|Year : 2021 | Volume
| Issue : 2 | Page : 61-66
Radiological aspects during cutting and removal of L-08 coolant channel from the core of 540 MWe TAPS-4 nuclear reactor
Durgaprasad Dakinedi1, S Adhya1, SK Pal1, K Venkataramana2
1 Health Physics Unit, Tarapur Atomic Power Station-3 and 4, Palghar, Maharashtra, India
2 Directorate of Technical (HSE), NPCIL, Mumbai, Maharashtra, India
|Date of Submission||21-Jun-2021|
|Date of Decision||19-Jul-2021|
|Date of Acceptance||28-Jul-2021|
|Date of Web Publication||23-Oct-2021|
Health Physics Unit, Tarapur Atomic Power Station-3 and 4, Tarapur, Maharashtra Site, TAPP, Palghar - 401 504, Maharashtra
Source of Support: None, Conflict of Interest: None
The pressurized heavy-water reactors (PHWR) consist of a low-pressure horizontal reactor vessel, calandria, containing heavy water as moderator. The calandria is pierced by a large number of coolant tubes (also called pressure tubes [PTs], 392 in 540 MWe PHWR), which contains fuel bundles, and through which pressurized heavy-water coolant circulates. During biennial shut down in 2017, in-service inspection of coolant channels in Tarapur atomic power station-4 (TAPS-4) had been carried out and after the review of results of inspection, it was recommended by various groups of experts that channel L-08 should be removed for postirradiation examination at Bhabha Atomic Research Center (BARC). Before the channel removal job, one special shielding flask was designed to shift the removed channel to BARC. The integrity test of special shielding flask was carried out by safely placing cobalt-60 (source strength 851000 MBq) capsule source inside the shielding flask with the help of cranking unit mechanism followed by dose rate mapping on the outer surface of the flask. To establish the hydrogen pickup rates in L-08 PT, sliver samples were collected and separately sent to BARC. Four metallic sliver samples were obtained at four different distances from north E-face. The activity content present in each sliver sample was also estimated. The maximum activity estimated was 2313.24 MBq. Subsequently, L-08 coolant channel was cut from both sides using a chipless tool. Jobs involving heavy water (D2O) collection work were carried out with a ventilated plastic suit. Derived air concentration (DAC) of tritium at the work location was maintained below 1DAC during the entire activity. Particulate DAC was found below the detectable limit. Floor contamination checks and floor decontamination were conducted at regular intervals to avoid buildup of contamination. As a result of such high-quality radiological safety measures, only 25 workers, out of 270 radiation workers, have received a cumulative dose of more than 3 mSv in direct reading dosimeter with a maximum individual dose of 8.45 mSv and maximum individual uptake of 0.39 GBq/m3. Job was completed with a total collective dose of 324.35P-mSv which is 14.5% lower than estimated. This article highlights some of the critical tasks involved in the cutting and the removal of irradiated coolant channel from the core of 540 Mwe TAPS-4 reactor which is a first of kind activity in nature.
Keywords: Coolant channel, derived air concentration, dose rate, shielding flask, sliver samples
|How to cite this article:|
Dakinedi D, Adhya S, Pal S K, Venkataramana K. Radiological aspects during cutting and removal of L-08 coolant channel from the core of 540 MWe TAPS-4 nuclear reactor. Radiat Prot Environ 2021;44:61-6
|How to cite this URL:|
Dakinedi D, Adhya S, Pal S K, Venkataramana K. Radiological aspects during cutting and removal of L-08 coolant channel from the core of 540 MWe TAPS-4 nuclear reactor. Radiat Prot Environ [serial online] 2021 [cited 2021 Dec 8];44:61-6. Available from: https://www.rpe.org.in/text.asp?2021/44/2/61/329136
| Introduction|| |
Tarapur atomic power station houses two units of boiling water reactors (TAPS-1 and 2) and two units of pressurized heavy-water reactors (PHWR) (TAPS-3 and 4). The PHWR is of capacity 540 MWe. The coolant channel assembly is the heart of the PHWR where the heat generated due to nuclear fission is removed from fuel bundles by means of pressurized heavy water. The assessment of healthiness of coolant channels is being carried out at regular intervals by in-service inspection using Bhabha Atomic Research Center (BARC) channel inspection system (BARCIS). ISI of coolant channels in TAPS-4 had been carried out during extended biennial shut down of 2017. Full volumetric examination was done in selected 30 representative channels including the area near the rolled joints on both sides. During the volumetric examination, the channel L-08 in TAPS-4 was observed to have four planar circumferential flaws (maximum depth ~ 1.8 mm, length: 14 mm) in the pressure tube (PT) main body besides multiple volumetric indications. The ISI results were reviewed by the expert group on coolant channels, unit safety committee (PHWR-SC-3), and Safety Review Committee for Operating Plants (SARCOP) in various meetings. During review of the ISI results, SARCOP noted that these observed flaws in L-08 could not be explained by any known degradation mechanisms (delayed hydride cracking and fatigue).
