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Measurements of radon around closed uranium mines
Journal of Environmental Radioactivity 62 (2002) 97–114 www.elsevier.com/locate/jenvrad
Measurements of radon around closed uranium mines Sadaaki Furuta *, Kimio Ito, Yuu Ishimori Ningyo-toge Environmental Engineering Center, Japan Nuclear Cycle Development Institute, 1550 Kamisaibara-son, Tomata-gun, Okayama-ken 708-0698, Japan Received 5 January 2001; received in revised form 3 September 2001; accepted 25 September 2001
Abstract There are several waste rock yards at closed uranium mines around Ningyo-toge, in the Western Honshu Island of Japan, and measurements of radon were carried out by both the passive method and the sampling method around these yards. As comparatively high radon concentrations were observed in two districts through routine measurements, more detailed measurements were made by the passive method in these districts. The impact of radon emanation from the waste rock yards was small for both residential districts and around these yards when considering the natural background level of radon. In addition, by simultaneous continuous measurements of radon and its progeny at two locations, it was estimated that the effective dose caused by the representative uranium waste rock yards was less than the public effective dose limit of 1 mSv year⫺1 at the fenced boundary of the waste rock site. 2002 Published by Elsevier Science Ltd. Keywords: Radon; Radon progeny; Closed uranium mine; Impact of radon; Dose
1. Introduction Uranium exploration in Japan was started in 1955, when an outcrop of uranium mineralization was discovered at Ningyo-toge near the boundary between Okayama and Tottori Prefectures, in the Western Honshu Island of Japan. Since then exploration and exploitation (test mining and uranium refining) activities were continued * Corresponding author. Tel.: +81-868-44-2211; fax: +81-868-44-2851. E-mail address: [email protected] (S. Furuta). 0265-931X/02/$ - see front matter 2002 Published by Elsevier Science Ltd. PII: S 0 2 6 5 - 9 3 1 X ( 0 1 ) 0 0 1 5 4 - 0
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there for nearly 20 years. The waste rocks were excavated until mining tunnels reached underground uranium deposits and were placed near the gallery entrances. Such waste rock yards can be found in both Okayama and Tottori Prefectures around Ningyo-toge. Since the Japanese mining safety regulation was amended in 1989 according to the ICRP Publication 26, radon measurements and the evaluation of radiation exposure have been required for the environment and general public around waste rock yards. Therefore, entry by the general public to such waste rock sites has been forbidden and controlled by way of a Keep Out sign and fences since then, and measurements of radon concentration in the environment were started with routine monitoring by passive and sampling methods. In this report, the fenced area is called a ‘waste rock (fenced) site’. Measurement points of radon concentration were chosen in and around waste rock fenced sites and in the residential area nearby. Long-term measurement of radon concentration was carried out by the passive method, and radon concentration and equilibrium equivalent radon concentration (EECRn) were measured by the sampling method. Continuous measurements of radon and EECRn by the active method were also carried out in a district where representative waste rock yards are located. The dose estimation for radon for the general public from the waste rock yards was also investigated using the various measurement techniques reported herein.
2. Material and methods 2.1. Location of the measurements At first, measurements were started by the passive method and the sampling method as routine monitoring for the purpose of management of waste rock yards. This monitoring covered a wide area in and around the waste rock sites and at residential districts nearby in four local municipalities, Kurayoshi City, Togo Town and Misasa Town in Tottori Prefecture and Kamisaibara Village in Okayama Prefecture, as illustrated in Fig. 1. In some districts, shown as hatched circles in the figure, there are several waste rock yards and relevant measurement points. Passive measurements were carried out by placing the monitors in two strategic locations, one near the center and the other near the boundary of the waste rock fenced site as shown in Table 1. This table summarizes the locations and numbers of waste rock sites and the relevant measurement points by method. Additional monitors were also placed near the boundary for relatively large waste rock fenced sites, such as Nakatsugo, Asabatake-2 and Katamo-1,2. As for outside the waste rock site, in the residential districts, the monitors were placed in the backyards of community centers or private houses, and some monitors were placed in the non-residential hilly area. Sampling measurements were also made at some of the same points as passive measurements. Since routine monitoring showed high radon concentrations at Katamo-1,2 waste rock fenced site in Togo Town by the sampling method, and at Ayumidani residential district in Kurayoshi City by the passive method, further measurements were carried
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Fig. 1. Location of waste rock yards and measurement points around Ningyo-toge. The two rectangles with broken lines represent locations of Figs. 2 and 3.
out as detailed measurements by the passive method in both districts and continuous measurement in Katamo district in order to estimate the influence of radon emanation from the waste rocks on the surrounding area. In this report names of measurement points at residential districts are called with the same names as the administrative area, so we put the names in quotation marks to prevent from confusion when the names mean the measurement points. At Katamo district in Togo Town the relatively large waste rock yards of Katamo1 and Katamo-2 are located almost in the same place, and they are grouped to be one fenced site as Katamo-1,2 as shown in Fig. 2. This site is situated on the steep slope of a hill about 1.5 km upstream from a residential district along a main ravine. There is a sprawling pear orchard approximately 1 km away from the residential district, and a forest extending up to the waste rock yards. The altitude of the yards is about 220–250 m above sea level, and that of the residential district is about 35 m above sea level. The total ore reserve of uranium mines in Togo Town including the Katamo deposit was estimated as 5.25 × 105 kg U3O8, with an average grade of 0.043% (PNC, 1994). The locations of the routine measurement by passive monitors are as follows: ‘Katamo’ in residential district, WR in the center of the waste rock fenced site, and WR(B1) at the east boundary, WR(B2) at the west, WR(B3) at the south and WR(B4) at the north of the fenced site, respectively. The locations of the detailed measurements are as follows: D-0 at the closed gallery entrance to the underground uranium mine, D-1 to D-5:Gate in the waste rock fenced site from the
Ningyo-toge ECc Nakatsugo Choja Kan’nokura-1 Kan’nokura-2 Asabatake-1 Asabatake-2 Asabatake-3 Asabatake-2B Katamo-1,2 Katamo-3 Katamo-S2 Ayumidani
Ningyo-toge Nakatsugo Choja Nan’nokura
c
b
a
1 1 1 1 1 1 1 1 1 1 (4) 1 1 1
5 2 1 1 1 1 4 1 1 4 (10) 1 1 1 (1) 6 (11)
1 (6)
1 1
3
Residential district
Center
Boundary
Outside WRb
Inside WRb
Passive measurementa
Numbers of detailed measurement points are in parentheses. WR means waste rock site. EC means Engineering Center.
Ayumidani Others (Controls)
Katamo
Asabatake
Waste rock site
Administrative area
Table 1 Summary of the waste rock sites and measurement points with measurement methods
(5) (4)
(6)
2
Non-residential district
2 2 2 2 2 2 1 1 5 1 1 1 9
Sampling measurement
2
Continuous measurement
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Fig. 2. Locations of measurement points around Katamo-1,2 waste rock site. Passive monitors at the center of the waste rock fenced sites (䊏), at the vicinity of the boundary and inside the waste rock fenced site (왖), at the vicinity of the boundary or influenced area outside the waste rock fenced site (왕), and at control point uninfluenced by the waste rock (䊐). Continuous monitors at D-5:Gate and Orchard (䊊).
waste rock yards to the gate of the site, D-14 to D-22 along the main ravine to the downstream direction from the fenced site, and D-6 to D-13 along the boundary of the fenced site. D-23, D-24 and D-25 are placed on the branch ravines as controls considering that the radon flows down along the main ravine from the waste rock yards as a result of down breezes along the hill slope at night. The locations of the continuous measurement were placed at D-5:Gate at the gate to the waste rock fenced site and in the pear orchard. Ayumidani district in Kurayoshi City is located on typical granitic terrain, where three private houses scatter along narrow ravines surrounded by steep hills as shown in Fig. 3. Though two waste rock yards are located along the main ravines, no uranium deposit was actually found in this area in spite of extensive uranium exploration. The locations of routine measurement by the passive method were placed at ‘Ayumidani’ in the backyard of a private house and at ‘Ayumidani WR’ in the center of the site. The locations of detailed measurement were placed at D-30 in the backyard of a private house and at some other points in this hilly area as controls. 2.2. Passive measurement For long-term outdoor radon measurement, a highly sensitive passive radon monitor equipped with an electrostatic collection system (Iida et al., 1988) was employed. The monitor was made by Aloka Co., Ltd (Japan). Radon concentrations were obtained from a solid-state track detector made of cellulose nitrate film by an automatic counting system. This type of monitor was also employed for the investigation of indoor radon concentration in Japan by Abe and Abe (1989). However, it should be noted that the authors switched to a CR-39 in April 1997 from cellulose nitrate
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Fig. 3. Location of measurement points in Ayumidani district. Passive monitors at the center of the waste rock fenced sites (䊏), at the vicinity of the boundary or influenced area outside the waste rock fenced site (왕), and at control point uninfluenced by the waste rock (䊐).
