MEASUREMENTS OF DOSE RATE AND RADON GAS CONCENTRATION IN SCHOOLS AND KINDERGARTENS CONSTRUCTED WITH USING COAL-SLAG AS BUILDING MATERIAL

Transcription

1 Radon in the Living Environment, 080 MEASUREMENTS OF DOSE RATE AND RADON GAS CONCENTRATION IN SCHOOLS AND KINDERGARTENS CONSTRUCTED WITH USING COAL-SLAG AS BUILDING MATERIAL Cs. NÉMETH*, J. SOMLAI**, B. KANYÁR** *Department of Physics, University of Veszprém, *Department of Radiochemistry, University of Veszprém, 8201 Veszprém, P.O. Box. 158 (Hungary) Coals mined in some regions of the Transdanubian Middle Mountains in Hungary have elevated concentrations of 22 Ra. The slags and ashes derived from these coals have been used as building material for schools and kindergartens. As the 22 Ra reached up in these waste products of coal-firing systems, the gamma dose contribution in the buildings, which contained them, was calculated. In these places the absorbed gamma dose rate could be reach the sevenfold of the global average values 80 ngy h -1 - but the annual effective doses are lower than 1 msv due to the relatively short time of occupancy even in the cases of highest level. The radon levels do not exceed the 102 Bq m -3 during the period when the people are in the rooms because of the frequent airing therefore, the assumed annual effective dose derived from radon not exceed 0. msv. INTRODUCTION In the last few decades the examination and restriction of the dose contribution deriving from natural radionuclides have attracted great attention. The biggest part of the dose is caused by the radon and its progeny but in some cases not negligible the dose originated from the elevated gamma dose rate. From the radiological point of view, the use of coal slag as building material, which may affect indoor doses from external irradiation and the inhalation of radon decay products, may be significant. Former measurements have proved that in the Transdanubian Hills in Hungary the natural radioactivity of coals, mainly the concentration of 22 Ra, exceed significantly up to fivefold or tenfold - of the world average level. [1] By burning these coals in small stoves and coal-fired power plants, the coal-slag and fly-ash become enriched in radium. [2] In spite of the fact that the use of these slags as building materials was banned since 190 in Hungary, [3] owing to poor public health control and lack of expert knowledge, it has not been stopped. The coal-slag itself from waste-heaps was applied as backfill or insulation material underneath the floors and in ceilings. In these buildings, external radiation due to excessive concentration of natural radionuclides in building materials as well as the internal dose contribution from radon exhaled from them, should be taken into account. The present investigations were made in schools and kindergartens in three towns (Ajka, Tatabánya and Várpalota) in this region, which all have coal mines, coal power plants and waste-heaps in their neighborhood, so it was likely of using the slag here. We examined the radionuclide concentrations of these slags, calculated the gamma dose rate and estimated the annual gamma dose contribution. The radon concentrations were measured by a device providing hourly average values in a period of - 18 days and the averages were calculated for the full period using these data. The difference in 85

