activity) to 4±1 Bq/kg, average 1.3±0.3 Bq/kg (95 % confidence), median 1.0 Bq/kg. Accoding Lithuanian legislation exemption level for Cs-137 is Bq/kg Reletive frequency 0,3 0,2 0,1 0, Cs-137 activity concentration, Bq/kg Distribution of Cs-137 activity concentration in the samples of Lithuanian wood

6 Contamination of wood imported to Lithuania Not many samples of imported wood from Belarus and Ukraine were investigated. Highest Cs-137 activity concentration was 406 Bq/kg in wood from Belarus. More samples of wood were wood fuel Fuel was burnt in laboratory for investigation of Cs-137 enrichment in ash Measured Cs-137 activity concentration was: Country of origin Sample type Cs-137 activity concentration, Bq/kg In wood fuel In ash Ukraine Wood briquettes Wood pellets Belarus Wood pellets Wood chips Clearance level for Cs-137 is 400 Bq/kg

7 Evaluation exposure in living environment Another way to evaluate exposure to the population due to radioactive contamination in wood is measurements in living environment. Measurements of gamma dose rate of wooden furniture, wood trim and wooden board were done in living houses. Gamma dose rate did not exceed background. Gamma dose rate indoors were measured in over 600 of wooden dwellings built in whole Lithuania. Increase of gamma dose rate was not identified. It means dwellings were built from not contaminated or very little contaminated wood and fallout after Chernobyl accident did not contaminate heavily surface of houses. It do not cause additional effective dose to the population

8 Contamination of Lithuanian wood fuel Wood fuel production is important industrial sector from radiation protection point of view due to possible contamination in wood ash. Burning wood concentrates activity of radionuclides in ash 118 samples of wood fuel and ash were taken from 13 energy power plants in Average Cs-137 concentration 4,2±1,8 Bq/kg, Sr-90 3,8±1,4 Bq/kg in wood fuel, Cs ±54 Bq/kg, Sr-90 22±15 Bq/kg in ash. It was investigated that ash treatment reduce Cs-137 concentration 2-3 times Activity concentration, Bq q/kg Cs-137 Sr-90 Samples Cs-137 and Sr-90 activity concentrations in the samples of Lithuanian wood chips and sawdust

9 Effective dose due to use of Lithuanian wood ash Energy power plants burn Lithuanian wood fuel Combustion temperature is C at energy power plants. At such temperature part Cs-137 and Sr-90 evaporate and discharge into the environment Discharge is one of the pathways of exposure to the population. Effective doses due to Cs-137 and Sr-90 discharge was evaluated and do not exceed 10 µsv per year. It do not need radiation protection measures. Another part of Cs-137 and Sr-90 remains in the ash. Effective dose to the population depends on the way of ash treatment and use as fertilizer in agriculture. If the maximum permitted quantity of ash, 2.5 tons per hectare per year would be used up to 25 cm in the depth, the ash would contain the maximum measured values of Cs-137 (103 Bq/kg) and Sr-90 (22 Bq/kg), the radioactive contamination of soil would increase by Cs Bq/kg, Sr Bq/kg after a single fertilization. Such contamination will not have significant influence to the exposure of the population.

10 Contamination of wood fuel Detail radiological monitoring of Cs-137 in wood pellets was performed at one of the biggest Lithuanian factory (where mixture of wood of Lithuanian origin and Belarus origin were used) taking daily integrated samples (each sample was composed from 24 samples taken on 60 min basis). The measurements of 122 integrated samples were done June 23 - October 21, Cs-137 activity concentration in the wood pellets was in range from 5 up to 40 Bq/kg. 50 Cs-137 activity concentration, Bq/kg Date Wood fuel production plant.

11 Estimation effective dose to population due to using wood as a building material Wood is used in the construction of houses Wood can be used only for floor, walls or ceiling. For indoor exposure evaluation standard room m 3 with wall thickness of 20 cm, density 0.6 g/cm 3 is used Exposure occur from walls, floor and ceiling, provided each of these is constructed of contaminated wood Gamma dose rate is caused by activity concentration of Cs-137 and was calculated with Visiplan-3D in the middle of the room at height 1.25 m For dose rate calculations Cs-137 concentration in wood were used: 1.3 Bq/kg and 4.0 Bq/kg (average and maximum concentrations found in Lithuanian wood) Gamma dose rate in the standard room made of wood with 4 Bq/kg of Cs-137 is 0.6 ngy/h. It cause effective dose for the population 3 µsv per year. This dose is low less than 10 µsv per year (effective dose to the population due to exempted practice should not exceed 10 µsv per year). Effective dose would be 0.3 msv per year when Cs-137 concentration is 406 Bq/kg (Belarus origin)

12 CONCLUSIONS 1. Radioactive contamination of wood from Lithuanian forests is very low and it does not cost any problem from radiation protection point of view and would not lead to increase exposure to the population and workers; 2. Ash of fuel produced from Lithuanian wood used for fertilizers can contaminate soil up to 0.1 Bq/kg for single fertilization; 3. Lithuanian wood used as building material cause the effective dose to the population 3 µsv per year.