film for etching quality improvement and procurement considerations in Japan, because at that time the quality of the film was changed slightly and some films became unstable during etching. The monitor was installed in the naturally ventilated storage box approximately 1–1.5 m in height. The etching and reading by the CR39 were performed quarterly. All monitors were calibrated on a yearly basis using a radon test chamber (Furuta, Ito, & Ishimori, 1999) developed by the authors. The cylindrical test chamber is 7.5 m3 in volume and the interior temperature and humidity can be controlled automatically. Radon concentrations can be kept up to
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about 1000 Bq m⫺3 with the controlled environment of the chamber. The calibration of the radon concentration in the test chamber was carried out by a gas charging type ionization chamber, using diluted radium solution as a primary standard source at Ningyo-toge Environmental Engineering Center. 2.3. Sampling measurement The objective of sampling measurements was to estimate the F value, an important factor to obtain the radiation exposure from radon concentration, which is a ratio of equilibrium equivalent radon concentration (EECRn) to radon gas concentration. The sampling measurements were carried out at the same points as where some of the passive monitors were placed. 2.3.1. Radon concentration The sampling measurements of radon concentrations were carried out using the gas charging type ionization chamber of 1.5 l volume. An air sample was collected into a plastic bag over a one-hour period in order to average out the temporal variation of radon concentration. The sample was then transferred into the vacuumed ionization chamber through calcium chloride as a drying agent. At this time, a small air pump was actuated with flow rate under 5 l min⫺1 for approximately one minute in order to sufficiently purge the chamber of the inside air. The ionization chamber was then measured at the Ningyo-toge Engineering Center by a vibrating reed electrometer three hours after the transfer to the ionization chamber. The detection limit by this method was about 5 Bq m⫺3. 2.3.2. Radon progeny concentration As for the radon progeny, a WLM200 monitor made by Tracerlabo Co., Ltd (Germany) was adopted for the sampling measurements of this study, as the portable size character of this monitor was convenient for outdoor measurements. The progeny were measured by alpha spectroscopy with a surface barrier semi-conductive detector for a few hours, being collected on the fixed membrane filter at the sampling head. The measurement results were printed out hourly by the monitor. A measurement for a comparatively short period was adequate by this method due to the fixed membrane filter used. As a result of the calibration check, good coincidence was obtained for the practical measurement by inter-comparison (Shimo et al., 1997) with other organizations. The measurement results by the monitor were recorded in a variety of measurement units which include: working level in WL, potential alpha energy concentration in J m⫺3 or MeV l⫺1 and EECRn in Bq m⫺3. In this study, the EECRn was used in accordance with Japanese regulations, and the detectable limit of 0.4 Bq m⫺3 was adopted from the catalog. 2.4. Continuous measurement The objective of the continuous measurement is to obtain long-term variable data of radon and its progeny concentrations around the waste rock yards, because their concentrations by the sampling method were measured over a limited time.
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2.4.1. Radon concentration Continuous measurements of radon concentrations were taken using either a PMTTEL (PYLON Co., Ltd, Canada) or an AlphaGUARD (Genitron Co., Ltd, Germany) type monitor. The PMT-TEL monitor was adopted initially, but the authors switched to the AlphaGURD in April 1998 due to drier maintenance considerations. The AlphaGURD monitor utilizes an ionization chamber as the detector. The air was sampled through the chamber using a pump, then measured by the detector. The records of radon concentration and other data such as date, time and temperature were collected hourly. These monitors were also calibrated beforehand by the radon test chamber (Furuta et al., 1999) as previously mentioned. 2.4.2. Radon progeny concentration It is important to measure the long-term radon progeny concentration because most of the radon dose is caused by its short-lived progeny nuclides. To accurately measure these long-term low radon progeny concentrations in the outdoor environment, a continuous monitor (Furuta, Ito, & Ishimori, 2000) was developed and employed. The important features of the monitor are a large sampling flow rate of 80 l min⫺1 and the use of alpha spectroscopy in a vacuum vessel. These features enabled a high sensitivity and a high resolution of the monitor. The use of a roll-type long membrane filter combined with a reliable automatic system enabled continuous measurement to be taken for a period of more than one month. The EECRn was measured hourly by the monitor.
3. Results and discussion 3.1. Measurement by the passive method 3.1.1. Routine measurement The measurement results were averaged over a period from fiscal years 1997 to 1999 (from April 1997 to March 2000) as shown in Fig. 4(a) and (b). Here Fig. 4(a) shows the data measured in both residential and non-residential districts outside the waste rock fenced sites, and Fig. 4(b) shows the data measured inside the fenced sites. The measurement locations were classified into the following four broad categories based on location to the waste rock yards and indicated with their labeling symbols as follows: at the center of the waste rock fenced sites (䊏), at the vicinity of the boundary and inside the waste rock fenced site (왖), at the vicinity of the boundary or influenced area outside the waste rock fenced site (왕), and at the control district uninfluenced by the waste rock (䊐). In some cases it was difficult to determine clearly the boundary between the influenced and uninfluenced area, but the downstream locations along the ravines outside the waste rock fenced site were treated as “influenced” to avoid an underestimation of the results. In the residential districts, except for Ayumidani, the measurement results were less than 30 Bq m⫺3 [Fig. 4(a)], which is almost the same level as reported previously for the general environment in Japan by Abe and Abe (1989). A relatively high
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Fig. 4.
105
Radon concentrations outside (a) and inside (b) waste rock sites by passive method.
concentration of radon was observed in Ayumidani compared with the other districts. In this district, further measurements were carried out and the results are described in Section 3.1.2. The concentrations in waste rock fenced sites, including Ningyo-toge Environmental Engineering Center, are shown in Fig. 4(b). Here ‘(B)’ in the location names means a point at the boundary of the waste rock fenced site. Concentration levels in excess of 100 Bq m⫺3 were observed in some sites. They were substantially higher than the levels observed in the residential districts. In addition, significant variations in concentration level were found within the same site, for example the Kan’nokura2 waste rock fenced site. This tendency suggests that the high concentration levels
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were localized. As for Ningyo-toge Engineering Center, in the same wide fenced site, there are uranium enrichment plants and other mining facilities in addition to waste rock yards. The concentrations were low at the main gate and administration building away from the waste rock yard. In the area of Ningyo-toge or Kan’nokura, monitor storage boxes were sometimes buried by snow of more than 1.5 m deep in winter. In these cases, very high radon concentrations were sometimes observed irrespective of their measurement locations. It seemed that radon emanating from the ground surface was streamed up along the stem and accumulated in the box, as the snow suppressed ventilation of the box. These values were therefore excluded from the data because they were not atmospheric radon concentrations. 3.1.2. Detailed measurement The results of detailed measurements around Katamo-1,2 waste rock yards are shown in Fig. 5(a). Generally higher concentrations were observed within the waste rock fenced site. The highest concentration was observed at the front of the temporarily closed gallery entrance, D-0, where radon seemed to emanate from some uranium ores left inside the gallery. High radon concentrations were also observed by the sampling method as well. Relatively high radon concentrations at D-0 decreased rapidly along the narrow ravine toward D-5:Gate, at the gate to the waste rock fenced site. However, since the radon concentrations were found to be lower between the gate and the residential district, further along downstream of the main ravine, it was concluded that the effect of the waste rock on this area was insignificant. Near the boundaries of the waste rock fenced site, the radon concentrations at D-12 and D13 in the north-western part of the site were at the same level as that found at the gate. The concentrations in the south and east (from D-6 to D-10) were as low as those measured in the residential district. As for the control locations, the radon concentration at D-24 was almost as high as that found at D-5:Gate on the boundary of the waste rock fenced site. It was probably due to air stagnation caused by the landform and vegetation around D-24, where the ravine narrows and the trees grow thicker, in addition to the granitic terrain. It is not to be denied that, to a greater or lesser degree, the results measured around D-5:Gate, the gate of the fenced site, might also be affected by the these same factors of geology, vegetation and especially the landform of a relatively narrow ravine. In short, it is possible to say that the effect of the waste rocks on the region outside the fenced site is limited. The results measured in Ayumidani district and other control areas are shown in Fig. 5(b). Relatively high concentrations were observed in control points at D-26, D-29 and D-31 irrespective of the non-existence of waste rock yard there. Those three points are located in a forest or a branch ravine away from the opened main ravine floor where a main road and paddy fields are located. It is considered that the relatively high concentrations observed were due to the specific locations, where diffusion of the atmosphere was suppressed by trees and hilly landform. It is also thought that the granitic terrain there contributed to the high radon concentrations. The range of radon concentrations at the control points in Kurayoshi City (two
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107
Fig. 5. Radon concentrations around Katamo-1,2 waste rock site (a) and in Ayumidani district and controls in other areas (b) by passive method.