2 080 Radon in the Living Environment, the average radon concentrations between the values calculated for the full periods and for the periods that the pupils really spent in the buildings were determined. METHODS OF MEASUREMENTS AND CALCULATION Determination of the external gamma-dose Methods Gamma dose rate measurements were performed in classrooms. Depending on the dimension of the classrooms, gamma radiation was measured at spots at 1 meter height above the floor. Additional measurements were taken at the floor (10 cm above the floor), and at the ceiling (10 cm below the ceiling) to determine the place of the slag. Monitoring of radiation level in the classrooms was carried out with a portable dosimeter with energy resolution of 30 kev at 7 MeV ± 20% (type of Berthold AUTOMESS 150ADB) calibrated by the Automation und Messtechnik GmbH Germany. Calculation of doses The annual effective dose was estimated using ICRP-0 [] and UNSCEAR- 93 [5] dose conversion factors. The external dose rate measurement in air at 1 m height above the floor and the 0.8 Sv/Gy conversion factor for children have been used. The annual dose contribution was determined by spending 180 school days with average of or 8 hours occupying of the exposed group. It means approximately 1100 or 1500 h/year. The 2000 h/year indoor occupancy can be applied at the case of kindergartens. Activity measurements of coal-slag Sampling and sample preparation Samples of slags, which probably were used as building materials, were taken primarily from waste-heaps, of coal-fired power plants, near Ajka, Tatabánya, and Várpalota. In some schools we were able to take samples of the built-in slags from under the floor or from the ceiling either. Coal-slag samples of about 1 kg were dried in air, later on at 105 o C. The dried samples were stored for 30 days in air-tight aluminum Marinelli beakers with a volume of 00 cm 3, to reach the radioactive equilibrium of 222 Rn with its progeny. The concentrations of natural radionuclides were determined by high resolution gamma ray spectrometry, using an n-type HPGe detector with an efficiency of 1% and energy resolution of 1.8 kev at kev. The gamma spectra were recorded by the Tennelec PCA-ME 8192-channel analyzer. The date collection time was 0000 seconds and the system was calibrated using an etalon containing natural uranium ore certified by the Hungarian National Authority of Measures. The 22 Ra concentration of the etalon was 28.3x10 3 ± 1.x10 3 Bq kg -1. In the slag samples the 22 Ra concentration were determined by measuring the activities of its decay products, 21 Pb (295 and 352 kev) and 21 Bi (09 and 1120 kev), that were in secular equilibrium with 22 Ra following the 30-day storage. The activity of 0 K was measured by the 11 MeV gamma ray, the 232 Th by the 911 kev gamma ray of 228 Ac and the 21 kev gamma ray of 208 Tl. Radon-222 measurements The device used was a PYLON AB-5 radon monitor equipped with a PCRD detector (calibrated, by a PYLON RN 2000A type radon source with activity of ± 0.% kbq in a Genitron EV calibration chamber with dm 3 volume) and a type of RADIM 2P, (calibrated by the Czech National Authority of Measures) equipped with semiconductor detector which stored the hourly average values in their memory. Each period of -18 days long measurements were carried out in 8

3 Radon in the Living Environment, 080 January and February. The radon concentration during the full measuring period and during the time that the pupil really spent in the buildings ( Monday - Friday) was determined. Calculation of dose The annual effective dose from radon was calculated using the directives of ICRP Publication 5. [] Assuming an annual exposure at home to radon progeny per unit radon concentration (7000 h y -1 indoors and 0. equilibrium factor) of 1.5 x 10-2 (mj h m -3 ) (Bq m -3 ) -1 and effective dose per unit exposure to radon progeny at home of 1.1 msv (mj h m -3 ) -1. In our case we calculated with 1500 h -1 y factor. Under these circumstances radon concentration of 1 Bq m -3 corresponds to an annual effective dose of 0.38 x 10-2 msv. RESULTS AND DISCUSSION Gamma doses The distribution of absorbed dose rate at a height of 1 meter of the classrooms in the three towns (Ajka, Tatabánya, Várpalota) are listed in Table 1. It can be concluded that in town Ajka the prohibitive decree of using these slags has been kept. There were find only several schools with relatively high levels of absorbed dose among the investigated ones, but these had been built before 190. In the town Várpalota the slag was used several sites but here the concentration of 22 Ra is lower than in the other two ones. Therefore, here there was no level measured above 250 ngy/h. It can bee seen, that in the town Tatabánya the slag was used widely and this caused significant raising of dose rate in several case. The values of absorbed dose rate measured at different heights are detailed in the Table 2. It can be seen that on the uppermost stories the dose rates measured at the floors are usually lower than the levels at the ceilings. The cause of this fact is that these buildings have been built mostly with deck-roof and it was needed to use thicker slag layer as insulation here than between the stories. The annual effective doses are calculated by dose rates given in the Table 1. and to consider the occupying time and the conversion factor of 0.8 Sv/Gy can be seen in the Table 3. The children could get much higher dose then the world average value calculated inside buildings (80 ng h -1 dose rate) but, because of the relatively short time that they spend inside, this not means considerable extra dose. Radionuclides in slags Among the 22 Ra, 232 Th and 0 K concentrations in slags used as construction materials, the highest values were for 22 Ra. The average concentrations of slag samples originated from the three towns are given in Table. The concentration of 22 Ra were significantly higher than the world averages of building materials ( 22 Ra: 50 Bq/kg;, 232 Th: 50 Bq/kg; 0 K: 500 Bq/kg; [UNSCEAR-93]). According to recommendations and restrictions used by certain countries the most part of these slags would not have been allowed to use as building material. [7] 87

