Radon Progeny Size Distribution in and around Closed Uranium Mine

Transcription

1 Radon Progeny Size Distribution in and around Closed Uranium Mine Kimio ITO, Yuu ISHIMORI, Sada-aki FURUTA JNC Ningyo-toge, 155 Kamisaibara-son Tomata-gun Okayama, , Japan INTRODUCTION The Japan Nuclear Cycle Development Institute (JNC) has uranium mines in Japan in order to develop the technique of uranium exploration. The JNC has already finished developing the exploration. But, the waste rock from the exploration activities has been left at the mine sites around Ningyo-toge. The radiation monitoring around the mine sites has been carried out because the JNC has to maintain the waste rock safely according to the Mine Safety Law. Radon is one of the most important radioactivity for the monitoring. Radon progeny attaches with ambient aerosol particles immediately after it is decayed from radon. Therefore, there are attached and unattached fractions of radon progeny in air (see Fig.1). A radon progeny size distribution of the attached fraction is affected by ambient aerosol and it is one of the important factors for the dose estimation due to radon progeny in air. A size distribution of unattached fraction is not so much affected by surrounding condition, but the ratio of unattached fraction to total radon progeny is an important factor for the dose estimation. Therefore, the measurements of the size distribution of attached fraction and the ratio of unattached fraction were carried out in and around mine sites. Aerosol particle Pb-214 Bi-214 Attachment Cluster formation Po-218 Recoil Rn-222 Exhalation Ground surface Fig.1 Rn-222 Ra-226 Basic processes of radon progeny behavior in air defining unattached and aerosol attached activities METHOD SIZE DISTRIBUTION OF ATTACHED FRACTION OF RADON PROGENY The outline of the measurement of a radon progeny size distribution is shown in Table 1 and Fig.2. The measurement of a radon progeny size distribution is based on the diffusion battery method that is often used to measure fine aerosol particles. The screen-type diffusion battery is used for our measurement because the equipment could be designed to make it portable. Portability is very important when we make outdoor measurements. A screen means a kind of mesh made of stainless steel. The collection efficiency on a screen was studied in detail by Cheng and Yeh (1). 1

2 Table 1 Outline of the measurement (a) Size distribution ITEMS COMMENTS Number of mesh density 25[-] Mesh diameter 4[mm] Stage of SDB 6 stages Number of screens (reference), 1, 1, 2, 46, 1, 2[-] Flow rate 4 Liter min -1 Iterative technique response matrix method (2, 3, 4) Measured item alpha radioactivity by SSB detectors (b) Unattached fraction ITEMS COMMENTS Number of mesh density 3[-] Mesh diameter 4[mm] front to total ratio Correction factors screen loss factor (5) Measured item alpha radioactivity by SSB detectors Ao[Bq] diffusion battery (a) measurement A[Bq] A/Ao number of mesh screens (b) penetration curve fraction particle diameter Fig.2 (c) radon progeny size distribution Schematic explanation for obtaining radon progeny size distribution 2

3 A screen-type diffusion battery consists of stack of the screens and seven kinds of diffusion battery are used for our measurement. The screen number of the seven diffusion batteries is, 6, 1, 2, 46, 1 and 2, respectively. The diffusion battery with screen is necessary to obtain a penetration rate. The penetration rate is the ratio of radioactivity through the battery with some screens to one through the battery with screen. The size distribution is calculated from the penetration curve by an iterative calculation based on the response matrix of the set of the diffusion batteries. Some kinds of the iterative calculation are developed by some researchers (2, 3, 4). These three methods were confirmed to give same results by some numerical experiment. In this study, the iterative calculation developed by Ikebe (2) is mainly used for our measurement. UNATTCHED FRACTION OF RADON PROGENY The outline of the measurement of the unattached fraction is shown in Table 1. A screen mesh and a filter are connected in series. After the unattached radon progeny is collected on a screen mesh, the attached radon progeny is collected on a filter. Two kinds of correction factors for measuring alpha particles on the screen mesh are recommended by Solomon and Ren (5). One is the front to total ratio and another is the screen loss factor. These correction factors are also considered in this study. RESULTS MEASUREMENT PLACES As mentioned in INTRODUCTION, there are some uranium mine sites with waste rock around Ningyo-toge. One of the most important uranium mine sites is the Katamo uranium mine site in Tottori prefecture. The radon and radon progeny concentration in air are high in the Katamo site. Especially, the concentrations in front of the old mining tunnel, which has been closed temporarily, are very high (6). Therefore, the Katamo site is selected as the measurement place. As the control place for the Katamo site, Ningyo-toge is selected because this place is considered to be the typical mine site around Ningyo-toge. The radon and radon progeny concentration are average and the most convenient place for our measurement. The measurement places are shown in Fig.3. 1km Tottori Hiroshima Katamo 1km Ningyo-toge Fig.3 Map around Ningyo-toge and Katamo MEASUREMENT RESULTS The obtained results are shown in Table 2 and Fig.4. The result of each place is an average of few measurements on one day. The geometric mean diameter (GMD), geometric standard deviation (GSD) and unattached fraction (fp) at Ningyo-toge were.26[µm], 2. and.1 respectively. The GMD, GSD and fp at the boundary of the Katamo site were.2[µm], 2.6 and.19 respectively. These results were not so different from 3

