INDOOR RADON CONCENTRATION MEASUREMENTS AT THE ...

INDOOR RADON CONCENTRATION MEASUREMENTS AT THE LOCATIONS OF THE FIRST NUCLEAR POWER PLANTS OF VIETNAM* B.D. DUNG1, T.V. GIAP1, T. KOVÁCS2, L.D. CUONG1, N.H. QUYET1 1

Institute for Nuclear Science and Technology, 179 Hoang Quoc Viet, Nghia Do - Cau Giay, Hanoi, Vietnam 2 Institute of Radiochemistry and Radioecology, University of Pannonia, H-8200, Veszprem Egyetem str 10, Hungary Received November 15, 2012

The Vietnamese government has decided to build the first two nuclear power plants in Ninh Thuan province. This study reports the indoor radon (Rn) survey of the area in the vicinity of the nuclear plants to be built in this province. The survey was conducted during the years 2010-2011 in order to establish the radioactive background database of the province before the nuclear power plant is built and put into operation. Fifty nine locations (representing the 59 most populated of the 64 communes in the province) were investigated. Indoor Rn concentration was measured using Solid State Nuclear Track Detector (SSNTD) LR-115 type II strippable with 3 months exposure for the dose assessment of the future population. Rn concentration values gained during the survey were rather low in general compared to the data of other international surveys. The average was 10±5 Bq/m3 (min. 4 Bq/m3; max. 27 Bq/m3). From the point of view of dosimetry even the highest estimated effective dose originating from the Rn concentration is negligible. After the completion of the power plants, however, the previous background data have crucial importance in order to assess the concentrations due to the power plants and to provide credible information to the public. Key words: radon, Vietnam, indoor air, dose assessment.

1. INTRODUCTION More than half of the radiation dose of natural origin comes from radon (Rn) [1]. Although there has not been a consistent Rn monitoring program in Vietnam, scattered investigations of the Rn level in the environment have been started in the early 1990s [2-5]. The first survey for indoor Rn was probably the one that started in 1993 for Hanoi dwellings [3]. Results of this survey showed that Rn concentration in the winter – spring period is a little higher (30 Bq/m3) than in the * Paper presented at the First East European Radon Symposium – FERAS 2012, September 2–5, 2012, Cluj-Napoca, Romania.

Rom. Journ. Phys., Vol. 58, Supplement, P. S108–S114, Bucharest, 2013

2

Indoor radon concentration measurements at the power plants of Vietnam

S109

summer – autumn period (25.5 Bq/m3). Rn concentration in dwellings depends on the nature of the construction material and the ventilation of the buildings and the rooms. Average indoor Rn concentration in Hanoi was found to be 27±3 Bq/m3 [3]. Subsequent surveys for Rn concentration in mining areas showed that indoor Rn concentration in offices and factories is relatively high (45-85 Bq/m3) [4]. In coal a mining area, for example, Rn concentration in dwellings was found to be 46±26 Bq/m3, while indoor radon concentrations measured at the coal storage field near the coastline were found to be 67±4 Bq/m3 [5]. These former surveys imply that considerable Rn concentration can be expected only at certain territories of small areas with limited numbers of inhabitants in Vietnam. Despite of the results of these surveys, further measurements of Rn is necessary in Vietnam. Surveys of thoron (Tn) also seem reasonable due to the relatively high thorium content of the soil. In 2008, the Directorate for Standards and Quality (Ministry of Science and Technology) had issued Vietnam Standard that set Rn action levels in dwellings for Rn concentration more than 200 Bq/m3. TCVN 7889: 2008 [6] also set that the recommended safety levels of Rn for new buildings should be less than 100 Bq/m3. The Vietnamese government has decided to build the first two nuclear power plants in Ninh Thuan province, in the southern part of Vietnam. According to the schedule, the construction of the first nuclear power plant will start in 2014, while generation of power will start in 2020. Many projects have been conducted in association with this governmental nuclear power program. One of these projects is a survey to establish the radioactivity background database of the province before the nuclear power plant is built and put into operation. This survey was conducted during the years 2010-2011. This paper reports the indoor Rn survey for Ninh Thuan province. However, latest surveys showed that in some areas [7-9] the presence of thoron is not negligible, therefore, later the discriminative measurement or radon/thoron will be important. Therefore, the Cr-39 passive (RADUET type) trace detector measurement of radon-thoron has been started in this area. 2. SITE DESCRIPTION Ninh Thuan is a coastal area, located in the South Central region of Vietnam and is one of the 63 provinces/cities of Vietnam. The land is within latitudes 11°18' and 11°10' North and longitudes 108°39' and 109°14' East, bordering with Khanh Hoa province to the north, Binh Thuan province to the south, and Lam Dong province to the west. It has a 105 km coastline at the east (Fig. 1). Land area is around 3360 km2 accounting for about 1 % of the total natural area of the country. The province's population is 573,925 people (as of June 10, 2009). Ninh

