Applied Radiation and Isotopes 69 (2011) 1079–1083

Contents lists available at ScienceDirect

Applied Radiation and Isotopes

0969-80

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/apradiso

Designing of an online system for radiocaesium measurements in themarine environment

M. Ahmadi a,n, P. Mir Ahmadpour b, M. Rabbani c

a Development of Reactors and Accelerators Research School, Nuclear Science and Technology Institute, P.O. Box 14155-1339, Tehran, Iranb Department of Chemistry, Islamic Azad University of Arak, Arak, Iranc Department of Chemistry, Islamic Azad University North Tehran Branch, P.O. Box 1913674711, Tehran, Iran

a r t i c l e i n f o

Article history:

Received 28 July 2008

Received in revised form

31 October 2010

Accepted 23 November 2010Available online 17 December 2010

Keywords:

Designing

Online System

NaI (TI) detector

MCNP.4C

Cs-137

Radiocaesium

43/$ - see front matter & 2010 Elsevier Ltd. A

016/j.apradiso.2010.11.022

esponding author. Tel.: +982182064265; fax:

ail address: mdahmadi@aeoi.org.ir (M. Ahmad

a b s t r a c t

An online radiocaesium measuring system (ORMS) is designed to determine the environmental

radiocaesium contamination of sea water near the Bushehr Nuclear Power Plant (BNPP). The System

consists of a 400-liter, spherical stainless steel water tank, including three aluminum (0.5 mm) enclosure

NaI (TI) detectors, which each are located in 1201 phase angle with size (6�3 in.) to confirm each other.

Responses of detectors to gamma radiations were calculated, using Monte Carlo code MCNP.4C. Moreover,

delay time of each response and the dead time of the whole system to be considered due to designing

decisions were calculated.

& 2010 Elsevier Ltd. All rights reserved.

1. Introduction

In situ gamma-ray spectrometry has been used widely in thepast years for monitoring natural and artificial radiations in themarine environments (Jones, 2001). The systems most commonlyused for gamma radiation spectroscopy in sea water, are based onNaI(Tl) detectors that are characterized by high-detection effi-ciency and low cost. However, they have the disadvantages ofrelatively poor energy resolution and high background mainlyoriginating from the Compton Effect (Vlastou et al., 2006). In themarine environment, natural radionuclides mainly result fromthe weathering and recycling of terrestrial minerals and rocks,where their distributions depend on their physical, chemical andgeological properties (Satyajit et al., 2000). The artificial radio-activity is extremely low compared to the concentration of naturalradionuclides. 137Cs is of special interest among the artificialradionuclides, because it is a long-lived radionuclide (30.05 years)used as a radionuclide tracer in sea water and constituents. 137Cshas the greatest radiological significance in the marine environ-ment (Papucci and Delfanti, 1999; Volpe et al., 2002; Delfanti et al.,2004). It persists in the environment from fallout from Chernobyl,weapon tests and nuclear power and processing facility discharges;

ll rights reserved.

+982182064222.

i).

it is transported over long distances by water currents andcontributes towards radioactive contamination of the marinefood chain (Aakenes, 1995; Wedekind et al., 1999; Van Put et al.,2004).

In order to analyze the spectra obtained from low-level envir-onmental radioactivity and having more reliable results, MonteCarlo simulations with the MCNP.4C code were used. Monte Carlosimulations have been extensively used during the last few years inorder to investigate the gamma-ray detector response features andefficiency calibration for different geometries, to validate newdesign and improve performances of new detectors useful fordifferent applications. (Vojtyla, 2001; Crespin et al., 2004; Vlastouet al., 2006). In the present work of designing Online RadiocaesiumMeasuring System MCNP.4C code was used to obtain optimumvolume and geometrical shape of water tank plus detector;applicable diameters.

2. MCNP code

One of the most commonly used radiation transport codes isMCNP. The interaction of nuclear particles with materials simula-tion is performed on a digital computer because the number oftrials necessary to adequately describe the phenomenon is usuallyquite large. The statical sampling process is based on the selectionof random numbers. It consists actually of the following each of

M. Ahmadi et al. / Applied Radiation and Isotopes 69 (2011) 1079–10831080

many particles from a source throughout its life to its death in someterminal categories (absorption, escape, etc.). The probabilitydistribution is randomly sampled using transport data to deter-mine the outcome at each step of its life. The desired Full Width atHalf Maximum (FWHM) is specified as follows (Heath, 1964):

FWHM¼ aþbðEþcE2Þð1=2Þ

ð1Þ

where E is the gamma-ray energy in MeV and a, b and c are user-provided constants. The FWHM is related to the Gaussian standarddeviation, s, by (MCNP, 2003)