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points), Togo Town (nine points) and Misasa Town (four points) is shown in Fig. 5(b). The maximum value in these results was 57 Bq m⫺3 in both Togo Town and Misasa Town. It seemed to reflect a typical granitic terrain in this area. 3.2. Measurement by sampling method The radon concentration and EECRn were measured by the sampling method at the same points where some passive monitors were placed. In relatively large waste rock fenced sites, the measurements were carried out basically every month. They were made every quarter at other sites and approximately once or twice a year in residential districts. These results are shown in Table 2. Here the arithmetic mean and F value were calculated using data where both the radon concentration and EECRn exceeded the respective detection limits by the simultaneous sampling. The highest values of radon concentration and EECRn were observed at D-0 in Katamo-1,2, located in front of the temporarily closed gallery entrance of the underground uranium deposit. The radon concentration was higher than that recorded by the passive monitor at the same point as shown in Fig. 5(a). Extremely high radon concentrations were sometimes recorded by this sampling method during the summer at this point. Probably one of the reasons is that the sampling was carried out in stable daytime weather conditions without rain, while the concentration by passive monitor was obtained through averaging data observed over a period of 24 hours. It is also assumed that radon in high concentrations could come out through the temporarily closed gallery entrance in the daytime. As the entrance is temporarily closed by stacked sandbags, it is not a fully airtight structure and is assumed to be like a kind of cave. It is well known that the radon concentrations in caves are generally high and especially high in the summer as reported by Tanahara, Iha, and Taira (1997) and Przylibski (1999). Tanahara et al. concluded that variation of air density resulted in the variation of radon concentration by their simultaneous measurements of radon concentration and air density both inside and outside a cave. It was also explained that the radon concentration in a cave became lower in the winter by outside air flowing into the cave, because the outside air density became higher in the winter. Conversely, in the summer the inverse phenomena would occur and cause the radon concentration to be higher in a cave due to air stagnation in the cave, and the high radon bearing air would go outside. As to diurnal variation, since air density became lower outside a cave in the daytime as observed in their study, it is reasonable to assume that the accumulated high radon in the closed gallery could leak out during the daytime through the sandbags stacked at the temporarily closed gallery entrance and result in the high concentration of radon at D-0. At Ayumidani residential district, higher radon concentration by the passive method was observed compared with the sampling method. It is generally said that a higher radon concentration is often observed from midnight to dawn, as atmospheric air becomes stable at night. At Ayumidani the reason for the higher concentration by the passive method seems to be due to this calm atmospheric stability at night leading to a higher radon concentration as mentioned above. On the other hand,
Waste rock site Nakatsugo WR Choja WR Choja WR(B) Kan’nokura-1 WR Kan’nokura-1 WR(B) Kan’nokura-2 WR Kan’nokura-2 WR(B) Asabatake-1 WR Asabatake-1 WR(B) Asabatake-2 WR
Location
35 127 91 107
147
1550
297
754 184
333
12 29 24 38
93
542
81
99 81
98
8
12 18
8
11
37
5 5 5 6
6.5
3.9 3.6
3.8
10.9
3.5
2.9 2.1 1.9 2.1
65.3
7.1 11.9
12.0
32.6
5.5
5.8 3.4 3.4 4.6
Max.
Mean
Min.
Mean
Max.
EECRn (Bq m⫺3)
Radon (Bq m⫺3)
Results by sampling method
0.9
1.1 0.9
1.5
0.9
0.8
0.7 0.6 0.8 0.6
Min.
0.10
0.10 0.06
0.17
0.05
0.04
0.29 0.19 0.18 0.12
Mean
F-valuea
0.71
0.28 0.16
0.37
0.24
0.07
0.70 0.60 0.52 0.36
Max.
0.01
0.01 0.01
0.02
0.01
0.02
0.05 0.02 0.01 0.02
Min.
63
16 15
8
12
12
37 10 14 11
Nb
68
37 70
19
233
67
10 12 17 25
6.9
3.7 4.3
3.2
11.9
2.7
2.9 2.3 3.1 3.1
0.37
0.20 0.23
0.17
0.63
0.14
0.16 0.12 0.17 0.16
Estimated Estimated EECRnc dose (Bq m⫺3) (mSv year⫺1)
(continued on next page)
Passive Rn (Bq m⫺3)
Table 2 Summary of the results of radon concentration and equilibrium equivalent radon concentration (EECRn) by sampling method, and estimation of long-term EECRn and annual dose
S. Furuta et al. / J. Environ. Radioactivity 62 (2002) 97–114 109
d
c
b
a
27 13 14 6 25 6 7 14 9 6 5 5 5
280 155
66,300
117 1580
184
81
101 217 48
42 87 191
3.7 3.0 3.3
2.2 3.7 3.5
3.1
4.9
3.6 26.0
70.5
3.8 3.9
3.3
12.3 12.8 8.4
4.3 10.5 5.9
6.5
33.0
7.9 97.3
343.7
8.0 10.4
9.9
0.5 0.9 0.8
0.8 0.7 2.0
0.9
0.9
0.7 2.6
0.6
1.1 0.6
0.7
0.29 0.22 0.36
0.06 0.09 0.21
0.16
0.15
0.22 0.05
0.07
0.05 0.10
0.12
0.74 0.77 0.69
0.15 0.21 0.44
0.40
0.65
0.73 0.12
0.52
0.14 0.23
0.43
0.07 0.03 0.03
0.01 0.03 0.07
0.03
0.03
0.04 0.02
0.00
0.01 0.01
0.01
64 39 60
11 13 17
28
46
50 22
73
14 13
57
A ratio between EECRn and radon gas concentration. Number of data. Estimated EECRn was obtained using F value from Rn concentration in fiscal years 1997–1999 by passive monitor. Others are Kan’nokura, Asabatake, Katamo, Kawakami, Yendani, Hirose, Anagamo, Ombara and Ningyo.
