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ORIGINAL ARTICLE
Year : 2018  |  Volume : 41  |  Issue : 2  |  Page : 66-69  

Identification of the contributors (Ag-110 m) for higher radiation field on primary heat transport system of Tarapur Atomic Power Station-3 and its impact on collective dose


1 Health Physics Unit, Tarapur Atomic Power Station-3 and 4, Tarapur Maharashtra Site, TAPP, Palghar, Maharashtra, India
2 Nuclear Power Corporation of India Limited, Head Quarter, Mumbai, Maharashtra, India

Date of Submission21-Jan-2018
Date of Decision21-Feb-2018
Date of Acceptance05-Mar-2018
Date of Web Publication24-Aug-2018

Correspondence Address:
Dr. Villas Mahadev Sonwalkar
Health Physics Unit, Tarapur Atomic Power Station -3and 4, Tarapur Maharashtra Site, TAPP, Palghar - 401 504, Maharashtra
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/rpe.RPE_5_18

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  Abstract 

Tarapur Atomic Power Station-3 is 540 MWe pressurized heavy water reactor in India, it achieved first criticality on May 5, 2006 and then operated at full power. The control of dose rates and collective dose of radiation worker is most important for the best performance of reactor. This article discusses the sudden rise in radiation levels on primary heat transport (PHT) pipelines and equipment. The radionuclide contributed to high radiation levels was Ag-110 m. Finding obtained during the route cause analysis used for the station operation and removal of Ag-110 m from PHT system using special type of resin (macroporous). The purification of the system using special type of resin has been enhanced. Ag-110 m activity in the PHT fluid has been controlled. PHT pipelines and equipment shielded for exposure control. The postirradiation examination of steam generator (SG) manhole gasket showed the presence of sulfur. The PHT fluid sample analysis revealed the presence of oil content in PHT fluid which was ingressed from buffer tank and FT D2O tank in PHT system. Oil containing sulfur was responsible for erosion of silver from SG manhole gasket in the form of silver sulfide and its ultimate transport to reactor core and neutron activation of silver.

Keywords: Activation, dose rate, in-situ gamma spectrometry, HPGe detector, oil content, postirradiation examination, primary heat transport system, silver (Ag-110 m)


How to cite this article:
Sonwalkar VM, Mohanta S, Pal S K, Rajagopalan H, Venkataramana K. Identification of the contributors (Ag-110 m) for higher radiation field on primary heat transport system of Tarapur Atomic Power Station-3 and its impact on collective dose. Radiat Prot Environ 2018;41:66-9

How to cite this URL:
Sonwalkar VM, Mohanta S, Pal S K, Rajagopalan H, Venkataramana K. Identification of the contributors (Ag-110 m) for higher radiation field on primary heat transport system of Tarapur Atomic Power Station-3 and its impact on collective dose. Radiat Prot Environ [serial online] 2018 [cited 2018 Oct 19];41:66-9. Available from: http://www.rpe.org.in/text.asp?2018/41/2/66/239685


  Introduction Top


Tarapur Atomic Power Station Unit-3 and 4 (TAPS-3 and 4), the first Indian pressurized heavy water reactor (PHWR) of capacity 540 MWe, attained the first criticality on May 21, 2006, and March 6, 2005, respectively. TAPS-3 and 4 are under commercial operation since August 2006 and September 2005.

This article presents the behavior Ag-110m radiochemical species in the primary coolant of PHWR plants. Managing these pollutants must lead to limit primary heat transport system (PHT) walls “over contamination” to decrease the dose rates during the maintenance operations. In Indian PHWR, cobalt (Co-60), silver (Ag-110 m), and antimony (Sb-124) represent the major radiochemical pollutants which require a good knowledge of the different phenomena to ensure the lowest contamination risks. The stakes include control, removal of Ag-110 m radionuclide, and the optimization of collective and individual doses.[1]

Normal contamination of circuits in Indian PHWR plants is due to contaminated walls in contact with the primary coolant because of activated corrosion products and fission products. Usual contaminants come mostly from materials/coolant interactions, leading to soluble, particulate, colloidal products' transport and corrosion products' activation. The dose rates can increase in case of more important corrosion products' transport or of incidental pollution. High radiation level was observed on PHT system pipes and equipment of unit-3 during radiation survey. With an aim to identify and quantify the radionuclide for high radiation field in the reactor building (RB), in situ gamma-ray spectroscopy measurement was carried out on PHT pipes and equipment in an accessible area, fuelling machine (F/M), and PHT equipment in pump room (during plant shutdown) using 10% relative efficiency coaxial HPGe portable detector of Baltic Scientific Instrument make. The present article gives the details of this investigation


