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ORIGINAL ARTICLE
Year : 2014  |  Volume : 37  |  Issue : 3  |  Page : 176-178  

Development of waste drum monitoring system for plutonium estimation


Radiation Safety Systems Division, Bhabha Atomic Research Centre, Trombay, Mumbai, Maharashtra, India

Date of Web Publication10-Apr-2015

Correspondence Address:
Vaishali M Thakur
Radiation Safety Systems Division, Bhabha Atomic Research Centre, Trombay, Mumbai - 400 085, Maharashtra
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-0464.154881

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  Abstract 

The screening based waste drum monitor system using neutron detectors was developed and evaluated. Present paper describes the studies carried out on the system evaluation for Plutonium Estimation. The system uses three BF 3 neutron detectors with 3.8 cm HDPE moderator for two extreme detectors and with 7 cm HDPE moderator for middle detector arranged all three detectors in linear geometry. The middle detector shows less background and better sensitivity than of the two. Present system can detect 85 mg of plutonium at counting time of minimum1000 second or more. With changing the detectors geometry of the system to triangular geometry and replacing all the detectors with 7 cm moderator systems, performance as well as MDL can be improved.

Keywords: Boron trifluoride neutron detectors, calibration factors, linear geometry, moderator thickness, waste matrix


How to cite this article:
Thakur VM, Jain A, Sawant P, Khuspe R R, Suresh Kumar M K, Gopalkrishnan R K, Chaudhury P, Pradeep Kumar K S. Development of waste drum monitoring system for plutonium estimation. Radiat Prot Environ 2014;37:176-8

How to cite this URL:
Thakur VM, Jain A, Sawant P, Khuspe R R, Suresh Kumar M K, Gopalkrishnan R K, Chaudhury P, Pradeep Kumar K S. Development of waste drum monitoring system for plutonium estimation. Radiat Prot Environ [serial online] 2014 [cited 2020 May 30];37:176-8. Available from: http://www.rpe.org.in/text.asp?2014/37/3/176/154881


  Introduction Top


The limit for alpha emitting radioactive element in solid waste going out of a nuclear facility has been set to 4,000 Bq/g by the Atomic Energy Regulatory Board (AERB). [1] The stipulated limit for waste drums of 200 L capacity (88 cm tall and 60 cm diameter) is 87.7 mg for the disposal of plutonium in the form of alpha radioactive waste. A neutron measurement based monitoring system has been developed in Bhabha Atomic Research Centre having minimum detectable limit (MDL) of 85 mg for research reactor grade plutonium oxide. The system has been developed using three BF 3 neutron detectors. This paper describes the methodology as how the system can be used and standardized so that it can be used in the field of plutonium measurements.


  Materials and methods Top


The system has been developed using three 100 cm long and 5.2 cm diameter BF 3 detectors. The required preamplifier and the amplifier/discriminator electronics module have been developed indigenously and integrated with the detectors. Microcontroller (P80C552) based counting and processing electronics have been developed and was interfaced with notebook/desktop PC using RS-232 serial communication. The detectors were arranged in a linear geometry [2] as shown in [Figure 1]. Though, linear geometry is inferior to triangular geometry, the linear geometry is used for ease of operation.
Figure 1: Experimental setup

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The outer two detectors are covered with high-density poly-ethylene (HDPE) moderator of 3.78 cm and central detector is covered with ~ 7 cm thick HDPE moderator. [3] This variation in the thickness is used to accommodate the nonuniform moderation by the waste in the drum, which might results changes in the calibration factors for the plutonium neutron average energy of 2.2 MeV. The background counts measured in outer detectors are more than the background counts in the central detector are due to more thermal neutrons absorption in the thicker HDPE moderator.

Experimental setup

The experimental setup was installed, and background counts were monitored for the system. Two extreme detectors background counts were within 0.08-0.1 counts per second (CPS), whereas the middle detector background count was within 0.04-0.05 CPS. Since the middle detector is surrounded by 7 cm thick moderator it's background is reduced from 0.08 to 0.04 CPS.