S110

B.D. Dung et al.

3

Thuan is surrounded by mountains at three sides and a sea coast. High mountains are to the west at Lam Dong border. At the north and south borders there are two ranges that run west – eastwart into the sea. The province land consists of about 22.4% of arid plains, 63.2% of mountainous areas, and about 15.4% of hilly regions.

Fig. 1 – Location of the study area Map of Vietnam and Ninh Thuan province 1st Nuclear power plant (NPP) at Phuoc Dinh Commune, 2nd NPP at Vinh Hai Commune, Dark squares are measuring points for Rn.

3. MATERIALS AND METHODS Indoor Rn concentrations at the location of investigation were measured using two LR-115 type II Strippable detectors put into the chamber called “Plastic bag sampler” [10] with 3 months exposure (Fig. 2). In total, fifty nine locations (representing the 59 most populated of the 64 communes in the province) were investigated. After the exposure period from August to October 2011, the detectors were brought back to the laboratory and were chemically etched for 100 minutes in 2.5M NaOH at 60°C. Track densities were counted on a spark counter device [11]. Indoor Rn concentrations were calculated as C Rn = D det k −1 H −1 , where C Rn is the concentration of Rn (Bqm-3), D det is the track density given by a spark counter device (counts cm-2), k is the calibration factor that equals to 6.75.10-4 (counts cm-2)/ /(h*Bq m-3), and H is the exposure time (h).

4

Indoor radon concentration measurements at the power plants of Vietnam

S111

Fig. 2 – Plastic bag sampler made of two LR-115 detectors.

4. RESULTS AND DISCUSSION The first, overall, conclusion is that the Rn concentration values gained during the survey are rather low in general. The average value is 10±5 Bq/m3 (min. 4 Bq/m3; max. 27 Bq/m3). Fig. 3 shows the Rn concentration distribution in Ninh Thuan province as depicted from the measured values. It is seen that most of the sampling locations have quite low (less than 10 Bq/m3) indoor Rn concentrations. According to TCVN 7889: 2008 (6), these Rn concentrations in Ninh Thuan province are negligible and the measured radon activity concentration does not reach any international suggested reference level (UNSCEAR, ICRP). These low values, however, make it easier to separate the impact of the power plants on Rn concentrations when the power plants are in use. 30

Frequency

25 20 15 10 5 0 4

8

12

16

20

24 3

Radon concentration distribution [Bq/m ]

Fig. 3 – Rn concentration distribution in Ninh Thuan province.

S112

B.D. Dung et al.

5

The spatial distribution of the indoor Rn concentrations across Ninh Thuan province is given in Fig 4. The figure shows that Rn is distributed nearly evenly and only a few spots can be observed with elevated concentration levels (which are, however, still below any action level).

Fig. 4 – Spatial distribution of Rn across Ninh Thuan province.