Hence, to determine the root cause(s) of the flaws, SARCOP has recommended that channel L-08 should be removed for postirradiation examination (PIE) at BARC. Hence the job of cutting and removal of coolant channel L-08 from the core of TAPS-4 reactor was carried out during the poison shut down of TAPS-4 in December 2018.
| The Challenges|| |
The job of cutting and removal of L-08 coolant channel is the first of its kind at TAPS-4 540 MWe reactor. Similar jobs were carried out in 220 MWe reactors during en-masse coolant channel replacement. Job was started with design and development of the tools and equipments such as sliver sampling tool, LASER cutting tool, chipless cutting tool, a platform for handling, and shielding flask for transportation. Before taking up the activities, each and every task of the job was reviewed. Mockup of all the tasks was carried out in the fueling machine service area (FMSA) and at the actual workplace at TAPS-3 and 4.
Around 22 skilled workers of a private firm were also involved in the platform handling and channel cutting jobs. Before the work at TAPS-4, these temporary workers were also involved in radioactive jobs at other stations. As a result, annual dose of some of these workers had already reached to about 9 mSv. Hence, controlling the occupancy of these workers in the high radiation field during the job is a challenge which would otherwise result in exceeding the dose constraint of temporary workers, i.e., 15 mSv.
The job of cutting and removal of L-08 coolant channel involves three critical tasks such as:
- Qualification of shielding flask by radiometry
- Sliver sampling and estimation of activity
- Application of dose-reduction techniques during L-08 channel removal from the core.
All the equipments and tools were indigenously designed and developed by BARC, Raja Ramanna Center for Advanced Technology (RRCAT), and NPCIL.
| Qualification of Shielding Flask by Radiometry|| |
To safely transport the removed PT for PIE at BARC, one special shielding flask was designed by NPCIL. Length and diameter of shielding flask are 6800 and 508 mm, respectively. Overall height is around 1160 mm (including pedestal and lifting hook height). The weight of the flask is 13,000 kg approximately. The shielding material used was lead with a thickness of 155 mm. The shielding material was sandwiched between carbon steel linings of thickness 15.06 mm.
The integrity test was done by safely placing cobalt-60 capsule source inside the shielding flask with the help of cranking unit mechanism followed by dose rate mapping on the outer surface of the flask. For ease of identification during mapping, the entire outer surface of the flask was divided into six regions (region one to six) of equal widths. Then, each region is further divided into five subregions, say, subregion A to E in region one and so on. Hence, thetotal number of subregion marked is 30 and labeled as subregion A to subregion DD [Figure 1]. Circumferential area in each subregion is further divided into five grids (say, A1–A5 in subregion A and so on) of equal dimensions 200 mm × 300 mm approximately.
|Figure 1: Part of the shielding flask showing grid marking on the outer surface in region one. Source was kept at the midpoint of region one inside the flask|
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The capsule source used in the integrity test was cobalt-60 of strength 851,000 MBq. This capsule source is stored inside the shield when not in use and can be easily transferred from the shield to inside the shielding flask using an extension probe of length about 100 m and the cranking unit mechanism. With this arrangement, the source can be moved in and out of the flask from a large distance and hence avoids exposure to the personnel operating it.
| Estimation of Dose Rate Over Outer Surface of Flask|| |
Thickness of lead shielding (155 mm) in terms of the TVL is 3.875 TVL. By keeping the source inside the flask at the midpoint of any region (region one to six) and using the shielding flask dimensions as mentioned above, the dose rate on the outer surface of shielding flask, directly above the source, was estimated using the equation,
Г = Specific gamma-ray constant for cobalt-60 at 1 m distance, for cobalt-60),
Srel = relative stopping power of tissue to air (1.12) used for converting air kerma to dose rate,
A = Source strength of cobalt-60 (851,000 MBq),
103.875 = Factor used for shielding thickness in terms of TVL,
d = Distance between source and outer surface of the flask (0.353 m).