  Measurement and Data Analysis Top


During routine surveillance in unit-3, the radiation dose rate on PHT D2O lines at 104 m elevation of TAPS-3 was showed sharp increasing radiation dose rate in relatively short period of time during October 2012. A detailed radiation survey of PHT pipelines, F/M lines, equipment, and pump room during biannual shut down (BSD) in RB was carried out. The radiation dose rate on PHT lines at 104 m elevation increased from 1.0 to 10 mSv/h, on F/M machine catenaries from 0.05–0.1 to 1.5.0–2.0 mSv/h, and on bleed cooler outlet line from 1.0–1.5 to 2.5–4.0 mSv/h. General radiation field in bleed cooler area was increased from 0.07 to 0.15 mSv/h in pump room and feed pump area from 0.01 to 0.03 mSv/h at 104 m elevation.

Specific program was set up at TAPS#3 to optimize diagnosis, surveillance, prevention, and remedies to keep doses ALARA during preventive maintenance and break down maintenance.

In situ gamma spectroscopy was carried out at selected location and equipment on PHT lines of TAPS-3. [Figure 1] shows the typical recorded gamma-ray spectrum of feed pump D2O lines at 104 m elevation. The spectrum was analyzed using the InterWinner multi-channel analyzer (MCA) program for qualitative analysis. Analysis showed the predominant radionuclide present in PHT fluid was Ag-110 m. It is formed by activation Ag -109 + 0n 1 = Ag -110m
Figure 1: In situ gamma spectroscopy of feed pump D2O lines

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The inner walls of PHT pipelines of TAPS-3 were contaminated with Ag-110 m which has a half-life 249.9 days. It is multiple gamma-emitting radionuclide and has around 60 number of gamma rays' energies ranging from 116 to 2004.7 KeV having effective energy 3009.1 KeV which is hard gamma emitter.[2]

PHT fluid (heavy water) samples were collected through sampling station located inside RB. Two milliliters of heavy water sample was taken in a aluminum planchette using micro pipette and heavy water was evaporated under infra red lamp. The planchette was counted using 30% relative efficiency coaxial-based MCA of EURISYS CANBERRA make HPGe. The system is calibrated for planchette geometry. Specific Ag-110 m silver activity in PHT fluid sample (from January 01, 2011, to March 31, 2014) of unit 3 is given in [Table 1]. The spectrum recorded on PHT lines and equipment during this period revealed Ag-110 m activity got deposited on inner walls of PHT lines and equipment though the PHT fluid sample did not show any presence of Ag-110 m radionuclide activity. Before October 2012 and after July 2013, Ag-110 m activity in PHT fluid was below detectable level.
Table 1: Specific Ag-110 m activity in primary heat transport fluid sample

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Various data collected from unit startup till 2014 were analyzed. Ag-110 m activity was observed in PHT crud filter from July 12 onward. Activity range was 4 MBq/g to 12 MBq/g. Before July 2012, Ag-110 m activity was <0.4 MBq/gm. This reveals that Ag-110 m activity started coming in PHT fluid from July 12 onward. In comparison to other location inside TAPS-3 RB, the observed dose rate was higher on F/M circuit. PHT feed lines and bleed cooler out let line due to deposition of Ag-110 m on inner surface of pipe lines. PHT IX columns removed during this period showed high radiation field, i.e. 1–2 Sv/h which is 20 times higher than normally removed PHT IX columns. Chemistry parameters such as pH value and iodine (I-131) activity in both units of PHT water were in comparable. There was no significant change in these parameters. In situ gamma spectroscopy of PHT lines and equipment showed the relative contribution of Ag-110 m in radiation field has varied from 35% to 88%.

Gamma spectroscopy of swipe samples of components removed during maintenance showed the Ag-110 m as predominant radionuclide. Relative contribution of Ag-110 m in radiation field on swipe has varied from 50% to 90%. The silver gets deposited at areas of low pressure and temperature circuits. Ag-110 m contributed around 30%–40% in collective dose during normal operation and BSD maintenance activities.

For identification of Ag-110 m, detailed analysis of above data was carried. Detailed discussion with various groups and study of documents from various sources were carried. Reviewed various activities carried out before and after the event. Components used in PHT system which have silver in them and suggested the mechanism for silver removal from PHT system.