4 080 Radon in the Living Environment, Radon concentrations The radon derived from the slag or from the soil at the buildings with single-story. According to the measurements the average radon concentrations during the investigated period were between Bq m -3. But according to the continuous radon measurements it can be seen (Figure 1) that during the time when the students stay inside the radon concentrations and the calculated annual effective dose were significantly lower (Table 5.) perhaps due to the frequent opening of doors and windows. Therefore, although our measurements were relatively short of periods, considering them, we can assume that the long time measurements with passive radon detectors, which provide a result related to the average radon concentration during the full time of exposure, could be misleading to calculate the dose contributions when the rooms are used only a certain period of the day. CONCLUSIONS According to our measurements it can be concluded, that the concentration of 22 Ra in the slags used as building material for schools and kindergartens in the town Tatabánya greadly exceeds the global average value. In town Ajka the slags were not used frequently for school buildings. In town Várpalota the 22 Ra concentration of the used slags were much lower. The absorbed gamma dose caused by these materials is not significant because of the relatively short time of occupancy inside the buildings. Calculating the dose caused by radon could result a significant error if one calculate using the data of long time measurements provided by passive radon detectors. REFERENCES [1] Nikl, I.;Végvári, I. (1992) The natural radioactivity of coal and by-products in Hungarian coal-fired power plants. Izotóptechnika, diagnosztika 35, 57-; (in Hungarian) [2] Bódizs, D.; Gáspár, L.; Keömley, G. (1993) Radioactive emission from coal-fired power plants. Periodica Polytechnica Ser. Physics 1, [3] 2/190 ÉM a Governmental Issue on Radiation Protection (in Hungarian) [] ICRP, International Commission on Radiological Protection. (1991) 1990 recommendations of the International Commission on Radiological Protection, Oxford, Pergamon Press; ICRP Publication 0, [5] UNSCEAR, United Nations Scientific Committee on the Effects of Atomic Radiation, (1993) Sources and effects of ionizing radiation. New York, United Nations [] ICRP, International Commission on Radiological Protection. 199 Protection against radon-222 at home and work. Oxford, Pergamon Press; ICRP Publication 5, [7] Risica, S. (1998) Legislation on radon concentration at home and at work. Radiation Protection and Dosimetry 78,

5 Radon in the Living Environment, 080 Table 1: Absorbed dose rates at 1 meter height from the floor in the schools and kindergartens of the three towns. Absorbed dose rate Number of investigated classrooms of the towns intervals [ngy h -1 ] Ajka Tatabánya Várpalota < >

6 080 Radon in the Living Environment, Table 2: Absorbed dose rates at different heights in the schools and kindergartens in the town Tatabánya. Number Levels Number of rooms Averages of absorbed dose rates at different heights [ngy h -1 ] 1 m Floor Ceiling Ground-floor 3 2. Ground-floor /A Ground-floor /B Ground-floor Ground-floor Ground-floor Ground-floor Ground-floor 8. Ground-floor Second floor 9. Ground-floor Second floor Ground-floor Ground-floor Ground-floor Ground-floor Second floor Ground-floor Ground-floor Ground-floor

7 Radon in the Living Environment, 080 Table 3: Calculated annual effective doses with the different dose rates and the different occupation time. Dose rate [ng h -1 ] Annual dose [µsv] 1100 hours 1500 hours 2000 hours Table : Concentration of the 22 Ra, 232 Th, and 0 K radioisotopes in slag samples derived from buildings and from waste-heaps from towns Ajka, Tatabánya and Várpalota. Slag samples from 22 Ra Radionuclide concentrations [Bq kg -1 ] 232 Th min.-max. average average average Ajka , 213, Tatabánya ,5 372 Várpalota K Table 5: Average radon concentrations during the full periods (A) and during the periods the students spent in the buildings (B) and the calculated annual effective doses. No. Period of measuring Average radon concentrations [Bq m -3 ] Annual effective dose [µsv] A B A B

8 080 Radon in the Living Environment, Radon concentration [Bq -3 ] Tim e [hours] Figure 1: Average values of a continuous measurement in a classroom. 92