Development of Radon Trap Device by Chemical Reaction in Corona Discharge Field

Y. Ya m a d a 1, A. Koizumi1, K. Ishikawa2, Y. Hishinuma2 and K. Tatenuma2

1 Radon Research Group, National Institute of Radiological Sciences, 4-9-1, Anagawa, Inage-ku, Chiba 263-8555, Japan 2 KAKEN Co, Mito-Institute, 1044, Hori-cho, Mito, Ibaraki 310-0903, Japan

Abstract Radon is noble gas and it is chemically inert. Thus it does not make any reactant in the common sense. But it seems that radon reacts with fluorine to form radon fluoride although the compound is not properly characterized. Based on the idea that radon can be adequately excited with a highly reactive property, radon might form some fluoric compounds. We succeeded in the confirmation of chemical reaction between radon and fluorine when corona discharge was used as a promoter. The reaction was reversible, and the radon fluoride was stable only under the discharge field. Applying this phenomenon to radon reduction technique, we developed radon trap device using tetra-fluoric carbon (CF4) gas. The trap efficiency of the device was measured to be over 99% at CF4 of 5%. When stopped the corona discharge, the trapped radon was immediately released. The mass balance between trapped and released radon was reasonable. Here we show our experimental results on trap and release.

Introduction Radon is the noble gas, and its high concentration exposure causes lung cancer1. Based on common sense, the noble gases are chemically inert and do not make any reactants. However, it seems that radon reacts with fluorine to form radon fluoride2, 3. Based on the idea that radon can be adequately excited with a highly reactive property, radon might form some fluoric compounds. To promote a chemical reaction between radon and fluorine, we used a corona discharge under coexistence with tetra-fluoric carbon (CF4). Some of CF4 would be decomposed, and radical fluorine (*F) would be produced as shown in Fig. 1. Radon also would be excited by the corona discharge, and active radon (*Rn) would be produced. If *Rn runs to *F, radon fluoride (RnFx) might be produced. When this RnFx is non-volatile and chemically stable, it deposits to surrounding wall and the radon trap could be realized. And thus radon is removed from air. On the hypothesis that stable RnFx exists, we tried to confirm the radon trap phenomenon.

Materials and Method Experimental apparatus was consisted of a gas flow Trap system for radon and fluoric compound gas, a corona RnFx CF4 Rn in Air reactor and a radon Reaction Excitation monitoring system (see Fig.2). Gas flow Gas flow Radon emanated from a *Rn *F *F ceramics disc source of 226Ra Surface electrode of 1.85 MBq was fed into the reactor. Radical fluorine was Inner electrode Electrode Aluminum cylinder of discharge supplied by discharging tetra-fluoric carbon (CF4). The total flow rate of radon-laden air and CF4 gas was constantly maintained to Fig. 1 Reaction image of radon with fluoric compound gas 100 cm3/min. Aluminum cylinder type of surface discharge was used to obtain a corona discharge. The average residence time of sample air in the reactor was calculated to be 4.7 sec. The discharge of corona reactor was operated by high frequency of 10 kV of peak to peak. White color along to surface electrode shows the strong field of plasma reaction. The radon concentrations were continuously measured with two scintillation cell type radon monitors located at upstream and downstream of the reactor. The radon Fig.2 Radon trap apparatus concentration was determined by the alpha counts measured during each 5 min.

Results and Discussion 1) Radon trap phenomenon: 107

Sample air containing radon was ) 3 6 continuously introduced together with 10 Discharge Upstream CF4 to corona reactor. When turned on the discharge, radon concentration began . (Bq/m 105 Downstream to decrease gradually and was reached to 4 Upstream a steady state as shown in Fig. 3. The don conc 10 Downstream radon concentration was obviously Ra reduced to less than one-hundredth. 103 2) Dependence of trap efficiency on CF4 02468101214 concentration: TiTimeme (hr(hr)) The trap efficiency depended discretely on CF4 concentration. Over 5% Fig.3 Radon concentration change CF4 was necessary to radon trap. by corona discharge treatment 3) Mass balance between the trapped and the released radon: When stopped the corona discharge, the radon concentration in downstream immediately increased. The amount of the released radon was close to that of the trapped radon.

Conclusion 1) The trap efficiency over 99.5 % was observed and its validity of the corona discharge technique was confirmed. 2) Chemical form of radon fluoride was not specified, but the formation was confirmed by mass balance of the trapped and released radon. It is assumed that a radon compound produced in the circumstance of coexistence with reactive fluorine should be meta-stable and be very easy to decompose. 3) This trapping technique of radon is expectedly applicable to other radioactive noble gases such as xenon and krypton emitted from nuclear power plants.

References 1) National Research Council, in Health Effects of Exposure to Radon (BEIR VI) (National Academy Press, Washington DC, 1999). 2) http://www.webelements.com/webelements/elements/text/Rn/chem.html 3) http://www.exresult.com/article/Radon