5
371
Results by sampling method
Asabatake-2 54 WR(B4) Asabatake-3 WR 118 Asabatake-2B 56 WR Katamo-1,2 3624 WR(D-0) Katamo-1,2 WR 26 Katamo-1,2 606 WR(D-1) Katamo-1,2 43 WR(D-5:Gate) Katamo-1,2 27 WR(B4) Katamo-3 WR 53 Katamo-S2 WR 68 Ayumidani WR 20 Residential district Ayumidani 14 Nakatsugo-2 21 Othersd 16 (including controls)
Location
Table 2 (continued)
63 21 14
20 46 60
8
62
25 263
694
90 55
121
Passive Rn (Bq m⫺3)
18.1 4.6 5.0
1.2 4.0 12.5
1.3
9.2
5.5 14.2
47.2
4.1 5.5
13.9
0.97 0.25 0.27
0.06 0.21 0.67
0.07
0.49
0.29 0.76
2.52
0.22 0.29
0.74
Estimated Estimated EECRnc dose (Bq m⫺3) (mSv year⫺1)
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111
the measurement by the sampling method was carried out basically in the daytime, which tends to indicate a lower concentration. Table 2 indicates that the EECRn values were not so high inside the waste rock fenced sites, contrary to the respective high radon concentrations there. In other words, the tendency toward a low F value was obtained in the waste rock fenced sites. A large fluctuation of the F values is also shown in the table as a result of a large variation in the respective radon concentrations. Therefore, in the vicinity of the waste rock yards as radon source, a phenomenon was observed that the progeny did not grow sufficiently. Such a phenomenon has been reported in other places (Yamasaki, Yoshimoto, & Tsujimoto, 1993). Using these F values obtained by the sampling method, long-term EECRn values were derived from the radon concentrations by passive method. Consequently lower EECRn values were obtained compared with the respective radon concentrations, as shown in Table 2. 3.3. Continuous measurement of radon and its progeny Radon concentrations and the EECRn were obtained around the Katamo-1,2 waste rock fenced site using active-type continuous monitors. Two sets of continuous monitors for radon and its progeny were placed at the gate to the site and in the orchard about 1 km away downstream of the main ravine from the gate as shown in Fig. 2. Simultaneous measurements were carried out at these two locations from October 1992. The results are shown in Fig. 6. As this figure indicates, the radon concentration at the gate was higher than that at the orchard, but the variation corresponded between those measured at the gate and the orchard. The arithmetic mean radon concentrations were 23 Bq m⫺3 and 65
Fig. 6. Radon concentration and equilibrium equivalent radon concentration (EECRn) by continuous measurements around Katamo-1,2 waste rock site. Numbers in the legend represent mean values.
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Bq m⫺3 at the orchard and the gate, respectively. The EECRn values were low compared with the high radon concentration. The mean F values were 0.20 and 0.12 at the orchard and the gate, respectively. Through these results, it is estimated that the effect of radon dispersion from the limited source like the waste rock yard was significant, because the radon progeny did not grow sufficiently at the gate compared with those at the orchard. The arithmetic mean EECRn, 8.0 Bq m⫺3, obtained by this method at the gate is higher than that of 4.9 Bq m⫺3 obtained by sampling method at the same place as shown in Table 2. But the former value is in relatively good agreement with the estimated EECRn value of 9.2 Bq m⫺3 at the same location as shown in the same table, because the estimated EECRn indicates a long-term concentration. As a seasonal variation, higher concentrations were observed in the summer. In general it is known that radon concentrations are lower in regions facing to the Sea of Japan in the winter and higher in the summer (Nishikawa, Aoki, & Okabe, 1985). The data measured in Katamo district tend to show a similar pattern of variation as observed in those regions. 3.4. Dose estimation First, the estimation of annual effective dose contributed by the uranium waste rock was made using the estimated long-term EECRn data indicated in Table 2, assuming that a person would live there continuously for one year. Effective dose E (mSv year⫺1) is derived as follows: E ⫽ S·T·EECRn / B, where S: T: B:
a conversion factor for public described in ICRP Publication 65 (3.88 mSv WLM⫺1) hours in one year (8760 h year⫺1) a conversion factor described in ICRP Publication 65 [ 6.37 × 105 (Bq h m⫺3) WLM⫺1]
Even in the waste rock fenced sites, the effective doses were less than the public dose limit of 1 mSv year⫺1 except for 2.5mSv year⫺1 obtained at Katamo-1,2 WR (D-0) as shown on the extreme right in Table 2. Consequently, the impact of the waste rocks on the surrounding area is small considering these results include the natural background component. Next, the dose estimation was carried out using the results of continuous measurement of EECRn. The mean value of the data was 4.6 Bq m⫺3 at the orchard, and 8.0 Bq m⫺3 at the gate as shown in Fig. 6. According to the literature, the following values are cited as the results of outdoor measurements: an average value of 4.1 Bq m⫺3 from August 1991 to June 1992 in Nagoya, Japan (Yamasaki, Iida, Shimo, & Ikebe, 1995), and an arithmetic mean of 8.6 Bq m⫺3 and a geometrical mean of 6.9 Bq m⫺3 from 1982 to 1992 in Neuherberg Germany (Hoetzl & Winkler, 1994).
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Judging from these data, it may safely be said that the result at the orchard is recognized at least as a normal level. Therefore, the elevated component of 3.4 Bq m⫺3 is obtained for the gate area by deducting the EECRn value at the orchard from that at the gate (8.0⫺4.6) that seems to indicate the long-term effect of the waste rock, as the value at the orchard is assumed to be a background level of this region. The effective dose of 0.18 mSv year⫺1 is estimated as the influence of the waste rock yards, using the equation described above and the EECRn value of 3.4 Bq m⫺3. Therefore, the result recorded on the boundary of the uranium waste rock fenced site is less than the public dose limit of 1 mSv year⫺1.
4. Conclusion Radon concentrations were measured in and around the uranium waste rock fenced sites, and their corresponding environmental impacts have been assessed. Although relatively high radon concentrations were observed in the waste rock fenced sites, on the other hand low EECRn and F values were obtained because the progeny nuclides did not grow sufficiently in the vicinity of a limited radon source like these uranium waste rock yards. Therefore, it was determined that the environmental impacts of radon from the waste rock yards were small around such sites, using the estimated long-term EECRn. It was further estimated, by simultaneous continuous measurements at two locations, that the effective dose caused by the representative waste rock yards was less than the public effective dose limit of 1mSv year⫺1 on the site boundary.
Acknowledgements The authors would like to express sincere gratitude to Professor Iida of Nagoya University for his help and useful suggestions in conducting this study.
References Abe, M., & Abe, S. (1989). A nation-wide survey of radon indoors and outdoors in Japan. In: S. Kabayashi, & T. Kankura (Eds.), Radon in the living environment — levels and risks, Proceedings of the 15th NIRS Seminar (pp. 79–88). (in Japanese). Furuta, S., Ito, K., & Ishimori, Y. (1999). Radon reference chamber for calibration of the monitors. Radioisotopes, 48, 725–731 (in Japanese). Furuta, S., Ito, K., & Ishimori, Y. (2000). A continuous radon progeny monitor with a vacuum vessel by alpha spectrometry. Radiation Protection Dosimetry, 90, 429–435. Hoetzl, H., & Winkler, R. (1994). Long term variation of outdoor radon equilibrium equivalent concentration. Radiat. Environ. Biophys., 33, 381–392. Iida, T., Ikebe, Y., Hattori, T., Yamanishi, H., Abe, S., Ochifiji, K., & Yokoyama, S. (1988). An electrostatic integrating 222Rn monitor with cellulose nitrate film for environmental monitoring. Health Physics, 54, 139–148. Nishikawa, T., Aoki, M., & Okabe, S. (1985). Diurnal and seasonal variation of the atmospheric radon
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and thoron daughters concentration. In: S. Okabe (Ed.), Atmospheric radon families and environmental radioactivity (pp. 107–114). (in Japanese). PNC (1994). Uranium resources in Japan. In: PNC TN7420 94–006. Power Reactor and Nuclear Fuel Development Corporation (in Japanese). Przylibski, T. A. (1999). Radon concentration changes in the air of two caves in Poland. Journal of Environmental Radioactivity, 45, 81–94. Shimo, M., Fujitaka, K., Furuta, S., Iida, T., Iimoto, T., Iyogi, T., Kurosawa, R., Kojima, H., Sanada, T., Tokonami, S., Fujimoto, K., Miyano, K., Yamasaki, K., & Yonehara, H. (1997). Radon intercomparison at EML USDOE. Hoken Butsuri, 32, 285–294 (in Japanese). Tanahara, A., Iha, H., & Taira, H. (1997). Factors controlling the changes in 222Rn concentration in Gyokusen Cave on Okinawa Island. Journal of the Speleological Society of Japan, 22, 98–105 (in Japanese). Yamasaki, T., Iida, T., Shimo, M., & Ikebe, Y. (1995). Continuous measurements of outdoor radon and its progeny concentrations. Hoken Butsuri, 30, 149–154 (in Japanese). Yamasaki, K., Yoshimoto, T., & Tsujimoto, T. (1993). A mysterious spot on the outdoor concentrations of radon and thoron. KURRI Progress Report, 418–419.
