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Year : 2010  |  Volume : 33  |  Issue : 3  |  Page : 123-127  

Development of a laundry monitor for contamination control

1 Health Physics Section (Directorate of KM & HSE, NPCIL), Kaiga Generating Station-3&4, Nuclear Power Corporation of India Limited, Uttar Kannada, Karwar, Karnataka, India
2 Pla Electro Appliances Pvt. Ltd., Mumbai, India

Date of Web Publication22-Oct-2011

Correspondence Address:
Vishwanath P Singh
Health Physics Section (Directorate of KM & HSE, NPCIL), Kaiga Generating Station-3&4, Nuclear Power Corporation of India Limited, Uttar Kannada, Karwar, Karnataka
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Source of Support: None, Conflict of Interest: None

Rights and PermissionsRights and Permissions

The radiation dose due to contamination can be avoided / minimized as a result of good practices, adherence to Radiation Protection Procedures and controlling by monitoring level of contamination of the personnel, in areas, on equipments and PPEs. Radioactive contamination on the materials, human body or other undesirable places is extremely harmful to the personnel at Nuclear Power Plants. Spread and cross contamination of radioactivity from the controlled areas is a very complicated problem for power reactor plant management. Protective cloths and PPEs used by workers during normal operation and maintenance as well as bieannual outage of the power plant are found contaminated by 60 Co, 90 Sr, 124 Sb and 137 Cs mainly. The contamination control and proper monitoring is a key of radiation protection to avoid superfluous local exposure which may result in local body radiation effects. A economical, simple to use and self contained new developed Laundry Monitor having 12 trays, each containing two energy compensated GM detectors with lead shielding has been installed at Kaiga Generating Station-3&4 with average monitor efficiency 0.23% (for 90 Sr/ 90 Yt plate source), LLD and MDA 7.97 cps and 34.65 Bq respectively. The operational experience of user-friendly Laundry Monitor provided very good results for contamination measurement and useful in controlling the spread of contamination.

Keywords: Contamination, laundry monitor, DWL, PPE, 60 Co and 137 Cs

How to cite this article:
Singh VP, Managanvi S S, Shah TN. Development of a laundry monitor for contamination control. Radiat Prot Environ 2010;33:123-7

How to cite this URL:
Singh VP, Managanvi S S, Shah TN. Development of a laundry monitor for contamination control. Radiat Prot Environ [serial online] 2010 [cited 2022 Jan 17];33:123-7. Available from: https://www.rpe.org.in/text.asp?2010/33/3/123/86278

  1. Introduction Top

The Kaiga site is PHWR 4x220 MWe, a Unit of Nuclear Power Corporation of India Limited is engaged in production of the electricity by the nuclear fission reactions of Uranium inside the reactor core. The Kaiga Generating Station-3&4 is a twin power reactor station for electricity production. The nuclear fissions of the Uranium fuel with the neutrons produce energy in the form of kinetic energy of fission products with various types of ionizing radiations inside the reactor core. The wide spectrums of mass number of radio-nuclides are produced as a result of fission and activation reactions in the core, out of which some are short and others are long-lived. The design of the nuclear power plant (NPP) is such that the nuclides are contained inside the pipelines, nuclear systems and does not come out during normal operation and may comes out during the maintenance expected. As good practices the workers engaged in operation and maintenance of the plant uses the personnel protective equipments (PPEs) to avoid self and as well contamination.

The ALARA principle for the contamination control is one of the major areas for protection of the occupational workers and member of public can be achieved by good practices, lesson learned, zoning philosophy, change room procedures, rubber change procedures, inter-zonal monitoring and follow of the radiation protection procedures (RPP). Health Physics professional also assist in controlling the contamination by proper monitoring of the working areas for airborne activities and the radiological survey of the equipments and PPEs. The gamma dose rate characterization of equipments and PPEs by portable multichannel analyser and gross gamma survey will help in proactive actions for dose and contamination control.

Among many nuclides, the long-lived 90 Sr, 134+137 Cs, 140 Ba, 131 I and 60 Co etc. (Severa and Jaromír Bár, 1991) are very serious for the contamination of materials, human body and protective cloth compared to short-lived. These nuclides are harmful because of their long half life as well as very high energy particle emitters. The contamination may occur during normal operation or due to spillage of primary heat transport (PHT) or moderator water by which the volatile nuclides (Cs, Iodine) may contaminate the working areas by suspended radioactive particulates and/or aerosols by settling slowly on the roof, walls, equipments and other areas. The PHT system water is having very large amount of radioactivity compared to moderator system so that spillage of PHT water is more potential for contamination.

