STUDY ON A NEW METHOD FOR REDUCING RADON LEVEL IN INDOOR AIR. Takao IIDA, Takayuki KATO and Hiroki MOCHIZUKI

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1 Radon in the Living Environment, April 1999, Athens, Greece 086 STUDY ON A NEW METHOD FOR REDUCING RADON LEVEL IN INDOOR AIR Takao IIDA, Takayuki KATO and Hiroki MOCHIZUKI Department of Nuclear Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, , Japan Tel: ; Fax: t-iida@nucl.nagoya-u.ac.jp The aim of this study is to develop a new technique for reducing high radon level in indoor air by method of membrane permeation using a hollow fiber module. The experiment has been carried out for MERASILOX-S module of fine hollow fibers of silicon rubber that has high permeability constant and MHF hollow fiber module made of thin segment polyurethane membrane. The radon permeability constants of hollow fiber membranes have been estimated from the decrease rate of radon concentration in a radon permeation system. The apparent permeability constant depends on the flow rate through the outside and the inside of hollow fibers. The apparent permeability constants of MERASILOX-S and MHF module membranes were m 2. s -1 and m 2. s -1 at the flow rate above 5.0 L. min -1. However, the permeation velocity of MHF module is grater than that of MERASILOX-S module. The radon concentration in a 50 m 3 room of concrete building could be reduced to about 1/10 by using a MHF module with membrane area of 18 m 2. The use of pressure different between the inside and the outside of hollow fibers might be applied to reduce the environmental radon concentration. Key words: radon removal, hollow fiber, silicon rubber, permeability constant, polyurethane, permeation velocity, radon concentration INTRODUCTION Indoor radon has at least two effects on human activities. One is the exposure due to inhalation of radon and its decay products. It has been reported that the effective dose equivalent in dwellings accounts on average for about one-half of that from all natural radiation sources (UNSCEAR Report 1993). Another is the effect on research activities or development of new products. Radon can enters into the clean rooms for production of semiconductor devices and research of molecular biology. Moreover, radon is the main cause of increasing background for radiation control and measurements. To reduce the effective dose equivalent due to radon and its progeny, it is necessary to remove radon in indoor air. For airborne radon progeny, Jonassen (1987) had already studied the effects of filtration and exposure to electric fields. Because radon is chemically inactive, charcoal trap method has been the only method until now. However, the method is affected by air humidity. In the present study, a membrane permeation method has been studied for radon removal in air. The air of low radon concentration could be obtained by permeating radon through membrane. This method had already been applied to separation of krypton and xenon from nuclear reactor atmospheres (Stern et al. 1980). The characteristics of the membrane permeation method for radon have been investigated and the validity for radon removal has been considered. 751