The experiment was carried out using standard 20 plutonium oxide sources, each of 50 mg. The system calibration was carried out using the plutonium sources kept at 25 cm from the geometrical center of the system. To know variation in the counts due to positional variation of the source, drum is divided into three equal planes and in each plane source is moved at five different locations as shown in [Figure 2] to collect the data. The measurements were also taken by distributing 10 sources each of 50 mg in a drum filled with cotton as shown in [Figure 2].
Figure 2: Experimental set up to evaluate system response for 500 mg source at different positions inside the waste drum

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  Results and discussion Top


The calibration of the system was rechecked by keeping the sources (100-1000 mg) at 25 cm at the geometrical center of the system and found to be shown in [Figure 3]. The relation between background subtracted CPS (Y) and plutonium quantity of mg is shown in Eq. (1).
Figure 3: System calibration

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Y = 0.00095 * (Pu in mg) − 0.00653 (1)

Background of the system (summation of all detector background counts) is ~ 238 counts for 1,000 s. Minimum detectable plutonium gives counts 3*σ above the background where σ is √ 238 that correspond to 47 that is 0.047 CPS. Adding Y as 0.047 CPS in Eq. (1) gives 84 mg as MDL of the system for reactor-grade plutonium oxide. This corresponds to below AERB regulations as 87.7 mg inside the drum.

System response for 100 mg and 150 mg sources were studied due to non-availability of standard sources of 85 mg. The source was kept at the center of the waste drum filled with cotton and counts were noted for 1,000 s counting time. The system response was shown in [Table 1] and [Table 2] for 100 mg and 150 mg plutonium sources, respectively. The table gives the background counts and the counts with source including background counts.
Table 1. Recorded reading with 100 mg plutonium source with 1000 second counting time


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Table 2. Recorded reading with 150 mg plutonium source with 1000 second counting time


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The detector with 7 cm thick moderator shows better sensitivity to the 100 mg, but the detectors with 3.78 cm moderator shows less sensitivity to 100 mg. The average counts for 100 mg sources were within statistical variation of 90% confidence level, but for 150 mg system response is much above the statistical variation of 99% confidence level. Using the summation of three detectors counts, one can definitely detect presence of 100 mg plutonium as shown in [Figure 4].
Figure 4: Graphical representation of counts with 100 mg and 150 mg sources

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Variation of the counts due to the source position changes is as shown in [Figure 5].
Figure 5: Graphical representation of variation of counts positional variation of 500 mg source

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The source position near to the detectors and away from the detectors shows deviation of ± 30% w.r.t source at center position for the stationary drum. The rotating drum in the system will show <15% variation.


  Conclusions Top


The system installed at PP using BF 3 detectors for plutonium monitoring is working in the laboratory from 2010 onwards and in the PP from last 1-year.

  • The system has an MDL calculated 85 mg of plutonium at counting time of 1,000 s
  • With changing the detectors geometry of the system to triangular geometry, the systems performance can be improved
  • All the detectors are replaced with 7 cm moderator then the systems' MDL can be improved further
  • If the waste matrix in the waste drum is changed to gloves or wet cotton or more moderating material then the existing system might be able to apply the correct correction factor to estimate the plutonium in drum. The algorithm can be developed to estimate the calibration factor for different moderation waste matrix by removing the one of the 3.78 thick moderating sleeve. Then three detector counts are from bare detector, 3.78 cm moderator sleeve and 7 cm moderator sleeve. These three counts may lead to if the waste matrix is heavily moderator that is from wet tissue waste or having plastic material like gloves or dry cotton, tissue paper. These studies can be further carried out
  • The system cannot differentiate between the isotopic composition of plutonium and gives correct results only for known isotopic composition of plutonium. For the other compositions, MDL of the system will still reduce further.

  Acknowledgments Top


The authors are gratefully acknowledged the guidance and encouragement from Dr. D. N. Sharma, in this study. We thank our colleagues, Shri Satish Joshi and Ms. Yogita Mestry for their support and cooperation during the course of this development work. Services provided by our colleagues at research and scientific support department workshop in the fabrication of the mechanical assembly are thankfully acknowledged.

 
  References Top

1.
AERB Safety Guide. Classification of Radioactive Waste. Mumbai, India: AERB/SG/RW-1; 2005.  Back to cited text no. 1
    
2.
Thakur VM, Jain A, Anilkumar S, Babu DAR, Sharma DN. Feasibility study of BF3 neutron detectors for monitoring plutonium in waste packets, poster presentation in NUCAR; 2011.  Back to cited text no. 2
    
3.
Thakur VM , Jain A, Sunil C, Kumar BK, Anilkumar S, Babu DAR, et al. Studies on optimization of Moderator thickness for BF 3 detectors used for monitoring of fissile material. Indian J Pure Appl Phys 2012;50:874-5.  Back to cited text no. 3
    


    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
 
 
    Tables

  [Table 1], [Table 2]



 

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Abstract
Introduction
Materials and me...
Results and disc...
Conclusions
Acknowledgments
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