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 Table of Contents 
ORIGINAL ARTICLE
Year : 2016  |  Volume : 39  |  Issue : 3  |  Page : 128-131  

Development of methodology for validation of efficiency of 85Kr monitoring system by portable HPGe spectrometer


Health Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai, Maharashtra, India

Date of Web Publication30-Nov-2016

Correspondence Address:
Pankaj Kumar
Health Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai - 400 085, Maharashtra
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-0464.194965

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  Abstract 

85Kr a fission product noble gas is released into environment through stack at reprocessing plants during dissolution of spent fuel. Dual GM tube based online monitoring system having efficiency 0.5% for 85Kr is used for quantification of 85 Kr activity released through stack. Efficiency calibration of online monitoring system requires use of standard 85Kr source which is not readily available. As per safety regulations periodic efficiency calibration of Krypton monitoring system is required to be done at least once a year. This paper presents an indigenously developed methodology for efficiency validation of krypton monitoring system using portable HPGe gamma spectrometer.

Keywords: 85Kr monitor, efficiency validation, portable HPGe spectrometer, reprocessing plant


How to cite this article:
Pandey J, Kumar P, Talole GB, Ansari AA, Gupta VK, Ganesh G, Tripathi R M. Development of methodology for validation of efficiency of 85Kr monitoring system by portable HPGe spectrometer. Radiat Prot Environ 2016;39:128-31

How to cite this URL:
Pandey J, Kumar P, Talole GB, Ansari AA, Gupta VK, Ganesh G, Tripathi R M. Development of methodology for validation of efficiency of 85Kr monitoring system by portable HPGe spectrometer. Radiat Prot Environ [serial online] 2016 [cited 2018 Jan 18];39:128-31. Available from: http://www.rpe.org.in/text.asp?2016/39/3/128/194965


  Introduction Top


85Kr is a fission product noble gas produced in nuclear fuel during irradiation in nuclear reactors. It has fission yield of 0.32% and decays to 85Rb by emitting average beta particle of 251.4 keV with Emax 687 keV (99.56%) and gamma photon of 514 keV (0.434%). Half life of 85Kr is 10.76 years and after generation it remains trapped in the fuel matrix within fuel clad. During reprocessing of spent nuclear fuel most of the 85Kr get released along with other off gas streams to the environment through high stack. Although release of 85Kr in environment has insignificant dose contribution to public due of its low gamma abundance, yet 85Kr release needs to be measured continuously as a regulatory requirement.

There are many techniques which are used for 85Kr measurement. However activated charcoal traps, beta scintillation monitoring at high pressure, online monitoring by internal gas proportional counting [1],[2],[3] and GM tube based monitoring system are most commonly used. At reprocessing plant, Tarapur, GM tube based monitoring system is used for quantification of 85Kr released through stack which consists of a stainless steel (SS) chamber in cuboidal shape having volume 6.27 L and internal dimensions 140 mm × 140 mm × 320 mm containing dual GM tube detectors (LND 719) housed coaxially. This chamber is provided with inlet and outlet nozzles for suction and discharge of air stream taken from stack. Twenty millimeter lead shield is provided around the SS chamber to minimize the interference from background radiation. The integral counts of both the GM detectors are recorded and data is communicated to Central Radiation Protection Console (CRPC) system in control room. This dual GM tube based 85Kr monitoring system has shown 0.5% efficiency for 85Kr which was determined using 3.7 ml glass ampoule having known activity of 370 KBq.

In a reprocessing plant two Krypton monitors are used in which one remains online and another is kept as stand by. As per safety regulations efficiency calibration of 85Kr monitoring system has to be carried out once in a year for which minimum two standard sources of 85Kr are required.[4],[5] But 85Kr standard source procurement is very tedious and difficult. In order to overcome these difficulties and to ensure periodic efficiency calibration of krypton monitoring system with standard sources, a method was indigenously developed which can validate the efficiency of 85Kr monitoring system using portable HPGe gamma spectrometer [6] and that methodology is presented in this paper.


  Materials and Methods Top


In this experiment three SS chambers named Chamber-1, Chamber-2 and Chamber-3 all identical in shape and sizes are used. Chamber-1 is installed in present system for online monitoring of 85Kr and it is a part of the system. Chamber-2 is used to prepare a standard geometry source by filling it with known activity of 85Kr and Chamber-3 is used to determine unknown 85Kr activity of stack air sample of reprocessing plant.

Efficiency calibration of Chamber-1 using standard source

Chamber-1 is an integral part of online krypton monitoring system for which efficiency was checked experimentally using standard 85Kr source [7] procured from Czech Republic (Czech Metrology Institure-Inspectorate for Ionizing Radiation). During experiment leak tightness of Chamber-1 was ensured and a glass ampoule of 3.7 ml containing standard 85Kr of known activity was broken inside it. Count rate was noted down and efficiency of 85Kr monitoring system was determined. In this experiment two glass ampoules of 85Kr source having activity 370 KBq and 310 KBq were used in two different sets of observations and average efficiency was worked out. Results are mentioned in [Table 1].
Table 1: Average efficiency of Chamber-1

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Preparation of standard geometry 85Kr source (Chamber-2)

A glass ampoule of 3.7 ml containing 85Kr source of activity 310 KBq was placed inside Chamber-2 and the chamber was sealed by welding. Leak tightness of the chamber was ensured by carrying out Dye Penetrant test. Glass ampoule was broken inside by shaking the chamber for uniform distribution of activity. As initial activity of 85Kr contained in the Chamber-2 is known to us therefore it can be used as standard geometry source.