Measurements of radon around closed uranium mines Sadaaki Furuta *, Kimio Ito, Yuu Ishimori Ningyo-toge Environmental Engineering Center, Japan Nuclear Cycle Development Institute, 1550 Kamisaibara-son, Tomata-gun, Okayama-ken 708-0698, Japan Received 5 January 2001; received in revised form 3 September 2001; accepted 25 September 2001
Abstract There are several waste rock yards at closed uranium mines around Ningyo-toge, in the Western Honshu Island of Japan, and measurements of radon were carried out by both the passive method and the sampling method around these yards. As comparatively high radon concentrations were observed in two districts through routine measurements, more detailed measurements were made by the passive method in these districts. The impact of radon emanation from the waste rock yards was small for both residential districts and around these yards when considering the natural background level of radon. In addition, by simultaneous continuous measurements of radon and its progeny at two locations, it was estimated that the effective dose caused by the representative uranium waste rock yards was less than the public effective dose limit of 1 mSv year⫺1 at the fenced boundary of the waste rock site. 2002 Published by Elsevier Science Ltd. Keywords: Radon; Radon progeny; Closed uranium mine; Impact of radon; Dose
1. Introduction Uranium exploration in Japan was started in 1955, when an outcrop of uranium mineralization was discovered at Ningyo-toge near the boundary between Okayama and Tottori Prefectures, in the Western Honshu Island of Japan. Since then exploration and exploitation (test mining and uranium refining) activities were continued * Corresponding author. Tel.: +81-868-44-2211; fax: +81-868-44-2851. E-mail address: [email protected] (S. Furuta). 0265-931X/02/$ - see front matter 2002 Published by Elsevier Science Ltd. PII: S 0 2 6 5 - 9 3 1 X ( 0 1 ) 0 0 1 5 4 - 0
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there for nearly 20 years. The waste rocks were excavated until mining tunnels reached underground uranium deposits and were placed near the gallery entrances. Such waste rock yards can be found in both Okayama and Tottori Prefectures around Ningyo-toge. Since the Japanese mining safety regulation was amended in 1989 according to the ICRP Publication 26, radon measurements and the evaluation of radiation exposure have been required for the environment and general public around waste rock yards. Therefore, entry by the general public to such waste rock sites has been forbidden and controlled by way of a Keep Out sign and fences since then, and measurements of radon concentration in the environment were started with routine monitoring by passive and sampling methods. In this report, the fenced area is called a ‘waste rock (fenced) site’. Measurement points of radon concentration were chosen in and around waste rock fenced sites and in the residential area nearby. Long-term measurement of radon concentration was carried out by the passive method, and radon concentration and equilibrium equivalent radon concentration (EECRn) were measured by the sampling method. Continuous measurements of radon and EECRn by the active method were also carried out in a district where representative waste rock yards are located. The dose estimation for radon for the general public from the waste rock yards was also investigated using the various measurement techniques reported herein.
2. Material and methods 2.1. Location of the measurements At first, measurements were started by the passive method and the sampling method as routine monitoring for the purpose of management of waste rock yards. This monitoring covered a wide area in and around the waste rock sites and at residential districts nearby in four local municipalities, Kurayoshi City, Togo Town and Misasa Town in Tottori Prefecture and Kamisaibara Village in Okayama Prefecture, as illustrated in Fig. 1. In some districts, shown as hatched circles in the figure, there are several waste rock yards and relevant measurement points. Passive measurements were carried out by placing the monitors in two strategic locations, one near the center and the other near the boundary of the waste rock fenced site as shown in Table 1. This table summarizes the locations and numbers of waste rock sites and the relevant measurement points by method. Additional monitors were also placed near the boundary for relatively large waste rock fenced sites, such as Nakatsugo, Asabatake-2 and Katamo-1,2. As for outside the waste rock site, in the residential districts, the monitors were placed in the backyards of community centers or private houses, and some monitors were placed in the non-residential hilly area. Sampling measurements were also made at some of the same points as passive measurements. Since routine monitoring showed high radon concentrations at Katamo-1,2 waste rock fenced site in Togo Town by the sampling method, and at Ayumidani residential district in Kurayoshi City by the passive method, further measurements were carried
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99
Fig. 1. Location of waste rock yards and measurement points around Ningyo-toge. The two rectangles with broken lines represent locations of Figs. 2 and 3.
out as detailed measurements by the passive method in both districts and continuous measurement in Katamo district in order to estimate the influence of radon emanation from the waste rocks on the surrounding area. In this report names of measurement points at residential districts are called with the same names as the administrative area, so we put the names in quotation marks to prevent from confusion when the names mean the measurement points. At Katamo district in Togo Town the relatively large waste rock yards of Katamo1 and Katamo-2 are located almost in the same place, and they are grouped to be one fenced site as Katamo-1,2 as shown in Fig. 2. This site is situated on the steep slope of a hill about 1.5 km upstream from a residential district along a main ravine. There is a sprawling pear orchard approximately 1 km away from the residential district, and a forest extending up to the waste rock yards. The altitude of the yards is about 220–250 m above sea level, and that of the residential district is about 35 m above sea level. The total ore reserve of uranium mines in Togo Town including the Katamo deposit was estimated as 5.25 × 105 kg U3O8, with an average grade of 0.043% (PNC, 1994). The locations of the routine measurement by passive monitors are as follows: ‘Katamo’ in residential district, WR in the center of the waste rock fenced site, and WR(B1) at the east boundary, WR(B2) at the west, WR(B3) at the south and WR(B4) at the north of the fenced site, respectively. The locations of the detailed measurements are as follows: D-0 at the closed gallery entrance to the underground uranium mine, D-1 to D-5:Gate in the waste rock fenced site from the
Ningyo-toge ECc Nakatsugo Choja Kan’nokura-1 Kan’nokura-2 Asabatake-1 Asabatake-2 Asabatake-3 Asabatake-2B Katamo-1,2 Katamo-3 Katamo-S2 Ayumidani
Ningyo-toge Nakatsugo Choja Nan’nokura
c
b
a
1 1 1 1 1 1 1 1 1 1 (4) 1 1 1
5 2 1 1 1 1 4 1 1 4 (10) 1 1 1 (1) 6 (11)
1 (6)
1 1
3
Residential district
Center
Boundary
Outside WRb
Inside WRb
Passive measurementa
Numbers of detailed measurement points are in parentheses. WR means waste rock site. EC means Engineering Center.
Ayumidani Others (Controls)
Katamo
Asabatake
Waste rock site
Administrative area
Table 1 Summary of the waste rock sites and measurement points with measurement methods
(5) (4)
(6)
2
Non-residential district
2 2 2 2 2 2 1 1 5 1 1 1 9
Sampling measurement
2
Continuous measurement
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Fig. 2. Locations of measurement points around Katamo-1,2 waste rock site. Passive monitors at the center of the waste rock fenced sites (䊏), at the vicinity of the boundary and inside the waste rock fenced site (왖), at the vicinity of the boundary or influenced area outside the waste rock fenced site (왕), and at control point uninfluenced by the waste rock (䊐). Continuous monitors at D-5:Gate and Orchard (䊊).
waste rock yards to the gate of the site, D-14 to D-22 along the main ravine to the downstream direction from the fenced site, and D-6 to D-13 along the boundary of the fenced site. D-23, D-24 and D-25 are placed on the branch ravines as controls considering that the radon flows down along the main ravine from the waste rock yards as a result of down breezes along the hill slope at night. The locations of the continuous measurement were placed at D-5:Gate at the gate to the waste rock fenced site and in the pear orchard. Ayumidani district in Kurayoshi City is located on typical granitic terrain, where three private houses scatter along narrow ravines surrounded by steep hills as shown in Fig. 3. Though two waste rock yards are located along the main ravines, no uranium deposit was actually found in this area in spite of extensive uranium exploration. The locations of routine measurement by the passive method were placed at ‘Ayumidani’ in the backyard of a private house and at ‘Ayumidani WR’ in the center of the site. The locations of detailed measurement were placed at D-30 in the backyard of a private house and at some other points in this hilly area as controls. 2.2. Passive measurement For long-term outdoor radon measurement, a highly sensitive passive radon monitor equipped with an electrostatic collection system (Iida et al., 1988) was employed. The monitor was made by Aloka Co., Ltd (Japan). Radon concentrations were obtained from a solid-state track detector made of cellulose nitrate film by an automatic counting system. This type of monitor was also employed for the investigation of indoor radon concentration in Japan by Abe and Abe (1989). However, it should be noted that the authors switched to a CR-39 in April 1997 from cellulose nitrate
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Fig. 3. Location of measurement points in Ayumidani district. Passive monitors at the center of the waste rock fenced sites (䊏), at the vicinity of the boundary or influenced area outside the waste rock fenced site (왕), and at control point uninfluenced by the waste rock (䊐).