4 the results at Ningyo-toge where was selected as the control place. The fp was slightly larger than the result at Ningyo-toge. Table 2 Results of radon progeny size distribution and unattached fraction Place GMD[µm] GSD[ - ] fp Season for measurement Boundary of Katamo site Autumn in Front of the old tunnel in.5 Summer in 1998 &.48 & 2. Katamo site Autumn in 1998 Ningyo-toge Summer in 1996 Boundary of Katamo Old tunnel of Katamo(1998 summer) Old tunnel of Katamo(1998 autumn) Ningyo-toge(1996 summer) Fig.4 Radon progeny size distribution ( x-axis: particle diameter[µm], y-axis: normalized frequency[-] The results obtained in front of the old tunnel in the Katamo site, however, were very different from the results at Ningyo-toge. The radon progeny size distribution with two peaks was observed in the summer in The GMD and GSD of the smaller peak are.2[µm] and 1.4. Those of the larger peak were.48[µm] and 2.. The radon progeny size distribution in the autumn in 1998 had only one peak. The GMD and GSD were.11[µm] and 2.3. The GMD was smaller than the results obtained at Ningyo-toge or the boundary of the Katamo site. The fp in front of the old tunnel was much higher than other results. The distance between the boundary and the old tunnel is only few hundreds meters, but the difference of the characters of the radon progeny at both places was observed clearly. 4

5 DISCUSSION COMPARISON WITH REFERENCE DATA The characteristic parameters of radon progeny such as the size distribution, the unattached fraction, etc. have been studied by many researchers (7, 8, 9, 1). The GMD and GSD of these results were ranged from.1 to.4[µm] and from 2. to 3.[-], respectively. These measurements were carried out mainly indoor environment. Our results obtained in this study were almost included in the range of the referenced results. As mentioned in previous section, however, the results obtained in front of the old tunnel were clearly different from the results obtained at Ningyo-toge as the control. While the results at the boundary of the Katamo site was relatively similar to the control. A radon progeny size distribution depends on ambient aerosol size distribution strongly. The difference among the radon progeny size distributions obtained in our study means that the environment around each measured place may affect the results. The place measured in front of the old tunnel is surrounded by many trees and far from residential area where much ambient aerosol are released from many kinds of source, such as automobiles, human activities etc. While other places, Ningyo-toge and the boundary of the Katamo site, are located in relatively spacious environment and near to residential area. It is easily considered that the concentration of the ambient aerosol in front of the old tunnel is lower than the concentration at other places. While the radon concentration in front of the old tunnel is very high. It means that the unattached fraction in front of the old tunnel is higher than that of other places as shown by the results in this study. This difference in condition around each place may affect the results of radon progeny size distribution also. AFFECT ON DOSE ESTIMATION Here is consideration about the difference of the dose estimation due to the difference of the radon progeny characters. The International Committee on Radiological Protection (ICRP) has recommended the dose conversion factor of 5[mSv/WLM] for worker in the latest publication about radon (11). From the previous publication, however, the ICRP recommended the dose conversion factors that depend on fp. These factors are obtained as follows (12), ICRP model fp fp fp where is the Jacobi-Eisfeld model with the lung model of Weibel(A) and the James- Birchall model with the lung model of Yeh-Shum. These factors are obtained in the case of 1.2[m 3 /h] for breathing rate. The ICRP recommends these factors to be used for the AMAD (activity median aerodynamic diameter) of.2-.3[µm]. The Nuclear Energy Agency of Organization for Economic Cooperation and Development (OECD NEA) published a report for radon (13). The and are recommended again. In this report, however, the conversion factors are prepared for each AMAD of.1,.2,.3[µm] as follows, for.1[µm] for.2[µm] for.3[µm] fp fp fp fp fp fp Otherwise, the new lung model of the ICRP had been recommended. The LUDEP, which is a convenient software to calculate lung dose on the new respiratory tract model due to radioactive nuclides, has been developed (14). For the present, the ICRP does not recommend the application of the new model for radon 5