2 086 Radon in the Living Environment, April 1999, Athens, Greece EXPERIMENTAL METHOD When there is a concentration gradient of radon in a membrane, radon permeates along the gradient. The value of permeability constant depends mainly on the membrane materials. At the beginning, the radon permeability constants have been measured for various rubber membranes. Rubber is well known for permeable materials to radon (Jha et al.). The obtained radon permeability constants were (5.1±1.9) m 2. s -1, (5.9±2.0) m 2. s -1, and (3.8±0.5) 10-9 m 2. s -1 for natural rubber, synthesis rubber, and silicone rubber, respectively. It was found that silicone rubber membrane has the highest constant from the preliminary experiment. Then, MERASILOX-S (Senko Ika Kogyo Co., Ltd.) was used for the radon removal experiments. The MERASILOX-S is the module for an artificial lung that is composed of 9,000 fine hollow fibers of silicon rubber. Each hollow fiber is 200μm in inner diameter, 100μm in thickness of silicon rubber membrane. The module of silicon rubber hollow fibers has the effective surface membrane area of 0.8 m 2, and is set in the 50 ml cylindrical polycarbonate vessel. Figure 1 shows the schematic diagram of radon permeation system set up with MERASILOX-S. The system of radon removal experiment consists of the MELASILOX-S module, a flow meter, an electrostatic radon detector, a diaphragm pump, and a filter holder which are connected in series. The air of high radon concentration was circulated in the system through the outside space of the hollow fibers in the MERASILOX-S. On the other hand, radon free air was passed through the inside of the hollow fibers. Then, the radon concentration in the system decreases since radon permeates through the hollow fiber membrane. The air flow-rate of the inside of hollow fibers is equal to the outside, and the direction was reversed to each other. The flow rates were changed from 0.5 L. min -1 to 6.0 L. min -1. The radon concentration in the system was measured with an electrostatic radon detector developed by Tasaka et al. (1994). The radon detector has a volume of about 0.9 L. Radon decays to 218 Po, and most 218 Po atoms are positively charged. The positive 218 Po ions are collected on the electrode of PIN photodiode. The pulse height spectrum for α-particles from radon daughters on the electrode is obtained on the personal computer. Therefore, the time variation of radon concentration in the system could be measured by using the α-counts from 218 Po atoms which have the half-life of 3.05 min. The membrane of silicon rubber hollow fiber in MELASILOX-S module is comparatively thick. As the permeation velocity is the product of permeability constant and membrane thickness, it is expected that the hollow fiber module with thin membrane had high permeation velocity. The same experiment was performed for an another hollow fiber module, MHF (Mitsubishi Rayon Co., Ltd.) made of segment polyurethane. The MHF hollow fiber module is used for degassing. Each hollow fiber is 200μm in inner diameter, 0.5μm in thickness of the membrane. The module of the fine hollow fibers has the effective surface membrane area of 0.6 m 2, and is set in the cylindrical polycarbonate vessel of 200 mm in length and 48 mm in diameter. The experiment was carried out in the same radon permeation system set up with MHF hollow fiber module. The flow rates were changed from 2.0 L. min -1 to 5.5 L. min -1. RESULTS AND DISCUSSION Figure 2 shows the time variations of radon concentration in the system with MERASILOX-S. The radon concentration in the system decreased exponentially. The time variation depends on the total volume of the system, the flow rate, and the radon permeability constant and the surface area of 752

3 Radon in the Living Environment, April 1999, Athens, Greece 086 hollow fiber membrane. The time variation of radon concentration in the system could be expressed theoretically as follows. It is supposed that there is no difference of radon concentration by place of the inside/outside of hollow fiber. A flow of radon in a hollow fiber membrane is given by Fick s first law as follows. n J = - P, (1) x Where, J is radon flux in Bq. m -2. s -1, P is permeability constant in m 2. s -1, and n is radon concentration in Bq. m -3. Assuming that the concentration gradient of radon in a membrane is constant and the radon concentration inside the hollow fiber is zero, Eqn (1) could be expressed as follows. P.n in J = -, (2) δ where, δ is the thickness of hollow fiber membrane in m and n in is the radon concentration in the system in Bq. m -3. The time variation of radon concentration in the system is given by dn in S. J = - λ. n in, (3) dt V Where, S is the surface area of hollow fiber in m 2, V is the total volume of the experimental system in m 3, and λ is radon decay constant in s -1. Defining radon exchange rate γ in the system as the following solution is obtained from eqn (3). S. P γ =, (4) δ. V where, n 0 is initial radon concentration in the system in Bq. m -3. n in = n 0 exp{-(γ+λ)t}, (5) The apparent permeability constants of radon for silicon rubber hollow fiber could be calculated from the results shown in Figure 2. As described above, since the time variation depends on flow rate, the apparent permeability constant depends also on the flow rate. Figure 3 shows the dependence of apparent permeability constant of silicon rubber membrane on the flow rate. As shown in Figure 3, the apparent permeability constant is fixed value at the flow rate above 5.0 L. min -1. It may be due to the difference of radon concentration by the place of the inside/ outside of hollow fiber at the low flow rate. The apparent permeability constant at the flow rate above 5.0 L. min -1 is m 2. s -1 that is same value as silicone rubber. Figure 4 shows the time variations of radon concentration in the system with MHF module 753

