INSPECTION OF RADIATION DOSE CAUSED BY RADON AND ITS PROGENIES IN CAVE AND BUILDINGS Thesis (PhD)

Transcription

1 INSPECTION OF RADIATION DOSE CAUSED BY RADON AND ITS PROGENIES IN CAVE AND BUILDINGS Thesis (PhD) Gábor Szeiler Doctoral School of Chemistry and Environment Consultant: Dr. János Somlai university lecturer Pannon University Institute of Radiochemistry and Radioecology Veszprém, 2012

2 1. Introduction, objectives Two major sources of the radiation dose of natural origin are the external gamma dose coming from radioisotopes in the crustal, and radon (and its progenies) making up more than half of the radiation dose. Nowadays, stricter and stricter recommendations are made up by international radiation protection organizations in order to prevent the occurrence of extreme radiation dose. It is proved that radon is in the second place as a cause for the occurrence of illnesses of pulmonary cancer. Therefore, it is reasonable to set up so-called activity levels both at workplaces and in residential buildings. However, even in case of complying with the activity level given in the recommendation of ICRP a 100 times greater radiation dose may be found in a flat than a resident living near a nuclear power plant receives from the nuclear power plant. Unfortunately, the regulations concerning radon concentration in Hungary are only concerning workplaces, therefore, it may easily happen that a unsuspicious citizen receives a radiation dose greater in their own flat than a worker receives at their workplace dangerous of radiation, for example in a nuclear power plant. All these support the reasonability of measurements, regulations, and the inspection and elaboration of interventions reducing radon concentration. During my work the values of radon concentration and gamma dose rate generated in such places were measured, and also the influence of the ventilation frequency on the reduction of radiation dose was inspected. An objective, on the one hand, is to certify that the use of slag at building construction works, produced during burning coal with radium-content in Ajka causes a considerable excessive radiation dose in many cases on residents of the buildings related to those of the residents of houses made up of normal building material. The influence of the method that seems to be the simplest of the possible ways of reducing radon concentration was inspected, that is the influence of frequent ventilation. In opposition to previous literature measurements, however, not only the changes in radon concentration were identified, but also the changes in the concentration of progenies causing the actual radiation dose. High level of radon concentration may occur in underground workplaces, therefore, identifying radon concentration and the radiation dose originating from radon and its progenies on those working there may be especially important in mines and caves. During the work measurements concerning the latter were performed in the Lake Cave of Tapolca. 2. Inspection methods and tools I. Seasonal changes of radon concentration in residential houses The annual radon activity concentration average value and the seasonal changes were identified using integral measurement methods (using solid body trace detectors) in 182 buildings in three settlements, and also the gamma dose rate in flats was identified (Automess 6150ADB ambient dose rate meter). The bound effective dose originating from radon and the effective dose originating from gamma radiation were assessed from the measurement data. Besides, the radon concentration changes per part of the day were also inspected in some flats (Radim3 and Radim2P radon monitors). In 25 buildings of one of the inspection locations it is proved that slag was built in as building material, therefore, the ratio of seasonal average radon concentration values were inspected

3 related to the annual average both in buildings containing slag and in buildings not containing slag. II. Ventilation inspections It is known that frequent ventilation reduces average radon concentration. However, the actual radiation dose is not caused by radon but its progeny concentration. In case of ventilations with different frequencies the equilibrium factor was measured and the value of EEC causing actual radiation dose was identified (Aphaguard PRQ 2000 radon monitor, and WLx radon progeny meter). III. Measurements in caves Measurements identifying radon activity concentration have been going on in the Lake Cave for several years, and since the date of putting into effect of the regulation No. 16/2000 (VI.8.) of the Ministry of Health Care also the tracing of the radiation dose of workers has become due. The annual average radon concentration in the cash-desk, waiting room in the building above-ground also exceeds the average value 1000 Bq/m 3 regulated by law. Monthly average values, especially in winter months showed great fluctuation. Monthly average radon concentration was measured at the constant workplace of those working in the cave, that is by the port and in the waiting room above-ground using trace detectors, and the personal radiation dose using a personal radon detector worn by the workers each. The influence of the annual average pressure and temperature measured outdoors was inspected related to the annual average radon concentration in the cave, and also the relationship between the bound annual effective dose estimated from the annual average air-space radon concentration and from the values actually measured with the personal radon dosimeters. 3. Theses I. Seasonal changes in the radon concentration in the flats Based on the radon concentration surveys performed with replacement every 3 months in three settlements (Bakonycsernye, Nádasdladány, and in Ajka) using common building material, and in Ajka using also coal slag during the building works of buildings, and on the results of gamma dose rate measurements the following was found: the external extra radiation dose originating from gamma radiation in flats with slag (1,79mSv/year) twice exceeds the values measured in buildings not using slag as building material (0,64-0,87). the annual average radon concentration in the surveyed buildings with slag was 668 Bq/m 3, and was minimum than four times in another settlements measured ( Bq/m 3 ). It exceeded the 300 Bq/m 3 activity level set out in the newest WHO and ICRP 2009 recommendation. Both the gamma dose rate and the distribution of radon concentration are completely differ in houses if they contain coal slag or not. Studying the daily fluctuation it is apparent that in the evening and at night, when people are sure to be home, the level of radon concentration is higher than in the daytime, therefore, underestimation can occur when calculating radiation dose from the average value, especially in case of higher radon concentration values.