6 progeny (11). Here is shown a dependency on the character of radon progeny by relative comparison among the results from the LUDEP. The relative comparison normalized to the results of the control is shown in Table 3. In this table, the and the are referred from the report of OECD NEA. These models are prepared for each AMAD. The AMAD of the boundary of the Katamo site, the front of the old tunnel and Ningyo-toge is approximately represented as.2,.1 and.3[µm] respectively except for the calculation by the LUDEP. The measured results of the GMD and GSD are used for the calculation by the LUDEP. Table 3 Relative comparison of the dose conversion factor Place ICRP Season for LUDEP* model measurement Boundary of Katamo site Autumn in 1996 Front of the old tunnel in ** 4.1** 2.9 Summer in 1998 Katamo site Autumn in 1998 Ningyo-toge(control) Summer in 1996 Note) * The calculation by LUDEP was carried out for the nuclide of Pb-214 approximately. The GMD and GSD of the unattached fraction of radon progeny are approximately 1[nm] and 1.4[-] (9). ** The AMAD was assured to be.1[µm]. Although each model has different dependency on the AMAD and fp, the dose conversion factor tends to be larger when the AMAD and fp becomes to be smaller and larger respectively. As a result of this tendency, the dose conversion factors for the Katamo site included both the boundary and the front of the old tunnel are evaluated to be twice or four times larger than the control. Even on the average, the factor for the Katamo site is estimated to be twice or more. The average concentration of radon progeny in front of the old tunnel was estimated to be about 25[Bq/m 3 ] (6). It would be not important that the dose conversion factor is larger than the factor at the control because the limit to set up a radiation controlled area by law is 3[Bq/m 3 ]. However, it is important to keep the concentration of radon progeny on the present level. CONCLUSION According to the obtained results, the radon progeny size distributions at the boundary of the Katamo site were not so different from the result at Ningyo-toge as the control area. The fp was slightely larger than the result at Ningyo-toge. The radon progeny size distribution in front of the temporarily closed and old mining tunnel are very peculiar. Here is surrounded by many trees and far from residential area where much ambient aerosol is released from many kinds of source. The radon concentration is also very high. This difference in condition around the measured place may affect the results of size distribution and fp. Some models for dose estimation due to radon progeny are recommended in several reports. The obtained dose conversion factor for the measurement results in the Katamo site is evaluated to be larger than the factors for the control. This influence to the radiation monitoring on the Katamo site is insignificant because it is already proven by previous our investigations that the estimated concentration of radon progeny is enough low to the limit to set up a radiation controlled area by law. REFERENCES 1. Cheng, Y. S.; Yeh H. C. Theory of a screen-type diffusion battery. Journal of Aerosol Science 11: (198). 2. Ikebe, Y. Determination of the size distribution of polydisperse submicron aerosols by a response matrix method. Pure Application Geophysics 98: (1972). 3. Twomey S. Comparison of constrained linear inversion and an iterative nonlinear algorithm applied to the indirect estimation of particle size distributions. Journal of Computational Physics 18:188-2 (1975). 4. Maher E. F.; Laird N. M. EM algorithm reconstruction of particle size distributions from diffusion battery data. Journal of Aerosol Science 16: (1985). 5. Solomon, S.B., Ren, T Aerosol Science Technology 17:69 (1992). 6

7 6. Ito, K.; Ishimori, Y.; Furuta, S., Environmental effects of radon and its progeny from uranium waste rock piles. In: Radon and Thoron in the Human Environment, proceedings of 7th Tohwa University International Symposium. Fukuoka, Japan, pp (1998). 7. Hillamo, R.E., Kauppinen, E., Aerosol Science Technology 14:33. (1991). 8. Kesten, J., Butterweck, G., Porstendoefer, J., Reinerking, A., Heymel, H.J., Aerosol Science Technology 18:156 (1993). 9. Reineking, A.; Kunutson E.A.; George A.C.; Solomon S.B.; Kesten J.; Butterweck G.; Porstendoefer J., Size distribution of unattached and aerosol-attached short-lived radon decay products: Some results of intercomparison measurements. Radiation Protection Dosimetry 56: , (1994). 1. National Research Council (NRC). Comparative dosimetry of radon in mines and homes (1991). 11. International Commission on Radiological Protection. Protection in work place and home Oxford: ICRP Publication 65 (1994). 12. International Commission on Radiological Protection. Limits for inhalation of radon daughters by workers. ICRP Publication 32 (1981). 13. Nuclear Energy Agency of Organization for Economic Cooperation and Development (OECD NEA). Dosimetry aspects of exposure to radon and thron daughter products (1983). 14. National Radiation Protection Board (NRPB). LUDEP 2.3 Personal computer program for calculating internal doses using the new ICRP respiratory tract model (1994). 7