Using the parameters of the shielding flask and using the above mentioned equation, the dose rate expected on the outer surface of shielding flask directly above the source is 0.319 mSv/h. If any flaw or defect exists in the interior of shielding material, it would result in a dose rate of more than 0.319 mSv/h.
Initially, the source was kept in the middle of region one (middle of region one is subregion C) and dose rate measurement was carried out in every grid of each subregion in region one (Subregion A to E). Ideally, the grids that are at an equal distance from source (Ex: B1 and D1 or A1 and E1 in region one) should show a similar value of dose rate. Thus, the dose rate profile over these grids should be like an inverted parabola with the maximum field near its vertex. This type of profile is noticed during mapping and is shown in [Figure 2]. A similar profile was also found on other grids in the region one. The data collected during mapping in region one are given in [Table 1]. When the measurement in each grid of region one is over, the source was then moved to region two and dose rate measurement was obtained again in every grid of each subregion in region two (Subregion F to J). Similarly, the source is kept in the middle of each region and dose rate measurement was carried out in each subregion.
|Figure 2: Radiation dose rate profile over grids of region one with equal distance from source|
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|Table 1: Data of measured dose rate (mSv/h) over the outer surface of the flask in region 1|
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From the data, it was noticed that dose rate in most of the grids directly above the source is found in the range 0.30–0.40 mSv/h [Subregion C in [Table 1]]. In some of the grids, the dose rate was found in the range 1–3 mSv/h. These values were noticed in those grids located nearer to the penetrations made for lifting hook. Hence, these grids were covered with additional lead shields of thickness 35 mm.
Dose rate measurement was carried out again in each subregion as per the same procedure mentioned above and it was found nearly 0.30 mSv/h. The reduction percentage achieved by placing additional lead sheets was in the range of 75%–90%.
This method has two major advantages.
First, since the source strength used was very high, 851,000 MBq, the expected dose rate on the outer surface is 0.319 mSv/h, which can be easily distinguishable from natural background radiation and can be easily measured by any survey meter. If any flaw or void is present in the interior of shielding material, it would cause an increase in dose rate beyond the expected value and can be easily detected using this method.
Second, even though the source strength was very high, 851,000 MBq, dose consumed by the personnel involved in the integrity test was negligible below recording level (BRL). This is because the cranking unit can be operable from a large distance of 100 m and personnel occupancy was restricted during movement of the source inside the extension probe.
| Sliver Sampling and Estimation of Activity|| |
To establish the hydrogen pickup rates in PT of L-08, hydrogen concentration measurement in PT L-08 is required to be carried out by taking sliver samples. Furthermore, to establish the baseline data of hydrogen pickup by quadruple melted, cold worked, and stress relieved Zr-2.5Nb alloy PTs, sliver sampling is required to be carried out as the data are not available for 540 MWe Indian PHWR.
The sliver sampling tool used is known as wet scraping tool (WEST-540) and was developed by BARC for the first time for taking out samples from the coolant channel of 540 MWe.
The final qualification trials of the WEST-540 tool have been carried out at a special facility made at calibration and maintenance facility and also at FMSA of TAPS-3 and 4 in the presence of experts from BARC.
Four metallic sliver samples were obtained at four different distances from north E-face (from 3600–6150 mm). Radiation dose rate measured on these four samples was ranging from 100 to 300 mSv/h. These samples were stored in a specially designed shielding flask and were transported later to PIE division (PIED), BARC. The details of sliver samples are mentioned in [Table 2].