FWHM¼ 2:35s: ð2Þ

The detector resolution would be specified in terms of standarddeviation of a Gaussian distribution. The standard deviation isgiven by the power-law form (Gardner and Sood, 2004):

s¼ dEe ð3Þ

where s is the standard deviation in MeV, E is the gamma-rayenergy in MeV and d and e are empirical constants. The broadenedspectrum is obtained by spreading the detector response functionmodel with a Gaussian distribution that has this form of thestandard deviation (Metwally1 et al., 2004). In this paper, MCNPcode was applied to obtain optimum geometrical shapes of water

Fig. 1. ORMS with three NaI detectors in 1201 phase angle equipments. (1—clar-

ification system to avoid suspended particles and sludge, 2—centrifugal pump, 3A

and 3B—filters, 4—flow meter, 5—water container (stainless steel tank) and 6A, B,

C—detectors and accessories).

Table 2Calculated result about (2 in.�2 in., 5.08 cm long and diameter) NaI detector, where r

receiving flux, which are calculated by multiplying a correction factor to software outputs

use A1 in (0.5, 1, 1.5, 2, 3, 4 and 5 mm).

Rad Vol(L) Fluxes

T-0.5 T-1 T1.5

46 407.5134 3.91E�02 3.89E�02 3.81E�02

49 492.5571 2.91E�02 2.90E�02 2.85E�02

52 588.6788 2.20E�02 2.19E�02 2.14E�02

55 696.5567 1.74E�02 1.69E�02 1.69E�02

58 816.8689 1.48E�02 1.43E�02 1.39E�02

62 997.7999 1.02E�02 1.06E�02 1.04E�02

65 1149.763 9.41E�03 9.05E�03 8.68E�03

Table 1Some characteristic of BGO, NaI and HPGE detectors.

Characteristics NaI BGO HPGe

Resolution Poor Poor Excellent

Efficiency Excellent Excellent Poor

Maintenance Excellent Excellent Poor

Cost Excellent Excellent Poor

Radiation resistance Excellent Excellent Poor

tank and volumes and the size of the detector. Simulation andsoftware calculations were performed using NaI (TI) crystalsof various dimensions (2�2, 3�3 and 6�3 in.). These dataare critical for the design of the online radiocaesium measuringsystem.

3. Detection system description

This system consists of three (6�3in., 15.24 cm long and7.62 cm diameter) NaI (Tl) scintillation crystals in 1201 phaseangle located in the middle of a stainless steel, 400-liter sphericalwater container. The whole provides high efficiency, which permitsdetection of low levels of gamma emitting radiocaesium present inthe water (Fig. 1).The detector is adopted with suitable photo-multiplier (PMT), preamplifier and power supply, as well aselectronics for data acquisition and data transmission. The wholeelectronic devices have been designed in such a way to beintergraded in the aluminum housing 0.5 mm thickness of thedetector system. The operating temperature varies from +10 to+60 1C and the gain shifts are automatically adopted with athermistor. Outside the system a watertight cylindrical enclosurehas been designed, which houses the above-mentioned NaI systemtogether with the appropriate electronics (Tsabaris, 2008).Thematerial of the enclosure was suggested according to the minimumgamma-ray absorption through the enclosure and to the pressuretolerance. 400-liter spherical water tank has a staggered inlet andoutlet to promote water mixing in the water container and thegeometry was designed in such a way that maximum detectorefficiency is obtained. The liquid handling system consists of scrubsand clarification system, centrifugal pump, filters, flow controlvalves, a flow meter and fresh water purge controllers.

3.1. Detection system selection criteria

Detector selection depends on the efficiency of peak resolution,material properties, hygrocopicity and cost. Some characteristics ofNaI (TL), BGO and HPGe detectors are illustrated in Table 1 (Proctoret al., 1999). NaI (Tl) is recommended for all nuclide identificationsbecause of high efficiency, being rugged to thermal and mechanicalshock, relatively low cost and availability in wide variety of sizes.The highlight output of NaI (Tl) results in a small contribution dueto photoelectron statistics in the measured resolution. The energyresolution of NaI detectors is weakly affected by the quality ofcrystals.