4 In case of buildings built using slag the correction factors recommended by different international literature cannot be applied for making an estimation of the annual average value from the seasonal radon concentration level, because in case of using slag as constructional material the fluctuation of the radon concentration is low. II. Ventilation efficiency as an intervention reducing radon level It is known that frequent ventilation reduces the average radon concentration level. However, the actual radiation dose is caused not by the concentration level of radon but that of its progenies. Therefore, the equilibrium factor was measured besides radon concentration during ventilation of different frequency (every 12, 8, 6, 4 hours), and the value of EEC giving the actual radiation dose was identified. It was found that the rate of radon concentration can be halved if the ventilation occur twice a day in spite of once a day. The further more increasement of the frequency of the ventilation effects lower average radon concentration decreasement ( probably thanks to the intensive radon inflow), the six times per day ventilation effects only 65% decreasement in the average radon concentration. However the increasement of the frequency of the ventilation significantly reduces the progenies concentration because during the ventilation the progenies ventilate out and the time needed for the equilibrium between inflow radon and progenies are longer. The value of the EEC giving the actual radiation dose defined the actual efficiency what is available with the different frequency of the ventilation. Based on the measurement results it was found that increasement in the frequency of the ventilation EEC was lower than it could be estimated from the decrease of the radon concentration level. In practice the frequent ventilation can be solved - with the use of an automatic ventilation system what is preventing heat-loss using heat-exchangers - without more difficult structural modification and continuously operating electrical equipments. III. Changes in the radon concentration in the cave, radiation dose of the workers in the Lake Cave in Tapolca The monthly average values of radon concentration and the radiation dose on those working there have been surveyed in the Lake Cave in Tapolca for 6 years. It was found that the annual average radon concentration both in the cave (average: 8641 [ ] Bq/m 3 ) and at the cash-desk (average: 1113 [ ] Bq/m 3 ) is high. At workers the bound effective dose originating from the radon were 10 msv/year, and those workers who worked throughout the year the maximum dose values exceeded the dose limit of 20 msv/year in three cases. Therefore not to exceed the dose limit (100 msv/5 years) the careful schedule and shifts of labour force is essential. Based on the measurement results it was found that there is close relationship between the bound annual effective dose estimated from the annual average air-space radon concentration and from the values actually measured using personal radon dosimeters in case of those working during the whole year in the cave and at the cash-desk. There is no single correlation between the annual average radon concentration and the annual average temperature and pressure measured outdoors.

5 4. Publication list Publications in foreign languages 1. J Somlai, G Szeiler, P Szabó, S Tokonami, T Ishikawa, A Sorimachi, S Yoshinaga, T Kovács: Radiation dose of workers originating from radon in the show Cave of Tapolca, Hungary. JOURNAL OF RADIOANALYTICAL AND NUCLEAR CHEMISTRY 279:(1) pp (2009) (IF: 0.631) 2. Norbert Kávási, János Somlai, Gábor Szeiler, Balázs Szabó, István Schafer and Tibor Kovács: Estimation of effective doses to cavers based on radon measurements carried out in seven caves of the Bakony Mountains in Hungary. RADIATION MEASUREMENTS- (ISSN: ) 45: pp (2010) (IF=1,019) 3. Somlai, J Hakl, N Kavasi, G Szeiler, P Szabo, T Kovacs Annual average radon concentration in the show caves of Hungary. JOURNAL OF RADIOANALYTICAL AND NUCLEAR CHEMISTRY 287:(2) pp (2011) (IF: 0.777) 4. V. Jobbágy, J. Somlai, J. Kovács, G. Szeiler, T. Kovács: Dependence of radon emanation of red mud bauxite processing wastes on heat treatment. JOURNAL OF HAZARDOUS MATERIALS , (IF=4,144) Conference proceedings 1. Somlai János, Hakl József, Kávási Norbert, Szabó Tibor, Kovács Tibor, Szabó Péter, Szeiler Gábor: Radonkoncentráció változása Magyarország turista barlangjaiban, XVIII. Környezeti ártalmak és a légzőrendszer Konferencia Kiadványa, 61-69, Hévíz, ISBN: J. Somlai, J. Hakl, N. Kávási, T. Kovács, G. Szeiler: Dolgozók sugárterhelése Magyarország túrabarlangjaiban, V. Magyar Radon Fórum Környezetvédelmi Konferencia Kiadványa 47-53, Pannon Egyetem, Veszprém, (2009). ISBN N. Gál, J. Somlai, G. Szeiler, Z. Sas, T. Kovács, N. Kávási, The change of radon activity concentration in Tapolca cave in the previous 4 years, VI th Radon Forum and Radon in Environment Satellite Workshop, Veszprem Hungary May J Somlai, Cs Földesi, G Szeiler, B Máté, T Kovács, Seasonal Changes of Radon Concentration Where Building Materials of High Radon Concentration were Used, In: Devasagayam TPA (szerk.), 7th International Conference on High Levels of Natural Radiation and Radon Areas: 7HLNRRA. Mumbai, India, Mumbai: p Somlai J, Kovács T, Kovács J, Sas Z, Szeiler G, Ajka környéki vörösiszap mező radiológiai vonatkozása, In: Szentmiklósi L (szerk.), Őszi Radiokémiai Napok Keszthely, Magyarország, , Keszthely: Magyar Kémikusok Egyesülete, p. 1.,(ISBN: ) 6. T. Kovács, Z. Sas, J. Somlai, G. Szeiler, Radiological Investigation of the Effects of the Red Mud Disaster, Radon Forum and Radon in Environment Satellite Workshop, Veszprem Hungary May 2011

6 Publication in foreign language under publication Gábor Szeiler, János Somlai, Tetsuo Ishikawa, Yasutaka Omori, Rosaline Mishra, B.K. Sapra, Y.S. Mayya, Shinji Tokonami, Anita Csordás and Tibor Kovács: Preliminary results from an indoor radon thoron survey in Hungary. RADIATION PROTECTION DOSIMETRY (under publication)