film for etching quality improvement and procurement considerations in Japan, because at that time the quality of the film was changed slightly and some films became unstable during etching. The monitor was installed in the naturally ventilated storage box approximately 1–1.5 m in height. The etching and reading by the CR39 were performed quarterly. All monitors were calibrated on a yearly basis using a radon test chamber (Furuta, Ito, & Ishimori, 1999) developed by the authors. The cylindrical test chamber is 7.5 m3 in volume and the interior temperature and humidity can be controlled automatically. Radon concentrations can be kept up to
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about 1000 Bq m⫺3 with the controlled environment of the chamber. The calibration of the radon concentration in the test chamber was carried out by a gas charging type ionization chamber, using diluted radium solution as a primary standard source at Ningyo-toge Environmental Engineering Center. 2.3. Sampling measurement The objective of sampling measurements was to estimate the F value, an important factor to obtain the radiation exposure from radon concentration, which is a ratio of equilibrium equivalent radon concentration (EECRn) to radon gas concentration. The sampling measurements were carried out at the same points as where some of the passive monitors were placed. 2.3.1. Radon concentration The sampling measurements of radon concentrations were carried out using the gas charging type ionization chamber of 1.5 l volume. An air sample was collected into a plastic bag over a one-hour period in order to average out the temporal variation of radon concentration. The sample was then transferred into the vacuumed ionization chamber through calcium chloride as a drying agent. At this time, a small air pump was actuated with flow rate under 5 l min⫺1 for approximately one minute in order to sufficiently purge the chamber of the inside air. The ionization chamber was then measured at the Ningyo-toge Engineering Center by a vibrating reed electrometer three hours after the transfer to the ionization chamber. The detection limit by this method was about 5 Bq m⫺3. 2.3.2. Radon progeny concentration As for the radon progeny, a WLM200 monitor made by Tracerlabo Co., Ltd (Germany) was adopted for the sampling measurements of this study, as the portable size character of this monitor was convenient for outdoor measurements. The progeny were measured by alpha spectroscopy with a surface barrier semi-conductive detector for a few hours, being collected on the fixed membrane filter at the sampling head. The measurement results were printed out hourly by the monitor. A measurement for a comparatively short period was adequate by this method due to the fixed membrane filter used. As a result of the calibration check, good coincidence was obtained for the practical measurement by inter-comparison (Shimo et al., 1997) with other organizations. The measurement results by the monitor were recorded in a variety of measurement units which include: working level in WL, potential alpha energy concentration in J m⫺3 or MeV l⫺1 and EECRn in Bq m⫺3. In this study, the EECRn was used in accordance with Japanese regulations, and the detectable limit of 0.4 Bq m⫺3 was adopted from the catalog. 2.4. Continuous measurement The objective of the continuous measurement is to obtain long-term variable data of radon and its progeny concentrations around the waste rock yards, because their concentrations by the sampling method were measured over a limited time.
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2.4.1. Radon concentration Continuous measurements of radon concentrations were taken using either a PMTTEL (PYLON Co., Ltd, Canada) or an AlphaGUARD (Genitron Co., Ltd, Germany) type monitor. The PMT-TEL monitor was adopted initially, but the authors switched to the AlphaGURD in April 1998 due to drier maintenance considerations. The AlphaGURD monitor utilizes an ionization chamber as the detector. The air was sampled through the chamber using a pump, then measured by the detector. The records of radon concentration and other data such as date, time and temperature were collected hourly. These monitors were also calibrated beforehand by the radon test chamber (Furuta et al., 1999) as previously mentioned. 2.4.2. Radon progeny concentration It is important to measure the long-term radon progeny concentration because most of the radon dose is caused by its short-lived progeny nuclides. To accurately measure these long-term low radon progeny concentrations in the outdoor environment, a continuous monitor (Furuta, Ito, & Ishimori, 2000) was developed and employed. The important features of the monitor are a large sampling flow rate of 80 l min⫺1 and the use of alpha spectroscopy in a vacuum vessel. These features enabled a high sensitivity and a high resolution of the monitor. The use of a roll-type long membrane filter combined with a reliable automatic system enabled continuous measurement to be taken for a period of more than one month. The EECRn was measured hourly by the monitor.
3. Results and discussion 3.1. Measurement by the passive method 3.1.1. Routine measurement The measurement results were averaged over a period from fiscal years 1997 to 1999 (from April 1997 to March 2000) as shown in Fig. 4(a) and (b). Here Fig. 4(a) shows the data measured in both residential and non-residential districts outside the waste rock fenced sites, and Fig. 4(b) shows the data measured inside the fenced sites. The measurement locations were classified into the following four broad categories based on location to the waste rock yards and indicated with their labeling symbols as follows: at the center of the waste rock fenced sites (䊏), at the vicinity of the boundary and inside the waste rock fenced site (왖), at the vicinity of the boundary or influenced area outside the waste rock fenced site (왕), and at the control district uninfluenced by the waste rock (䊐). In some cases it was difficult to determine clearly the boundary between the influenced and uninfluenced area, but the downstream locations along the ravines outside the waste rock fenced site were treated as “influenced” to avoid an underestimation of the results. In the residential districts, except for Ayumidani, the measurement results were less than 30 Bq m⫺3 [Fig. 4(a)], which is almost the same level as reported previously for the general environment in Japan by Abe and Abe (1989). A relatively high
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Fig. 4.
105
Radon concentrations outside (a) and inside (b) waste rock sites by passive method.
concentration of radon was observed in Ayumidani compared with the other districts. In this district, further measurements were carried out and the results are described in Section 3.1.2. The concentrations in waste rock fenced sites, including Ningyo-toge Environmental Engineering Center, are shown in Fig. 4(b). Here ‘(B)’ in the location names means a point at the boundary of the waste rock fenced site. Concentration levels in excess of 100 Bq m⫺3 were observed in some sites. They were substantially higher than the levels observed in the residential districts. In addition, significant variations in concentration level were found within the same site, for example the Kan’nokura2 waste rock fenced site. This tendency suggests that the high concentration levels
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were localized. As for Ningyo-toge Engineering Center, in the same wide fenced site, there are uranium enrichment plants and other mining facilities in addition to waste rock yards. The concentrations were low at the main gate and administration building away from the waste rock yard. In the area of Ningyo-toge or Kan’nokura, monitor storage boxes were sometimes buried by snow of more than 1.5 m deep in winter. In these cases, very high radon concentrations were sometimes observed irrespective of their measurement locations. It seemed that radon emanating from the ground surface was streamed up along the stem and accumulated in the box, as the snow suppressed ventilation of the box. These values were therefore excluded from the data because they were not atmospheric radon concentrations. 3.1.2. Detailed measurement The results of detailed measurements around Katamo-1,2 waste rock yards are shown in Fig. 5(a). Generally higher concentrations were observed within the waste rock fenced site. The highest concentration was observed at the front of the temporarily closed gallery entrance, D-0, where radon seemed to emanate from some uranium ores left inside the gallery. High radon concentrations were also observed by the sampling method as well. Relatively high radon concentrations at D-0 decreased rapidly along the narrow ravine toward D-5:Gate, at the gate to the waste rock fenced site. However, since the radon concentrations were found to be lower between the gate and the residential district, further along downstream of the main ravine, it was concluded that the effect of the waste rock on this area was insignificant. Near the boundaries of the waste rock fenced site, the radon concentrations at D-12 and D13 in the north-western part of the site were at the same level as that found at the gate. The concentrations in the south and east (from D-6 to D-10) were as low as those measured in the residential district. As for the control locations, the radon concentration at D-24 was almost as high as that found at D-5:Gate on the boundary of the waste rock fenced site. It was probably due to air stagnation caused by the landform and vegetation around D-24, where the ravine narrows and the trees grow thicker, in addition to the granitic terrain. It is not to be denied that, to a greater or lesser degree, the results measured around D-5:Gate, the gate of the fenced site, might also be affected by the these same factors of geology, vegetation and especially the landform of a relatively narrow ravine. In short, it is possible to say that the effect of the waste rocks on the region outside the fenced site is limited. The results measured in Ayumidani district and other control areas are shown in Fig. 5(b). Relatively high concentrations were observed in control points at D-26, D-29 and D-31 irrespective of the non-existence of waste rock yard there. Those three points are located in a forest or a branch ravine away from the opened main ravine floor where a main road and paddy fields are located. It is considered that the relatively high concentrations observed were due to the specific locations, where diffusion of the atmosphere was suppressed by trees and hilly landform. It is also thought that the granitic terrain there contributed to the high radon concentrations. The range of radon concentrations at the control points in Kurayoshi City (two
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Fig. 5. Radon concentrations around Katamo-1,2 waste rock site (a) and in Ayumidani district and controls in other areas (b) by passive method.
