Developing a database for Cs-137 radioactivity concentration and assessing residual levels in surface soils of Quang Ninh, Hai Phong, and Lang Son provinces

Nội dung text: Developing a database for Cs-137 radioactivity concentration and assessing residual levels in surface soils of Quang Ninh, Hai Phong, and Lang Son provinces

1. Environmental Sciences | Environmental Science Doi: 10.31276/VJSTE.63(3).92-96 Developing a database for Cs-137 radioactivity concentration and assessing residual levels in surface soils of Quang Ninh, Hai Phong, and Lang Son provinces An Trung Nguyen*, Thi Cam Ha Nguyen, Tien Thinh Lai, Hoang Giang Nguyen, Ha Vu, Thanh Long Nguyen Technical Support Center for Radiation and Nuclear Safety and Emergency Response, Vietnam Agency for Radiation and Nuclear Safety (VARANS TSC) Received 20 March 2021; accepted 10 June 2021 Abstract: The radioactivity concentration of Cs-137 was determined in 826 soil samples collected from Hai Phong, Quang Ninh, and Lang Son. The residual levels of Cs-137 have also been assessed and compared with previous publications to obtain a trend in Cs-137 in the surface soil. The actual measured average of the residual level of Cs-137 in the soil samples collected in this study was 236±146 Bq/m2, while the value calculated using the published experimental formula was 407±77 Bq/m2. The impact of Cs-137 deposition from the Fukushima nuclear accident (2011) in this area is confirmed to be negligible. Keywords: Hai Phong, Lang Son, Quang Ninh, radioactivity of Cs-137, residual level of Cs-137. Classification number: 5.3 Introduction To mitigate time constraints, sample processing and analyses were performed simultaneously by three Cs-137 is an artificial fission product with a relatively laboratories: VARANS TSC (526 samples; with 100 long half-life (30 years). As a result, Cs-137 can be used samples from Quang Ninh and 426 samples from Lang as an indicator of the environment impact of release Son), the Institute for Nuclear Science and Techniques from a nuclear power plant during normal operation (220 samples; including 80 samples from Hai Phong or emergencies. Although the residual level of Cs- and 140 samples from Quang Ninh), and the Center for 137 in soil in Vietnam was studied many years ago, Environmental Technology & Treatment - Ministry of recent developments of nuclear power plants near the North border of Vietnam, especially the operation of Defense (80 soil samples from Quang Ninh). Fangchengang Nuclear Power Plant (NPP) at about 54 Soil sampling and processing method km from the border, has brought out the need to establish Soil samples were taken from a flat-surfaced area a database of radioactivity concentration (Bq/kg) and a based on a 4.5 km x 4.5 km grid in all accessible areas to reassessment of residual levels (Bq/m2) of Cs-137 near ensure relatively uniform sample density over all three border areas. In this study, the radioactivity concentration provinces. All the soils, dug from a 30 cm x 30 cm x 30 and the residual level of Cs-137 in the soils of the Hai cm cubic hole, were mixed thoroughly. A depth of 30 cm Phong, Quang Ninh, and Lang Son provinces are was chosen to take into account the disturbance processes determined to fulfil this purpose. of soils. Then, the soils were reduced to about 1-1.5 Materials and methods kg by quartering and compartmentalization. Relevant information from the soil samples were also recorded. A total of 826 soil samples were collected over a period of 3 years from 2017-2019 following standard The sample processing procedures of all involved procedures by VARANS TSC. organizations were standardized and included the *Corresponding author: Email: natrung@most.gov.vn Vietnam Journal of Science, 92 Technology and Engineering September 2021 • Volume 63 Number 3