Radionuclide composition present in these sliver samples was estimated using ORIGEN-2 computer code and is given in [Table 3]. The maximum contribution to activity in sliver sample was found due to radioisotope Nb-95 (51.21%). Using this composition, the activity present in each sliver sample was also estimated. The maximum activity content of 2313.24 MBq was found in the sample with ID B2. The activity in other samples is mentioned in [Table 4].
|Table 3: Major isotopes (>3.7 MBq) present in the sliver samples after 5 days of cooling|
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|Table 4: Activity present in four metal sliver samples after 5 days of cooling at Tarapur atomic power station-4|
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The difference in dose rates measured over sliver samples can be attributed to two reasons:
- The distance from E-face or portion of PT from where the sample was collected and
- The weight of the sample.
The location of sample B3 is almost at the middle portion of the PT and this portion of the tube is expected to be the most irradiated portion. As a result, as expected, B3 sample had shown the highest dose rate, 300 mSv/h. A similar argument is valid for B4 and B1 as the location of these two samples is near to middle and near to end fitting, respectively. Although the location of sample B2 was away from the midpoint of the tube, it had shown a dose rate similar to B4 due to its highest weight (92 mg).
| Application of Dose-Reduction Techniques During L-08 Channel Removal from Core|| |
Before the job, a station ALARA meeting was convened for prejob planning of tasks involved in the job, dose control measures, and for estimation of the collective dose. An estimated collective dose of 380 person-mSv was planned for the execution of the job.
Each task in the job is planned for execution under high-quality radiological surveillance. As many as 270 radiation workers from various sections such as mechanical, fuel handling, control, electrical, operation, heavy water management, health physics, industrial safety, and R and D center at Tarapur were involved during different stages of the job that took nearly 15 days for execution of all planned tasks.
A group of experts from RRCAT, Indore, were also involved in the job for cutting different portions of coolant channel assembly with LASER tool. Another group of 22 skilled temporary workers from a contracting firm were involved during jobs of platform erection and removal and cutting and removal of coolant channel activities. Before their entry into reactor building, dose history of all these workers was checked to ensure that their annual dose was within the authorized dose constraint. Prewhole-body counting of all these workers was carried out. Training on basic radiation protection with emphasis on contamination control practices was imparted and familiarization of job area was done.
Job was started with the inspection of L-08 coolant channel with BARCIS followed by vacuum pulling of heavy water (D2O) present inside liner tube cavities. Subsequently, L-08 coolant channel was cut from both sides using chipless tool, followed by liner tube cutting, outboard end fitting cutting and removal, bellow lip and seal ring cutting, inboard end fitting removal, and installation of lattice tube protection sleeve and temporary shield plugs. Temporary shield plugs were placed inside PT and its face was thoroughly monitored for the identification of beaming field. Derived air concentration (DAC) of tritium at the work location was maintained below 1DAC during the entire activity. Particulate DAC was found below the detectable limit (BDL).
As a part of radiological surveillance program, multiple radiation monitors were installed at different locations inside fueling machine (FM) vaults to measure the dose rate during removal of inboard end fittings, garter springs, and coolant channel. Dose rate of 680,000 mSv/h was measured by a gamma radiation monitor installed at 15 cm distance from the coolant channel during its transfer to the shielding flask. Contact dose rate of 0.20 mSv/h was found on shielding flask containing coolant channel. Dose rates measured at various locations are given in [Table 5].
|Table 5: Data of observed dose rates at various locations during the retrieval of L-08 pressure tube|
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Persons working close to the channel were issued head and wrist thermoluminescent dosimeters (TLDs), alarm dosimeters in addition to the chest TLDs. All persons used lead jackets while working close to face of coolant channels. Dose received by individual at the end of each task was checked. Based on the dose registered by direct reading dosimeter (DRD) of individual, urgent processing of TLDs was carried out to ensure that consumed doses are below the authorized dose constraint. Heavy-water (D2O) collection work during opening and draining of feeder was carried out with a ventilated plastic suit. During the removal and insertion of laser tool into PT, workers were enforced to use particulate respirators for protection against possible particulate in air activity.
