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Evaluation of Large Sized Radioisotopes Source Storage and Transport Containers by NDT Techniques T.K. Jayakumar, Sudhir Singh, Prasanna Rao Board of Radiation and Isotope Technology,Vashi Complex. Navi Mumbai- 400 705. India Y.S. Mayya EAD,BARC, Trombay, Mumbai-400085, India Contact

Abstract

Large sized, intricate lead shielded containers are fabricated for carrying out en-masse replacement of coolant channels in PHWR type reactors, for in-house irradiators, for transport and storage of large amount of radioactive isotopes, for EB accelerator shields and for storage of nuclear waste material. Various IAEA / AERB codes specify stringent quality assurance procedures for fabrication of such vessels. Huge lead shielded containers were fabricated for such purposes and various NDT methods were used in for assuring the quality and system integrity of such containers during its stages of manufacture, use and revalidation process and these aspects are discussed in the paper.

1. Introduction:

Large sized lead shielded flasks are fabricated by various units of the Department of Atomic Energy for transportation of radioisotopes to various destinations and also for source housing of in-house Irradiators such as Gamma Chambers. These flasks are expected to pass through public domain, to be handled by untrained workers and subjected to rigors of wear and weather. Also intricate lead shielded containers are fabricated for carrying out en-masse replacement of coolant channel, for transport of radioisotopes, and storage of large amount of radioactive isotopes and nuclear waste material and shielding of Electron Beam Accelerators. In-service remote handling equipment at nuclear power station needs huge leaded systems. The demand for such intricate custom made containers is on increase with growth of nuclear and isotope programme in the country. In general such containers are usually designed in accordance with the following national and international standards in addition to any specific manufacturing recommendations and codes.
• IAEA Safety guide series No. 37, Advisory materials for the IAEA regulation for the safe transport of radioactive materials (1985) 3 ^rd edition as amended in 1990.
• The regulations for safe transport of radioactive materials IAEA safety series No. 6.
• Regulation for safe transport of radioactive material ST-1 (1996 edition)
• Atomic Energy Regulatory Board (India) codes for safety in transport of radioactive materials (1986) AERB code SC/TR-1.
• AERB guide for Quality Assurance for safe transport of radioactive material AERB-SG-TR-4
• ASME Boiler & Pressure Vessel Code Section III, & VIII, Section V, Div. 1, 1996
• Safe Design and use of Panoramic, Wet Source Storage Irradiators (Category IV), ANSI- N 43.10-1984. ANSI, New York
• Testing & Classification of Sealed Radioactive Sources. AERB SS-3 & ISO- 2919 (1980),ISO 1677 (1977) , ISO /TR 4826 (1979)
• IAEA Safety guide - Radiation Safety of Gamma & Electron Irradiation Facilities IAEA Safety Series 107 (1992)

Stringent Quality Control procedures as specified in the standards [1] are implemented during the fabrication, so as to provide total containment and adequate radiation shielding in case of accident under all conceivable normal & abnormal situations. Various NDT methods were used in for assuring the quality [2] and system integrity of large sized containers during various stages of manufacture and also revalidation of Co-60 transport containers such as Nordian F-168 Flasks.

2. Safety Evaluations:

Safety evaluations were carried out on following large sized radioisotope containers and shields to assess the radiation safety and AERB requirements.
• Cobalt absorber rod assembly removal cum transport flasks weighing 12 Metric Ton. 350mm-lead wall thickness and 2.8 meter long. Total 4 flasks
• Gamma chamber GC-5000 unit weighing 5 Metric Ton. A minimum of 272 mm lead thickness and 950 mm height. - Total 12 Numbers. , (Fig. 1)
• Coolant channel replacement machine pressure tube shielding flask. 105-mm lead thickness and 2.67 meter long, - Total 4 numbers.
• Blood irradiator flasks , Weight 5 Metric Ton , ( fig. 2)
• EB Accelerator Shields weighing 12 Metric Ton - 4 Main wall shields,15 trench shields & 1 dome shield, (Fig 3 & 4)
• Modified Gamma Chamber GC - 900 flasks,
• F-168 Transport Flasks Type B (U) for revalidation and AERB certification. - 2 No.
• MISC Waste Cask vessel for KARP, Kalpakkam,
• Cobalt - 60 Storage Flasks - Total 6 flasks, and
• Doughnut ( Flask mount) lead shield for cobalt replacement

Fig 1: GC -5000 Sectional View

Fig 2: Blood Irradiator

Fig 3: EB Accelerator Shield

Fig 4: Gammma Scanning in Progress

Safety analysis on these containers includes the applications of following NDT Techniques.

• Radiography,
• Ultrasonic Testing ,
• Liquid penetrant Techniques,
• Magnetic Particle Testing ,( For testing old version of F-168 flask with M.S. cladding) and
• Gamma Scanning.