2. Environmental Sciences | Environmental Science following steps: crushing the sample to reduce soil the higher energy region, self-absorption correction is particle size to 1 cm or less; removing foreign materials not necessary and thus was neglected [2]. Therefore, such as plant roots, pebble, and garbage; spreading the generation of the self-absorption correction curve using soil sample on tray then heating to about 100-105oC until RGU-1 is a suitable method. the samples were completely dried out (weight variation For VARANS TSC and INST, the quality assurance less than 1%); grinding and sieving the dry soil to obtain (QA) for sample analyses were performed by analysing particle sizes less than 1 mm; and finally weighing the IAEA reference sample (Sample No.4, provided in and compressing the samples into a confined box for measurements. the 2018 inter-laboratory comparison conducted by IAEA) with known activity of nuclides including K-40, Cs-137 analysis methods and quality assurance Cs-137, Tl-208, Pb-210, Pb-212, Pb-214, Bi-214, Ra- measures 226, and Ac-228, which followed IAEA’s assessment Technical specifications of each low-background method in which the trueness, precision of the analysis gamma spectrometer used by each laboratory for sample results is assessed separately [3]. For CTET, the QA was measurement and analysis are shown in Table 1. performed by inter-comparison of the analysis results The energy and efficiency calibration of the HPGE from three laboratories for a set of three samples. The detectors were developed using a multipurpose RGU-1 QA results show that all analysis results were evaluated reference source and RGU_IAEA.Lib library provided as “acceptable” (the highest grade among three levels of by IAEA. evaluation: A “Acceptable”, N “Not acceptable”, and W “Warning”). The geometry calibration file and correction curve for self-absorption due to sample density and composition Results were also generated using RGU-1 standard sources. Development of database on Cs-137 radioactivity The chemical composition of RGU-1 is similar to that concentration in surface soil of surface soil samples [1], which indicates that the contribution ratio of chemical elements in the two Figure 1 shows the spatial distribution of Cs-137 samples are similar. In addition, the self-absorption of radioactivity concentration in soil layers up to 30 cm in the samples with different chemical compositions mainly depth on the GIS maps of the Hai Phong, Quang Ninh, influences the energy region of less than 186 keV. For and Lang Son provinces. Table 1. Technical specifications of low-background gamma spectrometers used by involved laboratories. Institute for Nuclear Science and Center for Technology N0 Parameter VARANS TSC Techniques (INST) Environmental Treatment (CTET) ORTEC, Cylindrical, inner CANBERRA, Cylindrical, inner Cubical lead chamber with outer 1 Lead chamber diameter: 28.0 cm; outer diameter: 27.9 cm; outer diameter: dimensions: 100 cm x 80 cm and inner diameter: 50.4 cm 50.8 cm dimensions: 50 cm x 50 cm Detector 01 HPGe detector made by 02 HPGe detectors made by 01 HPGe detector made by 2 manufacturer CANBERRA CANBERRA CANBERRA Type of detector and - GC5019, ≥50% 3 GX4020, ≥40% GC1520, 15% relative efficiency - BE6530, ≥65% - GC5019: ≤1.9 keV at 1.33 MeV and ≤2 keV at 1.33 MeV and ≤1.0 keV at 122 keV 1.66 keV at 1.33 MeV and 0.77 keV at 4 Resolution ≤1.2 keV at 122 keV - BE6530: 1.9 keV at 1325 MeV and 122 keV 0.684 keV at 122 keV Minimum detectable 5 activity (MDA) to 0.18 Bq/kg 0.3 Bq/kg 0.6 Bq/kg Cs-137 Vietnam Journal of Science, September 2021 • Volume 63 Number 3 Technology and Engineering 93