Also, during bieannual shut-down (BSD), the maintenance jobs for ISI of steam generator, heat exchanger (HX) systems, coolant channels, feeders, and cleaning of strainers, handling of the spent ion-exchange regins are the major contributors for contamination of cloths PPEs. During BSD, the major source of contamination in steam generator for long-lived were 137 Cs & 60 Co (30%) & short-lived 95 Zr, 95 Nb, 99 Mo, 106 Ru, 122+124 Sb and 141 Ce whereas in moderator HX long-lived were 60 Co (40-45%) and short-lived 51 Cr, 95 Nb, 59 Fe, 99 Mo and 124 Sb observed (Internal Report, 2007). The radionuclide present in the systems may possibly contaminate the personnel clothing of the workers involved in maintenance also. The equipments of active laundry facility for decontamination of the laundries is also found highly contaminated by 60 Co (80-90%), 95 Nb (3-4%), 95 Zr (1-2%) and 125 Sb (1-2%) and 137 Cs (1-2%) etc deposited on in it's equipments (Ramesh et al., 2006). Contamination of the decontaminating laundry system showed the existence of the contamination of PPEs.

The contamination of equipment and PPEs may lead to spread and cross contamination with personnel cloths, material or equipments etc. Significant part of external dose received by personnel can be contributed by contamination PPEs and subsequently the Laundry workers. In order to eliminate/ minimize the unnecessary component of external dose, cross contamination, PPEs must be free from contamination within the derived working level (DWL) stipulated by AERB. As a good practice the level of contamination of each cloth is ensured by frisker before wearing or thrown out from rewashing. The allowable level of contamination of Laundry monitor requires the predefined monitor's alarm setting based on AERB DWL guideline (DWL=6Bqcm -2 ) (Safety Manual, 2005).

  2. Design Features Top

Laundry Monitor is an economical, simple to use, stand alone instrument for measurement of radioactive contamination level on protective clothing shown in [Figure 1] by block diagram and completely in [Figure 2]. The electronic circuit block diagram of typical detector tray is shown in the [Figure 3] and top view of the monitor with detector tubes, mesh along with shielding is shown in [Figure 4]. The Laundry monitor (model PLM-2) is completely self contained, mains operated unit is made of detector table (182×122cm 2 ) consisting of 12 trays (52×22cm 2 ) and each tray contains two GM detectors shown in [Figure 5]. All the trays are surrounded by 5 mm thick lead sheet from side and on bottom for shielding from the background radiation. The monitor is continuously measures the background radiation level and subtracts it from contamination level of activity if any.
Figure 1: Block diagram of laundry monitor

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Figure 2: Contamination check by laundry monitor

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Figure 3: Block diagram of typical detector tray

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Figure 4: Top view of laundry monitor

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Figure 5: Typical single tray of laundry monitor

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The energy compensated for gamma radiation, GM tube(s) are halogen quenched thin walled with overall length 280mm and diameter 25 mm and operating voltage: 600V with plateau voltage of 200V. The monitor facilitates with count rate alarm (audio and visual) in cps for integral counts of all the tubes. All the output from the 12 trays is mathematically added to check the contamination level of the cloth. The LED flashers are arranged in a graphical manner to show the radioactive material in vicinity of a particular tray by increasing the output of GM tubes to identify of contaminated part of cloth. The monitor is also having provision for sensing the presence of laundry by light operated sensors in tray no. 6 and 7. The cloth and detectors are separated by plastic sheets with iron mesh to avoid cross contamination of GM tubes.

  3. Calibration of Monitor Top

The alarm setting of the laundry monitor is the limit of level of the radioactivity above which, PPE is not permitted to wear by occupational workers. The alarm for radioactive contamination on cloth provides information for decontamination and proactive approach of investigation of occurrence incident or spread of contamination. The alarm is set by voltage corresponding to permitted radioactivity level (cps) on the cloths (PPEs). The alarm of the monitor is set after considering the background radiation.

The natural radiation background count and count rate near by the Laundry Monitor area are 156±12.25 and 15.6±0.48 cps for 10 sec with 95% confident interval. But the introduction of lead shielding around tray reduces average background count rate up to1.28±0.26 cps for 10 sec.