3. NDT methodology and material testing for evaluations:

The material, lead, being an effective shield, cheap and easy to fabricate, is most commonly used for shielding purposes. The cladding material is usually SS304L, because of its low carbon content (< 0.02%) and least inter-corrosion properties. The quality assurance written procedures in accordance with ISO-9000 are communicated to inter working groups and to the fabricator. The material procurement details, chemical & mechanical analysis reports were obtained before the fabrication of individual vessels. The QA system of the fabricator is scrutinized for evaluations and audit.

a) Industrial Radiography:
All the accessible weld joints are inspected by radiography techniques confirming to ASME Section V, Article 2. The Iridium -192 radioisotope & X-rays were used for inspection and a radiographic sensitivity of 2-2T was established in all radiographs. Agfa D-7 FW film with 2 to 2.5 D was viewed with an adjustable high intensity illuminator and defects were demarcated. The repaired joints were identified with R markings and re-inspected. Guidelines from ASME Section V, VIII and UW 51 acceptance standards were followed.

b) Liquid Penetrant Testing :
The single side access joints are inspected by liquid penetrant tests. Water washable Magnaflux Spot check SKL-SP penetrant was used with 15 minutes dwell time. MIL- STD 6866, ASTM E 165, & IS 3658-1981 standards were used for approval compliance. SKL-S2 developer, SKC-1 cleaner were used in the testing.

c) Ultrasonic Testing.
The thickness measurements were carried out by Ultrasonic thickness tester. The multiple echo technique, using 2 MHz, ultrasonic normal probe and EEC EX-100 ultrasonic Machine, was employed at random to determine the soundness of the sheets before fabrication.

d) Magnetic Particle Testing:
In the case of F-168 transport flasks the magnetic particle testing using Magnaflux yoke Model Y7 AC / HWDC, was also carried out on the welds of outer MS cladding, cooling fin joints, nuts & bolts in accordance with ASTM E-709-91. In addition to self load and jerk tests as required by AERB were also carried out.

e) Pressure Tests:
All the vessels were subjected to pressure tests as specified by ASME Sec VIII. Normally 1.5 times of the working pressure is applied except pneumatic where the value is 1.25 times. In our case the pressure ranging from 2-3 Kg/cm² (30 to 45 psi) was applied [3]

f ) Gamma Scanning:
During the lead pouring operations, inspite of best manufacturing techniques adopted today, voids, shrinkage, and separation of layers do occur in lead casting. The discontinuities are generated during the fabrication of lead containers due to change of certain parameters. The quantity of lead required to fill a certain volume of lead flask is calculated by volumetric method using water. The recheck of lead quantity and the final weight of the container including cladding material shall indicate any loss of weight. A loss of 200 kg of lead in 10 metric ton flask is not high (about 2 %); but in a weak spot, the alignment of void in a straight line along the radiation beam direction, might pose a danger of excessive radiation leakage. Therefore the careful evaluation of flask is required to monitor such discontinuities before the flask is put into use. The evaluation of large sized containers after lead pouring is very complex. Radiography suffers from excess attenuation of g - rays due to high absorption coefficient of lead and therefore could not be employed. Ultrasonic suffers from alloying grain consideration and interface conditions after lead pouring. The radiation transmission method otherwise known as Gamma Scanning, using a source and detector in opposite direction, with simultaneous movements, scores over these limitations. However a careful measurement and evaluation is required to identify and locate small defects.

g) Operational details of Gamma Scanning tests:
Cobalt - 60 Source with an activity of 112 GBq was employed and source was placed in specially fabricated source detector guide system and moved precisely along the vertical lines through the grids. Around 600 grid points were scanned in the case of absorber rod assembly removal flask and 200 grid points were selected in the case of GC 5000 flask. The Coolant channel replacement-shielding flask and accelerator shields were scanned using 14 MBq of Co-60 source and 600 grid points were scanned. In all cases NaI (Tl) detector was used as radiation detector, which was either connected to micro R meter or a count rate meter externally.

4. Evaluation by Gamma Scanning:

The computed dose rate for the above thickness was evaluated using the formula.

Where in
             S is source strength in Ci,,
             X is the distance between probe and detector
             t is shield thickness in cm
             a1, a2, A are Taylor's constants and
              C.F. is conversion factor i.e. photon/sec/cm² /µ R /hr

The dose rate as measured in each location under the particular position was evaluated for its thickness using various buildup factors. The loss of lead thickness in individual cases were exactly calculated and evaluated for the shielding loss. The physical verification, by removing the cladding at these defective locations as shown in fig.5(a) and (b), effectively proved that Gamma Scanning is an efficient NDT technique on such occasions and an accuracy of 3mm loss in 300 mm shield could be arrived in these methods.

Fig 5:	Fig 5a:	Fig 5b:
Gamma Scaning of Coolent Tube Shielding Flasks Fig 5(a) Flask 1. No significant voids present Fig 5(b) Flask 2. Voids present Estimated loss of lead 12 mm. Reading are in micro R/hr

5. Conclusions:

All NDT methods used in the evaluation are fast, cost effective, accurate and aid in ensuring biological safety of radiation and non-radiation workers. It also provided the vital data to analyse the safety requirements of individual vessels and inputs for further improvements in design and fabrication. The work carried out shows the defect detection could be carried out with a high degree of accuracy with suitable NDT technique for applications in radiation technology & safety analysis for regulatory agencies.

6. Acknowledgements:

The authors express their gratefulness to Dr. S. Gangadharan, Chairman & Chief Executive, BRIT for his encouragement and guidance during the work. The authors express their sincere thanks to Mr. D.R. Raipurkar, Nu Tech consultants, Mumbai for the technical support extended at different stages of gamma scanning & NDT services at various industrial sites.

7. References:

1. ISO - 9000 International Standard First Edition 1987-03-05
2. BRIT Quality Assurance Manual 1999
3. Safety analysis reports GC - 5000, 1994.

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