·Table of Contents
Evaluation of Large Sized Radioisotopes Source Storage and Transport Containers by NDT TechniquesT.K. Jayakumar, Sudhir Singh, Prasanna Rao
Board of Radiation and Isotope Technology,Vashi Complex. Navi Mumbai- 400 705. India
EAD,BARC, Trombay, Mumbai-400085, India
Stringent Quality Control procedures as specified in the standards  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  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.
|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.
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 
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.
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
|© AIPnD , created by NDT.net|||Home| |Top||