3. Environmental Sciences | Environmental Science The measured radioactivity concentrations of Cs-137 in Quang Ninh are higher than that of Hai Phong and Lang Son, albeit not much. The observation of high concentrations of Cs-137 is consistent with the high average annual rainfall in Mong Cai - Quang Ninh in comparison with that of the other surveyed areas [4]. Statistical distributions of Cs- 137 radioactivity concentration in the 826 soil samples collected in Hai Phong, Quang Ninh, and Lang Son provinces are shown in Fig. 2. The statistical parameters of the radioactivity concentration of Cs- Fig. 1. Spatial distribution of Cs-137 radioactivity concentration in soil samples 137 in the 826 soil samples collected collected in 03 surveyed provinces. from all 3 provinces are shown in Cs-137 radioac�vity Cs-137 radioac�vity concentra�on in Hai Phong soils concentra�on in Quang Ninh soils 60 35 50 ) 30 % ( 40 25 20 30 nc y 20 u e 15 e q r 10 10 F Frequency (%) 5 0 ] ] ] ] 2 1 ] ] ] ] ] ] 0 . 9 9 5 5 ] ] ] ] > 7 7 3 3 1 2 . . . . 2 1 ] ] ] ] ] ] 0 . . . . . . 9 5 9 5 , 0 1 0 1 > 7 3 1 7 3 2 . . . . 7 3 1 7 2 3 . . . . . 9 5 9 5 0 . . . . . > 7 3 1 7 3 2 . . . . , 1 1 0 0 0 . . . . . < 1 1 1 0 0 , 1 1 0 0 9 , , , , < 1 1 1 0 0 . , , , , , 9 , , , , 7 3 7 3 . , , , , , 5 1 9 5 1 1 . . . . 7 3 7 3 . . . . . [ 5 1 9 5 1 1 . . . . 1 1 0 0 . . . . . [ 1 1 0 0 0 [ [ [ [ 1 1 0 0 [ [ [ [ [ 1 1 0 0 0 [ [ [ [ [ [ [ [ [ Radioac�vity Concentra�on (Bq/kg) Radioac�vity Concentra�on (Bq/kg) (C) (D) Fig. 2.Fi Statisticalg. 2. Sta tdistributionsistical dist ofribu Cs-137tion radioactivitys of Cs-13 concentration7 radioacti vini t826y c soilon csamplesentrat collectedion in 82 from6 s o(A)il Haisamp Phong,les (B) Quangco Ninh,llect e(C)d Langfrom Son, (A )and Ha (D)i P thehong 3 provinces, (B) Qu asan ag whole. Ninh, (C) Lang Son, and (D) the 3 provinces as a whole. Vietnam Journal of Science, 94 TechnologyThe statistical and Engineering parSeptemberameters 2021of the• Volume radio 63a ctNumberivity 3concentration of Cs-137 in the 826 soil samples collected from all 3 provinces are shown in Table 2. For all samples in which Cs-137 radioactivity concentration was below LOD, the LOD values were chosen instead. Table 2. Statistical parameters on radioactivity concentration of Cs-137 in 826 soil samples collected in Hai Phong, Quang Ninh, and Lang Son.
4. Environmental Sciences | Environmental Science Table 2. Statistical parameters on radioactivity concentration of Cs-137 in 826 soil samples collected in Hai Phong, Quang Ninh, and Lang Son. Statistical parameters All 3 provinces Quang Ninh Lang Son Hai Phong Number of samples 826 320 426 80 Mean (Bq/kg) 0.68 0.84 0.60 0.46 Standard error (Bq/kg) 0.03 0.05 0.03 0.05 Median (Bq/kg) 0.41 0.60 0.37 0.34 Mode (Bq/kg) 0.30 0.30 0.15 0.05 Standard deviation (Bq/kg) 0.75 0.90 0.65 0.46 Kurtosis 9.86 8.64 5.03 2.26 Skewness 2.73 2.71 2.18 1.33 Minimum (Bq/kg) 0.03 0.06 0.03 0.05 Maximum (Bq/kg) 5.70 5.70 4.14 2.32 Confidence level (95%) 0.05 0.10 0.06 0.10 Table 2. For all samples in which Cs-137 radioactivity be approximately 60-63% of the published values in concentration was below LOD, the LOD values were 2001 (the addition of Cs-137 due to events such as the chosen instead. Fukushima accident could be ignored*). Comparison with results from previous publications The observed difference could be explained by processes that cause Cs-137 to redistribute to deeper layer In 2001, Prof. Pham Duy Hien and colleagues of soil, i.e., natural erosion or man-made disturbance of published on the residual density of Cs-137 in 292 the soil from cultivation or road renovation activities. undisturbed soil samples collected from 292 positions across Vietnam within 9.2-23.00N latitude [5]. The The Residualobserved diffe levelrence of couldCs-137: be explaine the researchd by processes by Prof. that Phamcause Cs -137 to principle purpose of this study is to assess the currentredistrib Duyute to Hien’s deeper layer group of soil [5], i. alsoe., natu proposedral erosion or an m an experimental-made disturbance of the status of Cs-137 in the surveyed area in order to establishsoil fromformula cultivation for or determiningroad renovation act theiv it residualies. level of Cs-137 a baseline for radioactivity concentration of this nuclide Raccordingesidual level toof Csthe-13 latitude7: the research of the by Psamplingrof. Pham