Floor contamination checks and floor decontamination were conducted at regular intervals to avoid buildup of contamination. Maximum floor contamination observed during the job inside FM vault (North/South) was 10 KBq/m2. Rubber change area procedure was strictly implemented. No event of spread of contamination has occurred during the job. After removal of PT, job of calandria tube inspection was carried out from FM vault (North) side. Channel L-08 was boxed up by lattice tube blanking assembly. Shielding flasks containing PT and inboards end fitting were removed from vaults and platform was dismantled. Out of 270 radiation workers, only 25 workers have received a cumulative dose of more than 3 mSv in DRD with a maximum individual dose of 8.45 mSv and maximum individual uptake of 0.39 GBq/m3.
With the help of such high-quality radiological surveillance methods, the collective dose consumed in the entire job, including the shifting of shielding flask loaded with coolant channel to PIED, BARC was measured as 324.35P-mSv against an estimated dose of 380P-mSv. The consumed collective dose includes 30.55P-mSv of internal dose due to tritium. Contribution of internal dose to total dose was below 10%.
| Conclusions|| |
As per the recommendation of group of experts, the removal of L-08 coolant channel of TAPS-4 and shifting to BARC for PIE is planned. One special shielding flask was designed and fabricated to transport the removed coolant channel by road. The integrity test of newly fabricated shielding flask was successfully carried out by placing cobalt-60 capsule source of strength 851,000 MBq inside flask followed by dose rate mapping on the outer surface of the flask. With the use of longest extension probe that facilitated the operation of the cranking unit from a large distance of 100 m and restriction of personnel occupancy during movement of the source, the dose received by the personnel involved in the task was found to be BRL.
To establish the hydrogen pickup rates in L-08 PT, sliver samples were collected and separately sent to BARC. Sliver sampling was carried out using WEST-540 designed by RED and RTD of BARC. Four metallic sliver samples were obtained at four different distances from north E-face. Isotopic composition in these sliver samples was estimated using ORIGEN-2 computer code. The maximum contribution to activity in sliver sample was found by radioisotope of Nb-95 (51.21%). The activity content present in each sliver sample was also estimated. The maximum activity content estimated was 2313.24 MBq. Surface dose rate measured on the four sliver samples was found to be ranging from 100 to 300 mSv/h. The difference in dose rates was attributed to the difference in degree of irradiation within the channel along its length and weight of the sample collected. Sample collected from the middle portion (B3) of the PT is found to have the highest dose rate (300 mSv/h) as this portion was expected to have experienced the highest irradiation. Due to its highest weight (92 mg) sample B2, was found to have a high dose rate (200 mSv/h) although it was collected away from the middle portion of the tube.
Before their involvement in the job, all radiation workers were imparted training on basic radiation protection with emphasis on contamination control practices along with familiarization of job area. Continuous radiological surveillance was ensured during cutting of PT with chipless tool, liner tube cutting, outboard end fitting cutting and removal, bellow lip and seal ring cutting, inboard end fitting removal, and installation of lattice tube protection sleeve, and temporary shield plugs. Temporary shield plugs were placed inside the PT to minimize the ambient radiation levels and its face was thoroughly monitored for the identification of beaming field. DAC of tritium at the work location was maintained below 1DAC during the entire activity. Particulate DAC was found BDL. Rubber change area procedure was strictly implemented and as result, no event of spread of contamination on the floor or personal contamination has occurred during the job. The coolant channel was successfully retrieved inside the shielding flask and sent to BARC for PIE. As a result of the high-quality radiological surveillance methods carried out during the job, job was completed with a total collective dose of 324.35P-mSv which is 14.5% lower than estimated. All personnel received doses below the dose constraint.
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Conflicts of interest
There are no conflicts of interest.
| References|| |
Cember H. Introduction to Health Physics. 4th
ed. New Delhi: The McGraw-Hill Companies; 2009.
Martin JE. Physics for Radiation Protection. 2nd
ed. Weinheim: WILEY-VCH Verlag GmbH & Co. KGaA; 2006.
Results of Radiometry Carried Out on Shielding Flask made for Shifting of L-08 Coolant Channel (TAPS3 and 4/09500/HPU/2018/S/054).
Report on Activity Estimation in L-08 Pressure Tube of TAPS-4 by RP and S, NPCIL, Mumbai.
[Figure 1], [Figure 2]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]