3.2. System simulation

MCNP.4C was used to model a simple geometry of NaI detectorsconsisting of 2�2, 3�3 and 6�3 in. in a water container.

ad column stands for radius in centimeter and fluxes column stands for detector

of each run. Factor T below the fluxes represent enclosure thickness, which choose to

T-2 T-3 T-4 T-5

3.77E�02 3.63E�02 3.53E�02 3.40E�02

2.83E�02 2.75E�02 2.75E�02 2.70E�02

2.12E�02 2.09E�02 2.09E�02 2.07E�02

1.67E�02 1.62E�02 1.61E�02 1.65E�02

1.40E�02 1.33E�02 1.29E�02 1.28E�02

1.03E�02 9.59E�02 9.51E�02 9.78E�02

8.68E�03 8.51E�03 8.18E�03 8.24E�03

Flux vs.V

Flux

(CPS

)

0.00E+00

2.00E-03

4.00E-03

6.00E-03

8.00E-03

1.00E-02

1.20E-02

1.40E-02

1.60E-02

1.80E-02

0v (t)

200 400 600 800 1000 1200 1400

Fig. 2. ORMS in 2�2 in. NaI detector.

Table 4Calculated result about (6 in.�3 in., 15.24 cm in height and 7.62 cm in diameter).

Fluxes

Rad Vol (L) T-0.5 T-1 T-1.5 T-2 T-3 T-4 T-5

46 407.5134 1.60E�01 1.58E�01 1.56E�01 1.54E�01 1.50E�01 1.47E�01 1.44E�01

49 492.5571 1.44E�01 1.42E�01 1.40E�01 1.38E�01 1.35E�01 1.31E�01 1.29E�01

52 588.6788 8.19E�02 8.14E�02 8.06E�02 8.03E�02 7.82E�02 7.67E�02 7.50E�02

55 696.5567 1.16E�01 1.15E�01 1.14E�01 1.12E�01 1.09E�01 1.08E�01 1.07E�01

58 816.8689 1.10E�01 1.06E�01 1.08E�01 1.03E�01 1.01E�02 9.85E�02 9.68E�02

62 997.7999 1.04E�01 1.02E�01 1.01E�01 9.82E�02 9.82E�02 9.61E�02 9.39E�02

65 1149.763 9.69E�02 9.40E�02 9.38E�02 9.12E�02 8.99E�02 8.80E�02 8.70E�02

Table 3Calculated result about (3 in.�3 in., 7.62 cm diameter and height).

Fluxes

Rad Vol(L) T-0.5 T-1 T-1.5 T-2 T-3 T-4 T-5

46 407.5134 1.18E�01 1.16E�01 1.15E�01 1.14E�01 1.11E�01 1.10E�01 1.08E�01

49 492.5571 8.94E�02 8.89E�02 8.71E�02 8.58E�02 8.54E�02 8.36E�02 8.15E�02

52 588.6788 7.11E�02 6.97E�02 6.84E�02 6.75E�02 6.71E�02 6.56E�02 6.51E�02

55 696.5567 5.60E�02 5.41E�02 5.31E�02 5.23E�02 5.15E�02 5.18E�02 5.11E�02

58 816.8689 4.66E�02 4.55E�02 4.61E�02 4.52E�02 4.29E�02 4.18E�02 4.13E�02

62 997.7999 3.56E�02 3.48E�02 3.42E�02 3.37E�02 3.25E�02 3.16E�02 3.02E�02

65 1149.763 2.98E�02 2.86E�02 2.88E�02 2.78E�02 2.70E�02 2.68E�02 2.63E�02

)

Flux vs.V

5.00E-02

6.00E-02

M. Ahmadi et al. / Applied Radiation and Isotopes 69 (2011) 1079–1083 1081

Volume of sea water samples from different depths and sedimentswere analyzed for 137Cs activity concentrations using HpGe and NaIcounting systems. Not only the same volumes of water wereaccumulated from different depths of sea, but also sedimentsand after pre-concentration of water analyzed samples of sea withHPGE and NaI counting systems. The results show that the amountof Cs-137 in water (1.7070.07 (mBq/L)) is lower than the LOQ(limit of quantification) of the system for liquid sample. Regardingbackground Busher sea water with (1.7170.36)�10�3 Bq/L isreported (marine radioactivity research for the ROPME, 2000). Forthe simulation, equivalent dose of source is converted to molefraction using the following relationship (Eq. (4)):

M¼N1:973� 10�16=mW ð4Þ

where M is the mole fraction of 137Cs, N is the Avogadro number,1.973�10�16 is converted coefficient of mBq to Ci, m is molecularmass and W is total mole fraction. Equipment of the detector isdefined according to real size and material. Other conditions suchas the radiocaesium source strength, its atomic and mass number,half life and 137Cs intensity and emitted particles, chemical andphysical characteristic of water are defined according to the realconditions of sea water.