Duylocation Hien’s gandroup [5] also proposerainfall:d an exper imental formula for determining the residual level of Cs-137 according as an initial reference value in case of future emergency.to the latitude of the sampling location and rainfall: Therefore, the priority for selecting samples is that the ( ) − = (3.53 ± 0.09) + (0.092 ± 0.004) + (0.62 ± 0.03) sampling points are evenly distributed over all accessiblewhere I is the residual level of Cs-137 in soil sample (Bq/m2); L is the latitude of the areas of the 3 provinces of interest (latitude from 20.61sa mpliwhereng locati Io n is(de thegree ) residual; AR is the annual level rainfall of Cs-137 at the sa inm pli soilng location sample (m ). 2 to 22.180N), which will take into account the land use in The(Bq/m variation); L ofis Lthen(I )latitude in the sur ofveyed the areasampling is shown location in Fig. 3. (degree);The residual level of Cs-137AR in soilis the sam annualples, calculated rainfall fro mat tthehe e xsamplingperimental folocationrmula with (m). rainf all data taken all localities. Thus, soil disturbance was not consideredfro m the nearest metrological stations, shows a smaller variation than the data collected in our study. under our Thestudy ,variation although the of upper Ln( liIm) itin of theboth surveyeddata sets are areaquite siism ilarshown. To make an appropriate comparison, only 264 soil in Fig. 3. The residual level of Cs-137 in soil samples, samples in our study were taken into account as samples calculated fromR estheidua l experimentallevel of Cs-137 vs laformula�tude with rainfall data7 taken from the nearest metrological stations, shows with Cs-137 radioactivity concentration below LOD 6 a smaller variation than the data collected underThM ựecoura tsếu red were neglected. 5 result study, 4 although the upper limit of both data sets are quite Lý thuyết Radioactivity concentration of Cs-137: according similar.ln(I) 3 Experi-mental formula to Ref. [5], the radioactivity concentration of Cs-137 2 in 292 soil samples varied from 0.5 to 18.0 Bq/kg with 1 *According0 to the annual measurement data under the Annual an average of 2.03 Bq/kg. In our study, these values 20.5 21 21.5 22 Vĩ độ ranged between 0.14 to 5.70 Bq/kg with an average of Environmental Monitoring TaskLa�tud edone by the Atomic Energy Institute of Vietnam, Cs-137 in fallout samples were detected at their Lang 1.14 Bq/kg, which is 56% of the average value from the Sơn city and Ha Long city stations over the period from 4/2011 to previous research. According to the exponential decay 9/2011. The residual level at this period was 0.14 Bq/m2. After that, principle, the expected residual levels of Cs-137 should Thethe residualactual av erlevelsage m ofeasure Cs-137d residual at the l estationsvel of Cs were-137 belowin soil LOD.samples collected in this study was 236±146 Bq/m2, while the one calculated from the experimental formula was 407±77 Bq/m2. Although the measured values were generally more dispersed and lower than the calculated values due to the erosion and disturbance processes as mentioned above, they are still in agreemVietnament when Journal one t ofak esScience, into account the standard September 2021 • Volume 63 Number 3 95 deviation of both datasets. Technology and Engineering Discussion and conclusions This study has provided comprehensive and detailed data on the current background level of Cs-137 radioactivity concentration in soil from Hai Phong, Quang Ninh, and Lang Son provinces. The data could be used as a reference value in case of future significant emergency from nuclear power plants near the north border of Vietnam. The residual level of Cs-137 in soil samples collected from the surveyed provinces demonstrate a decreasing trend when compared to previous monitoring results, which indicates that the contribution of nuclear accidents that occurred in the past 20 years (including the Fukushima accident) are negligible.