The high radiation field due to Ag-110 m on PHT piping and equipment which resulted in high-dose (P-Sv) consumption during unit outage (BSD) and maintenance job during normal operation. For establishing the root cause of PHT system over contamination due to Ag-110 m and to evolve the remedial measures for control of radiation dose. Visual inspection and scrutiny of primary cooling pump (PCP) seals, Internal relief valve (IRV) springs and steam generator (SG) manhole gasket (silver gasket) removed from PHT circuit were carried out.[3] The components of PCP seal, carbon bearing of PCP, mechanical seal of shut down cooling pump, and part of SG manhole silver gasket were sent to postirradiation examination (PIE) division, BARC for analysis.

The scanning electron microscopy – in the energy dispersive spectroscope, it is observed that the carbon seal does not possess any silver, but in SG silver gasket, the surface was having sulfur content up to 1.5 weight percentage which could have formed silver sulfide and went into PHT system. [Figure 2] shows the porous region of silver gasket at high magnification and [Figure 3] shows energy dispersive spectroscope of SG silver gasket.[4]
Figure 2: Porous region of silver gasket at high magnification

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Figure 3: Energy dispersive spectroscope of steam generator gasket

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The PHT fluid sample analyzed for its constituents. The result revealed the presence of oil content in PHT fluid which was ingresed from buffer tank and Fuel Transfer D2O tank in PHT system. Oil dripping from fuel handling (FH) system components found its way into the buffer tank containing D2O collected from the FH system. Later on, the ingress of oil into the tank was prevented by suitably modifying the Buffer tank. Oil containing sulfur was responsible for the erosion of silver from SG manhole gasket in the form of silver sulfide and its ultimate transport to reactor core and neutron activation of silver.


  Preventive Actions Top


  1. FT D2O tank (TK-1) sampled for total oil content (TOC) before pumping out to the PHT system
  2. High radiation area/equipment are regularly monitored and gamma spectroscopy is carried out for the purpose of trending
  3. Shielding of equipment/PHT pipelines for exposure control are done
  4. During outages, jobs carried on the equipment at low pressure and low temperature are carefully planned and work practice reviewed
  5. Review and trending of Ag-110 m is done continuously
  6. Any change in chemistry parameters analyzed in detail.



  Conclusions Top


  1. The source of silver in the PHT System of Unit-3 is from SG silver gasket. This is due to the oil ingress from FH system to the PHT which contains sulfur
  2. Macroporous resin in the purification circuit has resulted further reduction in release of silver
  3. Solubility of Ag-110 m reduces with a decrease in temperature; due to this, it gets stick to inner walls of pipelines at a low temperature zone
  4. Adherence to radiation protection procedures, administrative control, shielding of equipment/pipelines, and decontamination result in controlling the collective dose during planned shutdown, biannual shutdown, and normal operation.


Acknowledgments

The authors gratefully acknowledge Shri R. G. Godbole, CS, TAPS-3 and 4, Shri A. M. Deshnavi, SD, and TAPS-3 and 4 management for providing necessary support to carry out the work in the preparation of this article. Authors are indebted to the engineers of TAPS-3 and 4 who provided constant guidelines in understanding the system. Authors are very thankful to all PIE Division Staff, BARC, for their cooperation. The authors also greatly appreciate the active cooperation and assistance of health physics colleagues.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
  References Top

1.
Impact of Main Radiological Pollutants on Contamination Risks (ALARA), Optimization of Physico Chemical Environment and Retention Technics during Operation and Shutdown” by A Rocher, Electricite de France/GPR, Portoroz Workshop-2003.  Back to cited text no. 1
    
2.
Table of Isotopes CD-ROM Eighth Edition: 1998 Update by Richard B. Firestone, S.Y. Frank Chu, CD-ROM Editor of Lawrence Berkeley, National Laboratory, University of California.  Back to cited text no. 2
    
3.
Design Manual on Steam Generator (SG) Primary Manhole Cover Gasket Assembly for TAPS-3&4/33003/2657/DD Part 1503.  Back to cited text no. 3
    
4.
Report of PIED/SA/2014/1161 Analysis of Mechanical seal and Silver Gasket dated 12/06/2014, Post Irradiation Examination Division, Bhabha Atomic Research Centre Mumbai.  Back to cited text no. 4
    


    Figures

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

  [Table 1]



 

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Abstract
Introduction
Measurement and ...
Preventive Actions
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