4 086 Radon in the Living Environment, April 1999, Athens, Greece membrane. The apparent permeability constants could be calculated by using eqns (4) and (5). Figure 5 shows the dependence of apparent permeability constant of MHF module membrane. The apparent permeability constant of MHF module membrane was m 2. s -1 at the flow rate above 5.0 L. min -1. The constant is about one third of MERASILOX-S module membrane. However, as the membrane of MHF module is thinner than that of MERASILOX-S module, the permeation velocity of m. s -1 for MHF module is grater than that of m. s -1 for MERASILOX-S module. Then, the radon concentration in the system decreases more rapidly. CONCLUSION Radon in a membrane moves along the concentration gradient. The validity of membrane permeation method has been studied for radon removal from air. The experiment has been carried out for MERASILOX-S module and MHF module. The radon concentration in the system decreased exponentially. The apparent permeability constant of MERASILOX-S module membrane was m 2. s -1 that is same value as silicone rubber. The apparent permeability constant of MHF module membrane was m 2. s -1 that is about one third of MERASILOX-S module membrane. However, as the membrane of MHF module is very thin, the permeation velocity is larger than that for MERASILOX-S module. The radon concentration in the room of concrete building with airtight, which has 50m 3 volume and 50m 3 surface area, is saturated to be about 1000 Bq. m -3, assuming that the radon exhalation rate from concrete wall surface is Bq. m -2. s -1. The radon concentration could be reduced to about 1/10 by using a MHF module with membrane area of 18 m 2. However, the removal level is insufficient for practical application. The use of pressure different between the inside and the outside of hollow fibers might be applied to reduce the environmental radon. Further study is necessary as follows: (1) the improvement of module structure, (2) the use of pressure different between the inside and the outside of hollow fibers and (3) the test for other types of hollow fibers. REFERENCES [1] Jha G, Raghavayya M, Padmanabhan N. Radon permeability of some membranes. Health Phys. 1982; 42: [2] Jonassen N. The effects of filtration and exposure to electric fields on airborne radon progeny. In: Hopke P K editor. Radon and Its Decay Products in Indoor Air, 331 ACS Symposium Series. American Chemical Society, Washington,DC 1987, pp [3] Stern S A. Leone S M. Separation of krypton and xenon by selective permeation. AIChE Journal 1980; 26: [4] Tasaka S, Sasaki Y. Okazawa H, Nakagawa M. Development of an underwater radon detector. Radioisotopes 1994; 43: (in Japanese). [5] UNSCEAR Report. Sources and Effect of Ionizing Radiation, United Nations, New York,

5 Radon in the Living Environment, April 1999, Athens, Greece 086 filter pump AIR IN OUT hollow fiber flow meter filter pump flow meter Power supply detector data logger PC Figure 1: Schematic diagram of radon permeation system set up with MERASILOX-S. 755

6 086 Radon in the Living Environment, April 1999, Athens, Greece Figure 2: The time variations of radon concentration in the system with MERASILOX-S. 756

7 Radon in the Living Environment, April 1999, Athens, Greece 086 Permeability coconstant(x10-9 m 2 s -1 ) ) Flow rate(l min -1 ) Figure 3: The apparent permeability constants of the silicon rubber membrane in MERASILOX-S. 757

8 086 Radon in the Living Environment, April 1999, Athens, Greece Figure 4: The time variations of radon concentration in the system with MHF module membrane. 758

9 Radon in the Living Environment, April 1999, Athens, Greece Permeability constant(x10-9 m2 s -1 ) Flow rate(l min -1 ) Figure 5: The apparent permeability constants of MHF module membrane. 759

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