5. TheThe obseobserrvveded diffedifferrenceence couldcould bebe explaineexplainedd byby processesprocesses tthathat causecause CsCs 137137 toto redistribredistribuutete toto deeperdeeper layerlayer ofof soilsoil,, ii ee ,, natunaturralal erosionerosion oror mmanan mmadeade ddiisturbancesturbance ofof tthhee soilsoil fromfrom cultcultiivvationation oror roadroad renrenoovvationation actactiivvititiieses RResiduaesiduall levellevel ofof CsCs 131377:: tthehe rresearchesearch bbyy PPrrof.of. PhamPham DuyDuy Hien’sHien’s ggrorouupp [5][5] aalslsoo proposeproposedd anan experexperiimmentalental foforrmmulaula foforr detedeterrmmiininningg thethe residuresiduaall lleevvelel ofof CsCs 137137 accordiaccordinngg toto thethe latitudelatitude ofof thethe sasammplinplingg locatiolocationn andand raraiinfallnfall:: (( )) −− == ((33 5533 ±± 00 0099)) ++ ((00 090922 ±± 00 000044)) ++ ((00 6622 ±± 00 0033)) wherewhere II iiss thethe resiresiddualual lleevveell ofof CsCs 137137 iinn sosoiill ssaammpleple (B(Bq/q/mm22);); LL iiss thethe llaatitudetitude ooff ththee sasammpliplinngg locatilocatioonn ((dedeggreeree));; AARR isis thethe annualannual rainfallrainfall atat thethe sasammpliplinngg locationlocation ((mm)) TheThe vvariationariation ofof LLnn((II)) inin thethe sursurvveyedeyed areaarea isis shownshown inin FigFig 33 TheThe residualresidual lelevvelel ooff CsCs 137137 inin soilsoil ssaammplesples,, calculatedcalculated frfroomm tthehe eexxperiperimmenenttalal foforrmmulaula withwith rainfrainfaallll datadata ttaakkeenn frofromm thetheEnvironm nnearestearest mmeetroletrolntaloogg icalSciical e ssnctationstationses ,|, Environmshowsshows aa ssmmeallerallerntal variationvariation Scienc thanthane thethe datadata ccollecteollectedd underunder ourour sstudytudy,, althoualthougghh thethe upperupper lilimmitit ofof bothboth datadata setssets areare quitequite sisimmilarilar Pham Duy Hien and colleagues in his RReessiiduduaall lleevveell ooff CCss 137137 vsvs llaa��ttududee 77 publication in 2001, is still considered 66 valid for the recent data taking into TThhMựMựececa ta tsếsếu u rreded 55 rreessuultlt account the disturbed characteristics1 of 44 LLýý t thuhuyyếtết the collected soils. ln(I) ln(I) 33 EExxpeperri-mi-meennttaall ffoorrmmuulala 22 ACKNOWLEDGEMENTS 11 The study was conducted as a part 00 2020.5.5 2211 2121.5.5 2222 of the Ministerial Project titled Survey VVĩĩ độ độ LLaa��ttududee and Establishment of Environmental Fig. 3. Comparison of the measured residual level of Cs-137 with Radiation Database for localities calculation results from the experimental formula [5]. potentially impacted in the early period TheThe actualactual aavvereraaggee mmeasureeasuredd residualresidual lleevveell ooff CsCs 137137 iinn soilsoil ssaammplpleess collectecollectedofd i inn northern transboundary nuclear The actual average 22measured residual level of Cs- thisthis studystudy wawass 236236±±146146 BqBq//mm ,, whilewhile ththee ononee calculatedcalculated frfroomm tthhee experexperemergencyiimmentalental forfoforr mmtheulula a purpose of emergency response as 137 in soil samples22 collected in this study was 236±146 wawass 407407Bq/m±±77772 ,Bq Bqwhile//mm theAAlthlth oneoouugg hhcalculated thethe mmeasuredeasured from vv aluesthealues experimental werewere ggenerallyenerally assigned mmoreore disperseddispersed by Ministry aanndd of Science and Technology to lowerlower thanthan thethe calculatedcalculated vvaluesalues duedue toto tthehe erosionerosion andand disturbancedisturbance processesprocesses aass formula was 407±77 Bq/m2. Although the measured VARANS TSC from 2017 to 2019, with contribution by mmentioentionneded ababoovvee,, theythey areare stillstill iinn aaggrereeemmentent whenwhen oneone ttaakkeses intointo acacccountount thethe standastandardrd values were