|Year : 2011 | Volume
| Issue : 4 | Page : 242-245
Radiation exposure control by estimation of multiplication factors for online remote radiation monitoring systems at vitrification plant
Umesh V Deokar, VV Kulkarni, AR Khot, P Mathew, Kamlesh, RG Purohit, PK Sarkar
Health Physics Division, ORPRS, BARC Trombay, Mumbai, India
|Date of Web Publication||17-Jan-2013|
Umesh V Deokar
Health Physics Division, ORPRS, BARC Trombay, Mumbai
Source of Support: None, Conflict of Interest: None
Vitrification Plant is commissioned for vitrification of high-level liquid waste generated in Nuclear Fuel Cycle operations by using Joule Heated Ceramic Melter first time in India. Exposure control is a major concern in operating plant. Therefore, in addition to installed monitors, we have developed online remote radiation monitoring system to minimize number of entries in amber areas and to reduce the exposure to the surveyor and operator. This also helped in volume reduction of secondary waste. The reliability and accuracy of the online monitoring system is confirmed with actual measurements and by theoretical shielding calculations. The multiplication factors were estimated for remote online monitoring of Melter off Gas (MOG) filter, Hood filter, three exhaust filter banks, and overpack monitoring. This paper summarizes how the online remote monitoring system had helped in saving of 128.52 Person-mSv collective dose (14.28% of budgeted dose) and also there was 2.6 m 3 reduction in generation of Cat-I waste.
Keywords: Canister, Joule melter, melter off gas filter, online monitor, over pack, vitrification plant
|How to cite this article:|
Deokar UV, Kulkarni V V, Khot A R, Mathew P, Kamlesh, Purohit R G, Sarkar P K. Radiation exposure control by estimation of multiplication factors for online remote radiation monitoring systems at vitrification plant. Radiat Prot Environ 2011;34:242-5
|How to cite this URL:|
Deokar UV, Kulkarni V V, Khot A R, Mathew P, Kamlesh, Purohit R G, Sarkar P K. Radiation exposure control by estimation of multiplication factors for online remote radiation monitoring systems at vitrification plant. Radiat Prot Environ [serial online] 2011 [cited 2020 Apr 5];34:242-5. Available from: http://www.rpe.org.in/text.asp?2011/34/4/242/106097
| 1. Introduction|| |
All the installed monitors are connected to Centralized Radiation Protection Console (CRPC) in control room and in HP shift room to provide information continuously as well as on demand. In Vitrification Plant, 176 m 3 of high-level liquid waste was vitrified in 119 canisters. As the facility is new, constant HP surveillance is required during operation, to monitor the variation in radiation field on Melter off Gas (MOG) filters, Hood Filter, and exhaust filter banks. Also, overpack monitoring is required before interim storage. For monitoring, it required to make number of entries in amber areas of plants and in exhaust filter bank areas, which contributes to personal exposure and is time consuming. To minimize the number of entries in amber areas and to reduce exposure, we have developed online remote radiation monitoring system and multiplication factors were estimated by actual measurements and by shielding calculations based on Point Kernel method.  The radiological data, during Vitrification operation, is presented in [Table 1]. The work description and corresponding exposure/secondary waste saving at different stages of operation is given in [Table 2]. Because of remote online monitoring system, 128.52 Person-mSv (14.28%) of collective exposure was averted, which was represented in Pi, [Figure 1].
|Table 2: Work description and corresponding exposure/secondary waste saving|
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| 2. Online Remote Radiation Monitoring of Melter off Gas Filter|| |
During each Vitrification operation, there was built up of radiation field on MOG filter up to 350-450 mGy/h.  So, frequent radiation survey of MOG filter was required so that it would not exceed 500 mGy/h, which is the disposal limit in RCC trenches. Therefore, online monitor is installed on the shielding provided for MOG filter as shown in [Figure 2]. The online monitor gives continuous radiological status of filter in Control Room as well as in shift HP room. The multiplication factor of 200 was evaluated between contact radiation level on MOG and radiation level on shielding of MOG filter. This multiplication factor of 200 was confirmed by doing actual radiation surveys of bare MOG filter and also by theoretical shielding calculations. The data of correlation study carried out for estimation of multiplication factor for monitoring of MOG filter are presented in [Table 3] and [Figure 1]. Online monitor helped in changing over the MOG filter before it crosses the limit of radiation level of 2.0 mGy/h on shielding. Also, online monitoring system has reduced many entries in off gas room, which finally resulted in reduction in collective dose. Afterwards, to slow down the built up of radiation field on MOG filter, washable fiber glass filter assembly was installed before MOG filter, which resulted in reduction of changing frequency of MOG filter. The online remote monitoring of Hood filter was also done in similar way like MOG filter.