Flux

(CPS

0.00E+00

1.00E-02

2.00E-02

3.00E-02

4.00E-02

0v (t)

200 400 600 800 1000 1200 1400

Fig. 3. ORMS in 3�3 in. NaI.

3.3. Calculation

Using (2�2, 3�3 and 6�3 in.) NaI (TI) detectors, tank simula-tion and software calculations were performed three times to avoidaccidental errors. The results are given in Tables 2–4. Consideringseven radius sizes that give 7 volumes and seven thicknesses (0.5, 1,1.5, 2, 3, 4 and 5 mm) corresponding to each detector enclosureresults in a heptads square matrix. Diagram of flux vs. volume forproposed detector is illustrated in Figs. 2–4. Standard deviationof each column related to its enclosure is calculated and themean standard deviation of fluxes for each system is 0.010281,2.98E�02, 0.119012 for 2�2 in., 3�3 in. and 6�3 in. simulatedsystems, respectively. Calculated uncertainties for each simulated

system are extracted using the software numerical results. Stan-dard deviations related to the NaI systems of 2�2 in., 3�3 in.and 6�3 in. are 0.011265, 0.01112 and 0.011411, respectively.

Flux vs.V

Flux

(CPS

)

0.00E+002.00E-024.00E-026.00E-028.00E-021.00E-011.20E-011.40E-011.60E-011.80E-01

0v (lit)

200 400 600 800 1000 1200 1400

Fig. 4. ORMS in 6�3 in. detector

Fig. 5. Calibration curve.

M. Ahmadi et al. / Applied Radiation and Isotopes 69 (2011) 1079–10831082

From the CANNBERRA company the LOQ is 0.2–0.3 Bq/L for theselected NaI (TI) 150 detector characteristics (Bronson, 2007).Obtained results for 2�2 in. NaI detector are not in systemconsiderations. Optimum volume for 3�3 in. NaI detector iscalculated to be approximately 600–700 L and corresponding fluxis about 4�10�2 to 2�10�2 CPS (count/s). By increasing detectorvolume (6�3 in.) the flux is decreased .Maximum flux is measuredin 46 cm radius ,407.5 L volume and enclosure thickness of 0.5 mmin 6�3 in. NaI detector. Maximum flux is calculated to be around1.6�10�1 CPS with 400 L volume compared to 4.8�10�2 with600–700 L for 3�3 in. in NaI detector, which leads to optimalchoice of 6�3 in. NaI detector. On the other hand, delay time wascalculated using the following equation:

t¼½Vm=vp ð5Þ

where Vm is tank volume (L) and vp is centrifugal pump velocity(L/s) and t is time lags (s). 600 gpm (Ludwig, 1984) was used forvp. Time lag in 3�3 in. NaI and 6�3 in. detectors are 7.926 and5.284 s, respectively. According to the results we can recommendto use 6�3 in. NaI detector for shorter time lags (5.284 s),which leads to more online ability and trust ability in alertingsituation.

3.4. Calibration

Due to calculated activity values, the system efficiency has to bepre-evaluated. To this purpose an optimized water tank (liter involume) was designed .The tank was filled with low-level naturalsea water and 137Cs (1 Bq/L) standard was added to the water as acalibration source. The calibration source was counted for 5000 s

for 2�2, 3�3 and 6�3 in. NaI detector in optimized water tank(Fig. 5).

4. Conclusion

In the present work an online measuring system was designedto determine the Cs-137 contamination in sea water near the BNPP.In order to design ORMS and produce more reliable results, MonteCarlo simulation with MCNP.4C code has been used. It wasobserved that increase in volume results to reduction in flux,which is caused by absorption and emission phenomena. Increasein enclosure thickness leads to reduction in the flux. According tothe results and BNPP reactors scram, reactor period in power rangeis less than 10 s (IAEA, 2003). It is recommended to use 400-literspherical stainless steel water container including three 6�3 in.aluminum enclosure (0.5 mm thickness) NaI detectors in 1201position to confirm each other and worked separately for shortertime lag, which leads to more online ability and trust ability inalerting situation. Alarm system can work effectively due toacceptable delay time .ORMS does not include complicatedpilot preparations and has economical and industrial justifications.The system was designed in order to operate in situ for the powerplant outlet water to the sea and in stationary mode in variousregions in the marine environment. Therefore it was shown that theuse of ORMS can result in more efficiency and accuracy andlower costs.

Acknowledgments

The author would like to thank the Auxiliary & Utility Systemsgroup of Nuclear Science and Technology Research Institute forcontinuous support of investigation.

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