The calibration of the monitor requires the external source in the form of the realistic cloth. The size of single source will be very large and practically not possible for preparation and handling. Hence the small plate source of 90 Sr/ 90 Y of uniform specific activity 3 Bqcm -2 was prepared on Stainless Steel (SS) plate (3×6 cm 2 ) for calibration of the laundry monitor. The total area of the monitor is 182×122 cm 2 area of entire trays, so that total activity for the monitor is 66612 Bq. The monitor counting time for background radiation and plate source was taken same i.e. 10 sec. The net counts and efficiency of each tray are given in [Table 1] and [Figure 5] respectively, varies 0.30-0.50% (ave. 0.35%). Taking approximately area of protective cloth 12000 cm 2 for radioactivity measurement by the monitor, the average efficiency of the monitor is calculated.
Table 1: Tray-wise response of the laundry monitor

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The tray-wise source response of the individual tray is given in the [Table 1]. The net count of the entire tray with source of activity fully covered for all the trays is 154.9 cps. The efficiency of the monitor decreases by going from detector so that the distance between detector and mesh for cloth is optimized considering the design constraints with maximum possible efficiency. [Figure 6] The efficiency of the monitor is:

Figure 6: Efficiency of trays of the laundry monitor

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The alarm (cps) of the laundry monitor is calculated by well known efficiency equation as:

  4. Minimum Detectable Activity By Laundry Monitor Top

The minimum detectable activity (MDA) of the laundry monitor is the limit of activity below which monitor cannot detect. The MDA of monitor depend on natural background level where monitor is located for contamination measurement. The background radiation of that area is kept low as low as possible for measurement of lowest contamination level. This MDA of monitor controls alarm setting of the monitor as well as indication of increase in natural background of monitor.

According to famous "Currie equation, 1968" of lower limit of detection (Knoll, 1999) is:

This LLD minimum counts needed from source to ensure a false-negative rate no larger than 5% when the system is operated with critical level of L c =2.326 σNB or ensures a false-positive rate of no longer than 5%.The minimum detectable activity (Cember, 2010) is:

Where K is counting efficiency and t (=10sec) is sample counting time and background counting. The LLD and MDA of the Laundry Monitor are 7.97 cps and 34.65 Bq.

  5. Operation Top

Operational flow chart of monitor is shown in [Figure 7]. The operation is by turning "ON" mains in position for self test messages, laundry Monitor will check the internal memory, alarm activation, counts from each tray (Background).When all the tests are completed, laundry monitor will display message "Load Laundry". The monitor will sense presence of the cloth and "start counting" by pressing the start key will start counting for all 12 trays. After completion of counting the counts will be displayed one by one for the entire tray and then counts from all the trays are summed up along the background subtraction from the sum counts. If these net counts are higher than the alarm level set in the monitor audio and visual alarm turned ON "Laundry contaminated" and if these net counts are within the alarm level Laundry "Clean message" is displayed. On removal of the laundry, flasher LED will turn off and Laundry monitor is ready for new measurement of laundry.
Figure 7: Chart of monitor operation

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  6. Discussion and Conclusion Top

The laundry monitor installed is a specific and unique monitor to check the radioactive contamination of protective clothes and useful to prevent the cross contamination at station. Alarm is set for voltage corresponding to sum activity (cps) and background (cps). But monitor has provision for subtracting the background, so the alarm is set for 165.6 cps as whole monitor and 21.84 cps for individual tray based on the contamination limits. The efficiency of individual detector is very low still detectors are giving a very good response to detect the level of contamination. The operational experience of laundry monitor gives good results for contamination detection and useful in controlling the spread of contamination in operating island premise.

  7. Acknowledgements Top

The author V. P. Singh is thankful to Shri V. S. R. Krishna, Control Maintenance, KGS-3&4, for incorporation of suggestions during commissioning of the monitor. Special thanks to Shri Jagadish Valluri, SD, KGS-3&4, Shri J. P. Gupta, Site Director, Kaiga Site, Shri H. R. Bhat, CE(HP) and D.K. Goyal, ED (KM&HSE) for their valuable suggestions encouragements.

  8. References Top

  1. Cember H., Introduction to Health Physics, Third Edition, 407, 2010.
  2. Hurtgen C., Jeromeb S. and Woods M. (2000), Revisiting Currie - how low can you go?, Applied Radiation and Isotopes, Vol. 53, 45-50
  3. Internal Report (2007), Radiological Review of Unit-I, Biannual Shut-down Maintenance Activities, 20 June- 21 August, 2007.
  4. Knoll G.F. Radiation detection and measurement, Third Edition, 96, 1999.
  5. Ramesh R. et al. (2006), Source, Activity Contribution and Control Measures of Deposited Activity in Laundry Systems of KGS, OPENUPP, NRT-3, ISBN 81-8372-028-5.
  6. Safety Manual (2005), AERB/NF/SM/O-2 (Rev.4), Atomic Energy Regulatory Board.
  7. Severa J. and Jaromír Bár (1991), Handbook of Radioactive Contamination and Decontamination.


  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]

  [Table 1]


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  In this article
1. Introduction
2. Design Features
3. Calibration o...
4. Minimum Detec...
5. Operation
6. Discussion an...
7. Acknowledgements
8. References
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