|Table 3: The radiation level data of correlation study of melter off gas filter|
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| 3. Online Remote Radiation Monitoring of Exhaust Filter Banks|| |
Due to open pouring in canister in cell atmosphere, the radiation level of exhaust filter bank increases up to 1 mGy/h after each pouring operation, so frequent radiation survey of filter bank is required to measure the rise in radiation level on the filter. The limit for radiation level on filter bank was restricted to 2 mGy/h, so that they can be qualified as Cat-I waste for disposed in earthen trenches. For survey, we had to enter inside filter bank area with all necessary protective wears which results in personal exposure. Therefore, three online monitors were installed on duct of respective filter banks at a distance of 1.5 m from the center of filter banks as shown in [Figure 3]. The online monitor will monitor the radiation level on duct and which is correlated with respect to the contact radiation level on the filter bank. The multiplication factor of 25 is established between the radiation level on duct and radiation level on exhaust filter bank by actual radiation measurements on the contact of filter banks and by theoretical shielding calculations. The data of correlation study carried out for estimation of multiplication factor for monitoring of Exhaust Filter are given in [Table 4] and [Figure 2]. This online monitoring of exhaust duct of filters gives continuous radiological status of filter banks. Thus, it facilitated the plant operator to change over to the other filter banks when radiation field on filter banks exceeds 2.0 mGy/h. To reduce radioactive load on exhaust filter banks during pouring operation, an enclosure in the form of hood was provided in the space between drain point of Joule melter and canister. The hood was maintained under vacuum by blower and exhaust of hood was passed through Hood filter assembly. This has reduced stack release by eight times and radiation level on exhaust filter bank by five times.
|Table 4: The radiation level data of correlation study of exhaust filter bank|
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| 4. Online Remote Radiation Monitoring of Overpack|| |
Overpack containing two canisters having average gross β activity 153070 TBq per overpack was shifted to storage vault under HP coverage for interim storage. Before that, remote radiation survey of overpack was carried out. There is no direct access to monitor overpack. Therefore, for radiation level measurement of overpack online, wide range gamma monitor is installed at 1 m distance from the overpack inside shielded cubical as shown in [Figure 4]. The multiplication factor of 17 was established between online monitor reading and overpack contact radiation level. For estimation of multiplication factor, the radiation field at contact of the overpack was monitored by exposing TLD/DRD for known time. The multiplication factor and radiation levels obtained from remote online monitoring system were confirmed with TLD/DRD readings and also they are verified by theoretical shielding calculations,  whose results are given in [Table 5] and [Figure 3].
| 5. Conclusion|| |
Our online remote monitoring system has helped the plant management to plan in advance for replacement of these filters, which resulted in considerable saving in collective dose and secondary waste. The dose expenditure for vitrification was 2.548 μSv per TBq activity vitrified. Also, secondary waste generation was minimized to 12% of annual authorized limit. Because of remote online monitoring system, 128.52 Person-mSv collective dose (14.28% of budgeted dose) was averted and also there was 2.6 m 3 reduction in generation of Cat-I waste.
| 6. Acknowledgement|| |
Authors are thankful to Dr. A. K. Gosh Director H.S.E. Group and Dr. D. N. Sharma, Associate Director, H.S.E. Group for their guidance and encouragements. Authors express sincere thanks to Shri. S. Basu, CE NRB, Shri Y. Kulkarni, Chief Superintendent, and I. Vishwaraj, Plant Superintendent for their valuable suggestions in the course of implementation of online remote radiation monitoring system during operation of Vitrification Plant. Authors also wish to acknowledge the valuable contribution from S/Shri S. Sharma, A. Mishra, and other HP staff of TWMP, Tarapur.
| References|| |
|1.||H. Etherington. "Nuclear Engineering Handbook". 1 st ed. USA: McGrew-Hill; 1958. |
|2.||Hot Commissioning report of AVS-I on 119 VWP operations. |
|3.||Kulkarni VV. (Editor), Assessment of Radioactivity content of Waste Packages at Waste Management Facilities, Tarapur" Theme Meeting (Monitoring of Radioactive Waste Drums using Various Techiniques.). Organized by IANCAS Tarapur Chapter and BRNS Mumbai. |
[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]