5. DOSE CALCULATION Knowing the measured radon concentration (CRn) the committed effective dose was calculated using the equation: E= CRn × F × RT × DCF, -1

(1)

where E is the effective dose (mSv year ), CRn – is the Radon concentration (Bq m-3), RT is the residence time (h year-1: 7000 h), F is the equilibrium factor (typically 0.4 in dwellings), and DCF is the dose conversion factor (typically used from UNSCEAR: 9 nSv(Bq m-3 h)-1). The distribution of the yearly effective dose from radon and their progenies given in the Fig. 5. According to the low radon concentrations the yearly effective dose from the is not exceed the 0.7 mSv/year (min. 0.1 mSv/year; max. 0.65 mSv/year; average 0.28 mSv/year).

6

Indoor radon concentration measurements at the power plants of Vietnam

S113

25

Frequency

20 15 10 5 0 0.1

0.2

0.3

0.4

0.5

0.6

Effective dose distribution [mSv/year] Fig. 5 – Estimated effective dose distribution from Rn and their progeny.

6. CONCLUSION The Rn concentration values gained during the survey were rather low in general compared to data from other international surveys. The average was 10±5 Bq/m3 (min. 4 Bq/m3; max. 27 Bq/m3) and Ninh Thuan province is considered to be a low Rn background area. From the point of view of dosimetry, even the highest estimated effective dose originating from the Rn concentration is negligible. After the completion of the power plants, however, the previous background data are essential to separate the natural background radiation dose from the contribution of power plant. These data are also important from the point of view of providing credible information to the public, especially those living in the vicinity of the nuclear power plants. REFERENCES 1. UNSCEAR 2006 Report Volume II, Annex E: Sources-to-effects assessment for Rn in homes and workplaces, 2006. 2. N X Thang, Application of Solid State Nuclear Track Detector (SSNTD) and Isotope Geochemistry methods for radioactive ore exploration, Report of the Vietnam National Project 50B-01-03 Hanoi 1990. 3. T T Minh, Determination of radioactivity in hazardous environments directly effect the health, Report of the Vietnam National Project KC - 09 – 18 Hanoi 1996. 4. T B Trong, Investigation of the environment in a number of mines: Dong Pao; Then Sin - Tam Duong, Lai Chau; Muong Hum - Lao Cai; Yen Phu - Yen Bai; Thanh Son - Phu Tho; An Diem, Ngoc Kinh, Suon Giua - Quang Nam, Report of Vietnam Confederation of Geological Survey for Radioactive and Rare-Earth Elements Hanoi 2004.

S114

B.D. Dung et al.

7

5. D D Nhan, F P Carvalho, N T Thu Ha, N Q Long, D D Thuan, H Fonseca, Radon (222Rn) concentration in indoor air near the coal mining area of Nui Beo, North of Vietnam, Journal of Environmental Radioactivity 110 98–103 2012. 6. Vietnam Standard TCVN 7889: 2008, Natural Radon activity in buildings - Levels and general requirements of measuring methods Hanoi 2008. 7. Y Yamada, Q Sun, S Tokonami, S Akiba, W Zhuo, C Hou, S Zhang, T Ishikawa, M Furukawa, K Fukutsu, H Yonehara, Radon–thoron discriminative measurements in Gansu province, China, and their implication for dose estimates, Journal of Toxicology and Environmental Health Part A 69 723–734 2006. 8. T Kovacs, Thoron measurements in Hungary, Radiation Protection Dosimetry 141 328–334 2010. 9. M Rosaline, S P Tripathy, D T Khating, K K Dwivedi, An extensive indoor 222Rn/220Rn monitoring in Shillong, India. Radiation Protection Dosimetry 112 429–433 2004. 10. L. Tomasino, D.E. Chrouati, A Plastic Bag Sampler for Passive Radon Monitoring. Nuclear Track. 12, 681–681 1988. 11. G. Somogyi, L. Medveczky, I. Hunyadi, B. Nyako, Automatic Spark Counting of Alpha-Track in Plastic Foils. Nuclear Track Detection 1-2 131–138 1977.