ENVIRONMENTAL RADON MONITORING IN DWELLINGS NEAR THE RADIOACTIVE SITES

Transcription

1 ENVIRONMENTAL RADON MONITORING IN DWELLINGS NEAR THE RADIOACTIVE SITES B.S BAJWA and H.S. VIRK Deptt. of Physics, Guru Nanak Dev University Amritsar , India. In view of the of the fact that radon and its daughter product are a major source of natural radiation exposure, the measurement of radon concentration in dwellings is assuming ever increasing importance. It is known from recent surveys in many countries that radon and its progeny contributes significantly to total inhalation dose and its fairly established that radon when inhaled in large quantity causes lung disorder. Keeping this in view, the indoor radon level monitoring were carried out in the dwellings of Ramehra, Nukhel, Gallot, Kheri, Ropa and Samurkhurd villages, using the Nuclear Etch (passive) technique. These villages are known to be located in the vicinity of radioactive sites of Himachal Pradesh. The radon concentration in the dwellings of some villages has been found to be varying from Bq/m 3. The radon concentration has been found to be not only varying with seasonal changes, but also with the mode of construction of houses, building material utilized and the ventilation conditions. The houses with mud floors shows quite higher radon as compared to the houses with brick flooring. Even the radon survey in the soil-gas and the dwellings has also been carried out using the Alpha- Guard (Active ) technique, which is based on pulse ionization chamber. The soil-gas radon concentration shows a large variation from 1000 Bq/m 3 to Bq/m 3. The indoor radon concentrations levels which have been measured in the dwellings by the Alpha-Guard are quite different as compared to that measured in these dwellings by the Nuclear Etch technique (using the SSNTDs), indicating the importance of the SSNTDs in the long-term integrated measurement of radon levels in the dwellings. INTRODUCTION Measurement of indoor radon are of importance because the radiation dose to human population due to inhalation of radon and its daughters contributes more than 50% of the total dose from natural sources (1). The three radon isotopes ( 219 Rn, 220 Rn, 222 Rn ) are gaseous and they may be released from the ground, rocks and also from building materials and accumulate with their short lived daughters in closed spaces, and in particular in dwellings. 220 Rn and 219 Rn, mainly because of their short half-life are not as important as 222 Rn, which may reach levels of concentration in the air which are significant in terms of radiological protection. The dose deriving from the presence in the air of 222 Rn is linked to the inhalation of its short-lived daughters, which are deposited in the respiratory organs, if deeply inhaled, emit alpha-particles in direct contact with the bronchial and pulmonary epithelium. For these reasons, the dose deriving from the exposure to 222 Rn in closed spaces has been placed in direct relation to the risk of lung cancer (2). The epidemiological evidence for the induction of lung cancer following inhalation of radon comes from several cases studies of underground miners. During recent years, several reports have appeared in literature demonstrating the ever increasing interest in monitoring the radon in the dwellings all over the world (3-9) and the result of the studies show that some countries ( e.g. Sweeden, Norway, Hungary and some parts of USA ) has high radon concentrations in many of their dwellings ( 1, 10) and even in certain cases doses from this source for some people living in these areas may exceed those received by occupational workers. The main sources of indoor radon levels are the soil-gas, building materials, tap water and natural gas used for cooking. The topography, house construction type, soil characteristics, ventilation rate, wind direction, atmospheric pressure and even the life style of the people, also significantly influence it (11-14), which emphasizes the importance of long - term integrated measurements, thus indicating the importance of the SSNTD techniques in these measurements. In the present investigations, these Solid State Nuclear Detectors (SSNTDs) have been utilized to study the indoor radon levels of the dwellings of Ramehra, Gallote, Nukhel, Kheri and Samurkhurd villages, which are located in the vicinity of the uranium mineralized pockets of the Hamirpur District, Himachal Pradesh. EXPERIMENTAL METHODS Experimental methods for radon detection and measurements are based on alpha-counting of radon and its daughters. Both active and passive devices are available for this purpose. Etch technique The plastic track detector (LR-115 type -II) is a cellulose nitrate film of 12 m thickness manufacture by Kodak Path, France. Due to its ruggedness and a fine window for recording alpha particles emanating from radon progeny, it is highly useful for integrated measurements from few days to several months. These plastics 1