generally more dispersed and lower than dedevviatiiatioonn ofof bothboth datasetdatasetss technical staff from VARANS TSC, INST and CTET in the calculated values due to the erosion and disturbance sample collection, processing, and analysis. DiscDiscuussprocessesssiioonn aanndd cocoasn nmentionedcclluussiioonnss above, they are still in agreement We would like to express a special thanks of gratitude ThisThiswhen sstudytudy one has hastakes prproo vintovidedided account ccoommpprehensrehens the iistandardvvee anandd dde etaileddeviationtailed datadata onofon ththee currentcurrent babacckgrounkgroundd lelevvelel ofofboth CsCs -datasets.-113377 radioactradioact iivvityity concentratconcentratiionon iinn soilsoil fromfrom HaiHai PhPhonontogg,, Dr.QuanQuan Nguyengg NiNinnhh ,Hao, andand Quang for the valuable comments to elaborate this paper. LLaanngg SDiscussionSonon prproovincesvinces and The Theconclusions datadata couldcould bebe usedused asas aa referencreferencee vvaluealue inin casecase ofof futufuturree sisiggnificnificaantnt eemmeerrgencygency frfroomm nuclearnuclear powerpower plantsplants nearnear thethe nnorthorth borderborderCOMPETING ofof VietnamVietnam INTERESTS TheThe This residualresidual study lelevvee ll ofofhas CsCs 137137provided inin soilsoil ssacomprehensiveammplepless ccollectedollected frfrandoomm thethe susurrvveyeyeedd prproovvincinceess dedemmonondetailedsstratetrate aa decreasidecreasidata onnn ggthe trendtrend current whenwhen background ccoompmpaarreded to tolevel prpreev viousofious Cs- mmonitoonitorriinThengg resultsresults authors,, whicwhic declarehh that there is no conflict of interest indicatesindicates137 tha tharadioactivitytt thethe contrcontri ibutionbutionconcentration ooff nucleanuclea inrr soilacaccc identsidentsfrom Haithatthat Phong, occurreoccurre dd iregardinginn thethe pastpast the 2200 publication yearyearss of this article. (includi(includiQuangnngg thethe Ninh,FuFukkushush andiimama Langaccident)accident) Son areare provinces. nenegglliiggiibleble The data could be used as a reference value in case of future significant REFERENCES emergency from nuclear power plants near the north [1] N.T. Dang (2011), “Soil and nutrients for plants”, Thai border of Vietnam. Nguyen University of Agriculture and Forestry (Vietnamese version). The residual level of Cs-137 in soil samples collected from the surveyed provinces demonstrate a decreasing [2] Wafaa F. Bakr (2019), “Study the effect of sample’s trend when compared to previous monitoring results, chemical composition on the quality of gamma measurements which indicates that the contribution of nuclear accidents using Mont-Carlo simulation,” Int. J. Eng. Appl. Sci., 6(1), pp.2394-3661. that occurred in the past 20 years (including the Fukushima accident) are negligible. [3] IAEA (2009), Worldwide Open Proficiency Test on the Determination of Radionuclides in Spinach, Soil and Water, The processes of natural erosion and anthropogenic 258pp. disturbance of the topsoil are quite strong and widespread [4] Institute of Strategy and Policy on Natural Resources and in the survey area. The data should be further updated Environment (2009), Vietnam Assessment Report on Climate regularly to quantitatively estimate the relationship Change, ISBN: 0-893507-779124. between these processes and Cs-173 levels to provide a more accurate assessment of Cs-173 supplementary after [5] P.D. Hien, H.T. Hiep, N.H. Quang, N.Q. Huy, N.T. Binh, P.S. Hai, N.Q. Long, V.T. Bac (2002), “Derivation of 137Cs a significant emergency. deposition density from measurements of 137Cs inventories in The experimental formula, provided by Prof. undisturbed soils,” J. Environ. Radioact., 62(3), pp.295-303. Vietnam Journal of Science, 96 Technology and Engineering September 2021 • Volume 63 Number 3