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points), Togo Town (nine points) and Misasa Town (four points) is shown in Fig. 5(b). The maximum value in these results was 57 Bq m⫺3 in both Togo Town and Misasa Town. It seemed to reflect a typical granitic terrain in this area. 3.2. Measurement by sampling method The radon concentration and EECRn were measured by the sampling method at the same points where some passive monitors were placed. In relatively large waste rock fenced sites, the measurements were carried out basically every month. They were made every quarter at other sites and approximately once or twice a year in residential districts. These results are shown in Table 2. Here the arithmetic mean and F value were calculated using data where both the radon concentration and EECRn exceeded the respective detection limits by the simultaneous sampling. The highest values of radon concentration and EECRn were observed at D-0 in Katamo-1,2, located in front of the temporarily closed gallery entrance of the underground uranium deposit. The radon concentration was higher than that recorded by the passive monitor at the same point as shown in Fig. 5(a). Extremely high radon concentrations were sometimes recorded by this sampling method during the summer at this point. Probably one of the reasons is that the sampling was carried out in stable daytime weather conditions without rain, while the concentration by passive monitor was obtained through averaging data observed over a period of 24 hours. It is also assumed that radon in high concentrations could come out through the temporarily closed gallery entrance in the daytime. As the entrance is temporarily closed by stacked sandbags, it is not a fully airtight structure and is assumed to be like a kind of cave. It is well known that the radon concentrations in caves are generally high and especially high in the summer as reported by Tanahara, Iha, and Taira (1997) and Przylibski (1999). Tanahara et al. concluded that variation of air density resulted in the variation of radon concentration by their simultaneous measurements of radon concentration and air density both inside and outside a cave. It was also explained that the radon concentration in a cave became lower in the winter by outside air flowing into the cave, because the outside air density became higher in the winter. Conversely, in the summer the inverse phenomena would occur and cause the radon concentration to be higher in a cave due to air stagnation in the cave, and the high radon bearing air would go outside. As to diurnal variation, since air density became lower outside a cave in the daytime as observed in their study, it is reasonable to assume that the accumulated high radon in the closed gallery could leak out during the daytime through the sandbags stacked at the temporarily closed gallery entrance and result in the high concentration of radon at D-0. At Ayumidani residential district, higher radon concentration by the passive method was observed compared with the sampling method. It is generally said that a higher radon concentration is often observed from midnight to dawn, as atmospheric air becomes stable at night. At Ayumidani the reason for the higher concentration by the passive method seems to be due to this calm atmospheric stability at night leading to a higher radon concentration as mentioned above. On the other hand,
Waste rock site Nakatsugo WR Choja WR Choja WR(B) Kan’nokura-1 WR Kan’nokura-1 WR(B) Kan’nokura-2 WR Kan’nokura-2 WR(B) Asabatake-1 WR Asabatake-1 WR(B) Asabatake-2 WR
Location
35 127 91 107
147
1550
297
754 184
333
12 29 24 38
93
542
81
99 81
98
8
12 18
8
11
37
5 5 5 6
6.5
3.9 3.6
3.8
10.9
3.5
2.9 2.1 1.9 2.1
65.3
7.1 11.9
12.0
32.6
5.5
5.8 3.4 3.4 4.6
Max.
Mean
Min.
Mean
Max.
EECRn (Bq m⫺3)
Radon (Bq m⫺3)
Results by sampling method
0.9
1.1 0.9
1.5
0.9
0.8
0.7 0.6 0.8 0.6
Min.
0.10
0.10 0.06
0.17
0.05
0.04
0.29 0.19 0.18 0.12
Mean
F-valuea
0.71
0.28 0.16
0.37
0.24
0.07
0.70 0.60 0.52 0.36
Max.
0.01
0.01 0.01
0.02
0.01
0.02
0.05 0.02 0.01 0.02
Min.
63
16 15
8
12
12
37 10 14 11
Nb
68
37 70
19
233
67
10 12 17 25
6.9
3.7 4.3
3.2
11.9
2.7
2.9 2.3 3.1 3.1
0.37
0.20 0.23
0.17
0.63
0.14
0.16 0.12 0.17 0.16
Estimated Estimated EECRnc dose (Bq m⫺3) (mSv year⫺1)
(continued on next page)
Passive Rn (Bq m⫺3)
Table 2 Summary of the results of radon concentration and equilibrium equivalent radon concentration (EECRn) by sampling method, and estimation of long-term EECRn and annual dose
S. Furuta et al. / J. Environ. Radioactivity 62 (2002) 97–114 109
d
c
b
a
27 13 14 6 25 6 7 14 9 6 5 5 5
280 155
66,300
117 1580
184
81
101 217 48
42 87 191
3.7 3.0 3.3
2.2 3.7 3.5
3.1
4.9
3.6 26.0
70.5
3.8 3.9
3.3
12.3 12.8 8.4
4.3 10.5 5.9
6.5
33.0
7.9 97.3
343.7
8.0 10.4
9.9
0.5 0.9 0.8
0.8 0.7 2.0
0.9
0.9
0.7 2.6
0.6
1.1 0.6
0.7
0.29 0.22 0.36
0.06 0.09 0.21
0.16
0.15
0.22 0.05
0.07
0.05 0.10
0.12
0.74 0.77 0.69
0.15 0.21 0.44
0.40
0.65
0.73 0.12
0.52
0.14 0.23
0.43
0.07 0.03 0.03
0.01 0.03 0.07
0.03
0.03
0.04 0.02
0.00
0.01 0.01
0.01
64 39 60
11 13 17
28
46
50 22
73
14 13
57
A ratio between EECRn and radon gas concentration. Number of data. Estimated EECRn was obtained using F value from Rn concentration in fiscal years 1997–1999 by passive monitor. Others are Kan’nokura, Asabatake, Katamo, Kawakami, Yendani, Hirose, Anagamo, Ombara and Ningyo.