2 films of size 2cm 2cm were fixed on glass slides and then these slides were mounted on the walls of different dwellings at a height of about 2 m from the ground level with their sensitive surface facing the air. After an exposure time of three to four months, detector films were removed and etched in 2.5 N NaOH solution at 60 C for 80 minutes in a constant temperature bath. Then these films were washed, dried and scanned under a Carl Zeiss binocular microscope for track density measurements. The exposed area of the each film was scanned thoroughly and sufficiently large number of graticules were counted to get average track density and to minimize the uncertainty due to counting errors. The track density is converted to radon concentration in dwellings using a calibration factor tracks/cm/day = 1Bq/m 3. the calibration factor was obtained after two inter-laboratory intercomparison excersises carried out at the national level by the Environment Assessment Division, BARC, Bombay. Alpha logger technique Alpha Guard supplies by Genitron Instrument, Germany, was used for the short term radon measurements in soil-gas and the indoor radon levels. It is based on the principle of pulse ionization chamber and can be operated in the diffusion and flow modes. A specially designed soil-gas unit measured radon concentration in soil by connecting the Alpha Guard, Alpha pump and STITZ-soil gas probe. To measure the indoor radon concentration the Alpha Guard has been operated in the flow-mode. RESULTS AND DISCUSSION The results of indoor radon concentration measured in dwellings of the village Ramehra, where observations have been taken in different seasons are reported in the Tables 1 & 2. The results shows quite higher indoor radon levels, especially in the winter season (December to April), as compared to the other season, thus clearly indicating the seasonal variations in the radon levels. It can also be observed that the average radon concentration in the winter season is two times higher than the average radon level noticed even in the next six months. But even in the winters, one of the dwellings R2 is showing quite low concentration as compared to the other dwellings. This may be mainly because of the pucca(cemented) floor and walls of this particular dwelling. The observations of indoor radon levels measured in the villages Nukhel & Gallote in the winter are summersied in Table 3. In most of the dwellings, especially in the villages of Ramehra, Gallote and Nukhel the construction of the houses is almost identical, where the local sandstone has been used for the walls, dolomite slates for the roof and the mud clay for plastering the floors. Only few houses have cemented (pucca) floors. As indicated in the Tables 1 & 3, the dwellings in these villages shows quite higher concentrations of the indoor radon levels. Especially in the winters, in these villages most of the dwellings shows radon concentrations above 300 Bq/m 3 and even some of the dwellings have quite high concentration as 500 Bq/m 3 to 900 Bq/m 3. These large variation of the indoor radon concentration between different dwellings of these villages can be explained due to different ventilation rate, nature of the soil underneath and particular due to the geological considerations. Even it has been established that the radon activity in soil gas is highly variable and it differ dramatically from place to place e.g. takes value of 30 KBq/m 3 in one place to 900 KBq/m 3, 10 m away ( 15 ). On the otherhand, indoor radon levels measured in the villages of Kheri and Ropa (Table 4), even during the winter interval (December to March) are comparatively very low. This is mainly because the dwellings in which radon level has been measured in these two villages are completely made up of bricks and cement and no local material has been used in these dwellings. Secondly these particular dwellings have cemented (pucca) floors and the cemented walls as compared to the dwellings in the villages of Romehra, Gallote,Nukhel and Samurkurd, which results in the reduction of radon levels in these houses. The Table 3, shows the variation of the radon levels in the dwellings of Nukhel and Gallote villages from December 1997 to March The concentration in these two villages varies from 200 to 900 Bq/m 3, whearas the indoor radon concentrations during the same duration of December 1997 to March 1998 measured in one of the location of Amritsra City, Punjab (Table 5), which is non urani-ferrous area, shows variation from 40 to 90 Bq/m 3 only. Thus the higher concentration of indoor radon activity of the villages in the uranium mineralized areas is mainly due to the higher amount of radioactive radon gas emanating from uranium bearing rocks and soils of these villages and thus clearly due to the presence of uranium prospects beneath the soil of this area and also may be due to high radium content in the building materials like stones, slates, mud etc, which have been extensively utilized in construction of these dwellings. The results of the Alpha-Guard radon survey in soil-gas carried out in some of the villages, has been reported in the Table 6, showing quite wide variation from 1000 Bq/m 3 to Bq/m 3. Especially the villages of Romehra and Samurkhurd, which shows high indoor radon levels, also indicates high values of the soil-gas radon of 69 and 75 KBq/m 3 respectively. Thus overall the reasons for the quite large variation of indoor radon 2