Efficiency calibration of portable HPGe

Using Chamber-2 as 85Kr standard geometry source, efficiency calibration of portable HPGe gamma spectrometer (Falcon 5000) was carried out for 514 keV gamma energy. During the experiment portable HPGe was placed at a fixed distance from Chamber-2 and efficiency of detector was estimated using the formula:



Where, N is net counts under photo peak (region of interest), T is counting time in seconds, A is activity in Bq of 85Kr contained in Chamber-2 and AF is abundance factor (0.00434) for 85Kr gamma photon.

Estimated efficiency of portable HPGe for 514 keV gamma photon from 85Kr at a fixed distance from the Chamer-2 was worked out to be 0.0726%.

Measurement of 85Kr activities in Chamber-3

Chamber-3 was made with the provision of inlet and outlet nozzles for suction and discharge of stack air stream. Chamber-3 was connected in series with Chamber-1 between the discharge of Chamber-1 and suction of rotameter as shown in [Figure 1]. Same volume of stack air is passed through Chamber-1 and Chamber-3 with the help of rotameter adjustment. Portable HPGe was placed at same distance from Chamber-3 as it was placed during its efficiency estimation. Counts of HPGe spectrometer were recorded at regular interval of 300 s during chopping and dissolution of spent fuel when release of 85Kr is expected.
Figure 1: Schematic diagram of stack monitoring system with calibration set-up

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Total activity of 85Kr present in Chamber-3 was estimated by multiplying net counts under photo peak with derived value of activity multiplication factor 1057.34 Bq/count. The setup used for measurement of 85Kr activity of Chamber-3 is shown in [Figure 2].
Figure 2: Setup for Chamber-3 85Kr activity estimation by portable HPGe spectrometer

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  Results and Discussion Top


Determination of activity multiplication factor

Observed spectrum of 85Kr with standard geometry source (Chamber-2) is shown in [Figure 3]. This spectrum is used to derive activity multiplication factor by dividing total activity with net counts under the photo peak which was worked out to be 1057.34 Bq/count. This multiplication factor has been used to estimate the unknown activity of 85Kr activity present in Chamber-3.
Figure 3: Gamma spectrum of standard 85Kr at work place

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Validation of efficiency of 85Kr monitoring system

Chamber-1 and Chamber-3 are identical in all respects and same volume of stack air is passed through them, therefore total 85Kr activity present in both the chambers at any instant is considered to be equal. Ten observations were noted down for count rate from CRPC historian data of online 85Kr monitoring system and 85Kr activity present in Chamber-1 at each observation was estimated using 0.5% efficiency for 85Kr monitoring system. Simultaneously 85Kr activity present in Chamber-3 was also derived by HPGe spectrometer and both the activities were compared and presented in [Table 2]. Observed activities in Chamber-1 and Chamber-3 are found to be in good agreement with each other with a mean deviation of ± 8%. Hence this method validates the 0.5% efficiency of 85Kr monitoring system.
Table 2: Comparison of 85Kr activities in Chamber-1 and Chamber-3

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  Conclusions Top


  • Efficiency validation of 85Kr monitoring system can be done using calibrated portable HPGe gamma spectrometer
  • Assuming zero leakage from Chamber-2, it can be used as a standard geometry source which will ensure all time availability of 85Kr source at least for two half lives i.e., 21 years
  • This method of validation of efficiency can be performed online without taking outage of online 85Kr monitoring system.


Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
  References Top

1.
Desmond M, Glass RW, Chiles MM, Inman DJ, Watkin DC. β-scintillation monitor for krypton-85 at high pressures. Tennessee, U.S.A.: Oak Ridge National Laboratory; 1977.  Back to cited text no. 1
    
2.
Smith KJ, Murray M, Wong J, Sequeira S, Long SC, Rafferty B. Krypton-85 and other airborne radioactivity measurements throughout Ireland. Radiopro 2005;40:S457-63.  Back to cited text no. 2
    
3.
Seifert DA, Stuart RL. Calibration technique for counting krypton-85 in charcoal traps. Anal Chem 1968;40:2080.  Back to cited text no. 3
    
4.
AERB Safety Guide No. AERB/RF-RS/SG-2 Radioisotopes Handling Facilities; August, 2015.  Back to cited text no. 4
    
5.
IAEA Safety Report No. 64 Programmes and Systems for Source and Environmental Radiation Monitoring. 2010.  Back to cited text no. 5
    
6.
Goles RW. An automated krypton-85 γ-ray stack monitor. IEEE Trans Nucl Sci 1980;28(1);(PNL-SA-8642).  Back to cited text no. 6
    
7.
Phillips HC, Johansson L, Sephton JP. Standardization of Kr-85 monitor. Appl Radiat Isot 07/2010;68(7-8):1335-9. [DOI: 10.1016/2009.12.09].  Back to cited text no. 7
    


    Figures

  [Figure 1], [Figure 2], [Figure 3]
 
 
    Tables

  [Table 1], [Table 2]



 

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