5
371
Results by sampling method
Asabatake-2 54 WR(B4) Asabatake-3 WR 118 Asabatake-2B 56 WR Katamo-1,2 3624 WR(D-0) Katamo-1,2 WR 26 Katamo-1,2 606 WR(D-1) Katamo-1,2 43 WR(D-5:Gate) Katamo-1,2 27 WR(B4) Katamo-3 WR 53 Katamo-S2 WR 68 Ayumidani WR 20 Residential district Ayumidani 14 Nakatsugo-2 21 Othersd 16 (including controls)
Location
Table 2 (continued)
63 21 14
20 46 60
8
62
25 263
694
90 55
121
Passive Rn (Bq m⫺3)
18.1 4.6 5.0
1.2 4.0 12.5
1.3
9.2
5.5 14.2
47.2
4.1 5.5
13.9
0.97 0.25 0.27
0.06 0.21 0.67
0.07
0.49
0.29 0.76
2.52
0.22 0.29
0.74
Estimated Estimated EECRnc dose (Bq m⫺3) (mSv year⫺1)
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S. Furuta et al. / J. Environ. Radioactivity 62 (2002) 97–114
111
the measurement by the sampling method was carried out basically in the daytime, which tends to indicate a lower concentration. Table 2 indicates that the EECRn values were not so high inside the waste rock fenced sites, contrary to the respective high radon concentrations there. In other words, the tendency toward a low F value was obtained in the waste rock fenced sites. A large fluctuation of the F values is also shown in the table as a result of a large variation in the respective radon concentrations. Therefore, in the vicinity of the waste rock yards as radon source, a phenomenon was observed that the progeny did not grow sufficiently. Such a phenomenon has been reported in other places (Yamasaki, Yoshimoto, & Tsujimoto, 1993). Using these F values obtained by the sampling method, long-term EECRn values were derived from the radon concentrations by passive method. Consequently lower EECRn values were obtained compared with the respective radon concentrations, as shown in Table 2. 3.3. Continuous measurement of radon and its progeny Radon concentrations and the EECRn were obtained around the Katamo-1,2 waste rock fenced site using active-type continuous monitors. Two sets of continuous monitors for radon and its progeny were placed at the gate to the site and in the orchard about 1 km away downstream of the main ravine from the gate as shown in Fig. 2. Simultaneous measurements were carried out at these two locations from October 1992. The results are shown in Fig. 6. As this figure indicates, the radon concentration at the gate was higher than that at the orchard, but the variation corresponded between those measured at the gate and the orchard. The arithmetic mean radon concentrations were 23 Bq m⫺3 and 65
Fig. 6. Radon concentration and equilibrium equivalent radon concentration (EECRn) by continuous measurements around Katamo-1,2 waste rock site. Numbers in the legend represent mean values.
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Bq m⫺3 at the orchard and the gate, respectively. The EECRn values were low compared with the high radon concentration. The mean F values were 0.20 and 0.12 at the orchard and the gate, respectively. Through these results, it is estimated that the effect of radon dispersion from the limited source like the waste rock yard was significant, because the radon progeny did not grow sufficiently at the gate compared with those at the orchard. The arithmetic mean EECRn, 8.0 Bq m⫺3, obtained by this method at the gate is higher than that of 4.9 Bq m⫺3 obtained by sampling method at the same place as shown in Table 2. But the former value is in relatively good agreement with the estimated EECRn value of 9.2 Bq m⫺3 at the same location as shown in the same table, because the estimated EECRn indicates a long-term concentration. As a seasonal variation, higher concentrations were observed in the summer. In general it is known that radon concentrations are lower in regions facing to the Sea of Japan in the winter and higher in the summer (Nishikawa, Aoki, & Okabe, 1985). The data measured in Katamo district tend to show a similar pattern of variation as observed in those regions. 3.4. Dose estimation First, the estimation of annual effective dose contributed by the uranium waste rock was made using the estimated long-term EECRn data indicated in Table 2, assuming that a person would live there continuously for one year. Effective dose E (mSv year⫺1) is derived as follows: E ⫽ S·T·EECRn / B, where S: T: B:
a conversion factor for public described in ICRP Publication 65 (3.88 mSv WLM⫺1) hours in one year (8760 h year⫺1) a conversion factor described in ICRP Publication 65 [ 6.37 × 105 (Bq h m⫺3) WLM⫺1]
Even in the waste rock fenced sites, the effective doses were less than the public dose limit of 1 mSv year⫺1 except for 2.5mSv year⫺1 obtained at Katamo-1,2 WR (D-0) as shown on the extreme right in Table 2. Consequently, the impact of the waste rocks on the surrounding area is small considering these results include the natural background component. Next, the dose estimation was carried out using the results of continuous measurement of EECRn. The mean value of the data was 4.6 Bq m⫺3 at the orchard, and 8.0 Bq m⫺3 at the gate as shown in Fig. 6. According to the literature, the following values are cited as the results of outdoor measurements: an average value of 4.1 Bq m⫺3 from August 1991 to June 1992 in Nagoya, Japan (Yamasaki, Iida, Shimo, & Ikebe, 1995), and an arithmetic mean of 8.6 Bq m⫺3 and a geometrical mean of 6.9 Bq m⫺3 from 1982 to 1992 in Neuherberg Germany (Hoetzl & Winkler, 1994).
S. Furuta et al. / J. Environ. Radioactivity 62 (2002) 97–114
113
Judging from these data, it may safely be said that the result at the orchard is recognized at least as a normal level. Therefore, the elevated component of 3.4 Bq m⫺3 is obtained for the gate area by deducting the EECRn value at the orchard from that at the gate (8.0⫺4.6) that seems to indicate the long-term effect of the waste rock, as the value at the orchard is assumed to be a background level of this region. The effective dose of 0.18 mSv year⫺1 is estimated as the influence of the waste rock yards, using the equation described above and the EECRn value of 3.4 Bq m⫺3. Therefore, the result recorded on the boundary of the uranium waste rock fenced site is less than the public dose limit of 1 mSv year⫺1.
4. Conclusion Radon concentrations were measured in and around the uranium waste rock fenced sites, and their corresponding environmental impacts have been assessed. Although relatively high radon concentrations were observed in the waste rock fenced sites, on the other hand low EECRn and F values were obtained because the progeny nuclides did not grow sufficiently in the vicinity of a limited radon source like these uranium waste rock yards. Therefore, it was determined that the environmental impacts of radon from the waste rock yards were small around such sites, using the estimated long-term EECRn. It was further estimated, by simultaneous continuous measurements at two locations, that the effective dose caused by the representative waste rock yards was less than the public effective dose limit of 1mSv year⫺1 on the site boundary.
Acknowledgements The authors would like to express sincere gratitude to Professor Iida of Nagoya University for his help and useful suggestions in conducting this study.
References Abe, M., & Abe, S. (1989). A nation-wide survey of radon indoors and outdoors in Japan. In: S. Kabayashi, & T. Kankura (Eds.), Radon in the living environment — levels and risks, Proceedings of the 15th NIRS Seminar (pp. 79–88). (in Japanese). Furuta, S., Ito, K., & Ishimori, Y. (1999). Radon reference chamber for calibration of the monitors. Radioisotopes, 48, 725–731 (in Japanese). Furuta, S., Ito, K., & Ishimori, Y. (2000). A continuous radon progeny monitor with a vacuum vessel by alpha spectrometry. Radiation Protection Dosimetry, 90, 429–435. Hoetzl, H., & Winkler, R. (1994). Long term variation of outdoor radon equilibrium equivalent concentration. Radiat. Environ. Biophys., 33, 381–392. Iida, T., Ikebe, Y., Hattori, T., Yamanishi, H., Abe, S., Ochifiji, K., & Yokoyama, S. (1988). An electrostatic integrating 222Rn monitor with cellulose nitrate film for environmental monitoring. Health Physics, 54, 139–148. Nishikawa, T., Aoki, M., & Okabe, S. (1985). Diurnal and seasonal variation of the atmospheric radon
114
S. Furuta et al. / J. Environ. Radioactivity 62 (2002) 97–114
and thoron daughters concentration. In: S. Okabe (Ed.), Atmospheric radon families and environmental radioactivity (pp. 107–114). (in Japanese). PNC (1994). Uranium resources in Japan. In: PNC TN7420 94–006. Power Reactor and Nuclear Fuel Development Corporation (in Japanese). Przylibski, T. A. (1999). Radon concentration changes in the air of two caves in Poland. Journal of Environmental Radioactivity, 45, 81–94. Shimo, M., Fujitaka, K., Furuta, S., Iida, T., Iimoto, T., Iyogi, T., Kurosawa, R., Kojima, H., Sanada, T., Tokonami, S., Fujimoto, K., Miyano, K., Yamasaki, K., & Yonehara, H. (1997). Radon intercomparison at EML USDOE. Hoken Butsuri, 32, 285–294 (in Japanese). Tanahara, A., Iha, H., & Taira, H. (1997). Factors controlling the changes in 222Rn concentration in Gyokusen Cave on Okinawa Island. Journal of the Speleological Society of Japan, 22, 98–105 (in Japanese). Yamasaki, T., Iida, T., Shimo, M., & Ikebe, Y. (1995). Continuous measurements of outdoor radon and its progeny concentrations. Hoken Butsuri, 30, 149–154 (in Japanese). Yamasaki, K., Yoshimoto, T., & Tsujimoto, T. (1993). A mysterious spot on the outdoor concentrations of radon and thoron. KURRI Progress Report, 418–419.