3 levels in the adjoing dwellings as well as in the soil -gas radon levels of the different areas, alongwith the other factors, may lies primarily in the geology of radon -- the factor that governs the occurrence of uranium, the formation of radon, movement of radon, soil-gas etc. The indoor radon concentrations in the dwellings of village Ramehra have also been measured using the Alpha-Guard (active) technique, and these are reported in the Table 7 & 8. Comparison of the results with the active and passive techniques(table 1 and Tables7&8) indicates that although the same dwelling (R3) shows the higher level in both the techniques, but the long term measurements using the SSNTDs are more significant, thus indicating the importance of the long-term integrated measurements (passive techniques) in the indoor radon level measurements. REFERENCES 1. UNSCEAR, Sources, Effects and Risks of Ionizing Radiation, United Nations Reports, New York, USA (1988) 2. ICRP Publication 50, Lung cancer risk from indoor exposure due to radon daughters. Annals of the ICRP 17(1) ( Oxford : Pergemon)(1987). 3. F. Abu-Jarad and J. H. Fremlin, A working level monitor for radon measurements inside houses. Radiation Protection Dosimetry,1, (1981). 4. G.Keller and H.H.Folkerts,Radon-222 concentration and decay product equilibrium in dwelling and in the open air. Health Physics, 47, (1984). 5. T.V.Ramachandran, B.Y.Lalit, and U.C.Mishra, Measurement of radon and thoron present in the environment using nuclear track etch detector technique. Nucl. Radiat. Meas.11, (1986). 6. M.C.Subba Ramu, T.V.Ramachandran, T.S.Muralidharan and A.N.Shaik, Indoor levels of radon daughter in some high background areas in India. Radiation Prot. Dosim. 30, 41-44(1990). 7. G.Marx and E. Toth, Increasing radon exhalation in Hungery. Rare Gas Geochemistry (ed. H.S.Virk),Guru Nanak Dev University,Amritsar, (1997). 8. W.W. Nazaroff and S.M.Doyle, Radon entery into houses having a crawl space. Health Physics 48, (1985). 9. A.J. Khan, A.K.Singh and R.Prasad, The distribution of radon levels in some indian dwellings. Rare gas Geochemistry, Amritsar, , Guru Nanak Dev University Press(1997). 10. G.A.Swedjemark and L.Majones, Radon and radon daughter concentrations in Sweedish homes. Radiat. Prot. Dosim.7, (1984). 11. E.Stranden, L.Berteig and F.Ugletveit, A study on radon in dwellings. Health Phys.. 36, (1979). 12. F. Abu-Jarad and J.H.Fremlin, Effects of internal wall covers on radon emanation inside houses. Health Phys. 44, (1983). 13. M.C.Subba Ramu, A.N.Shaikh, T.S.Muraleedharan and T.V.Ramachandran, Measurements of indoor radon levels in India usinf solid state nuclear track detectors : need for standardisation. Proc. of Seventh National Conference on Particle in Solids( eds S.Kumar and A.R.Reddy ),pp , Defence Laboratory,Jodhpur,India (1993). 14. U. C. Mishra and T. V. Ramachandran, Indoor radon levels in India : A review. Rare Gas Geochemistry( ed.h.s.virk) Guru Nanak Dev University Press,Amritsar, (1997) E.Toth, F.Deak, C.S.Gyurkosza, Z.S.Kasztovsky,R.Kuczi, G.Marx, B.Nagy, S.Oberstedt, L.Sajo- 3

4 Bohus, C.S.Sukosed,G.Toth and N.Vajda, Radon variation in a Hungarian village. Environ. Geol. 31, (1997). Table 1. -etch radon concentration ( passive technique) for exposure period ( 5thDec thApril, 1998 ) Village No. Graticules Radon Conc. R R R Ramera R R R Table 2. -etch radon concentration (passive technique) for exposure period(10thapril thSept.1998) Village No. Graticules Radon Conc. R Ramera R R R Table 3. -etch radon conc.for exposure period (Dec.1997-Mar.1998) Vilage Code Graticules () Galot GTD GTD GTD GTD Nukhel NTD NTD NTD NTD NTD NTD

5 Table 4. -etch radon conc. for an exposure period of ( 5 th Dec., th April, 1998) Village No. Graticules Radon Conc. Kheri Kh Ropa Rp Samurkhurd S S Table 5. -etch radon conc. for Dashmesh locality of Amritsar for exposure period (Dec Mar.1998) Code Graticules A A A A A

6 Table 6. Alpha Guard radon survey in the soil-gas of some villages situated in the vicinity of uranium-mineralized zones Village (KBq/m 3) Temperature ( C ) Pressure (mbar) Rel. Humidity (%) Mehra 1.02 ± Asthota 6.98± Ramera ± Samurkhurd 75.40± Table 7. * Alpha-Guard radon concentration (active technique) in the living rooms of village Ramehra on 8 th Jan,1998 Code Temperature ( C ) Pressure (mbar) R R2 53± R3 297± R4 165± Rel. Humidity (%) * Observations recorded in the diffusion - mode for 10 minutes durations. Table 8. $ Alpha-Guard radon concentration (active technique) in the living rooms of village Ramehra on 20 th Sept.,1998 Code Temperatrure ( C) Pressure (mbar) R R2 44± R3 149 ± R4 95± Rel. Humidity (%) $ Observations recorded in the diffusion - mode for 24 hours (overnight) durations. 6