Ensuring Shielding adequacy in Lead shielded spent fuel transportation casks using gamma scanning

Transcription

1 Ensuring Shielding adequacy in Lead shielded spent fuel transportation casks using gamma scanning More info about this article: M.Ravichandra 1, P.Raghavendra 1, Dhiren Kothari 1, Dr.Yelgaonkar 2, 1-Heavy Engineering Division, L&T. 2. BRIT, Navi Mumbai Abstract Lead shielded casks are designed to transport and store nuclear spent fuel for definite period of time. Lead is a commonly used shielding material in such casks since it is relatively cheap and easily available. Also, it can be easily melted and poured in to the annulus of assemblies to give adequate shielding. However, such pouring process also generates several defects such as voids, porosity, foreign material entrapment, etc. inside the assembly. Any such defect would give rise to a localized inadequacy in radiation shielding when a radioactive material is placed inside, thereby posing a serious radiation hazard. Detecting such defects using conventional NDT techniques is a challenge as the defects in lead are usually subsurface in nature, also in this case lead is sandwiched by steel casing on either sides. Gamma scanning or gamma radiometry is one NDT technique which can be used for this application. The basic principle of gamma scanning is detection of differentially attenuated gamma radiation passing through the shielding material and correlation of the same with variations in density. Gamma scanning is a relative method of measurement and hence requires calibration prior to scanning. Selection of parameters is very critical in Gamma scanning as they greatly influence the scanning results. This paper gives a brief insight of the radiometry conducted on the spent fuel Storage/Transportation cask which was manufactured at L&T Heavy Engineering. It also discusses selection criteria of various parameters such as detector, counting statistics and collimation on both source and detector, etc. which can drastically affect the results of radiometry. Keywords : Gamma scanning, Lead shielding, Radiometry, Standard deviation, Counting statistics, Scintillation detector.

2 Introduction: A spent fuel transport cask is a container that is used to transport active nuclear materials between nuclear power station and spent fuel reprocessing facilities. Each cask is designed to maintain its integrity and avoid radiation leakage under normal transportation conditions and during hypothetical accident conditions. To provide robust structural integrity to cask and to withstand extreme environments in case of accident, steel-leadsteel walled casks is a common design. In steel-lead-steel design, lead would be sandwiched between the layers of steel. Such a sandwich design and high thickness of lead makes NDE of the cask a very big challenge since conventional NDE like UT, RT are not feasible. Gamma scanning or Radiometry serves as an excellent tool for inspection of the above discussed case. Principle The transmission of Gamma radiation through any material is governed by the exponential equation I = I0 e -µx Where: I = Intensity of radiation transmitted through a material of thickness X mm Io = Intensity of radiation from the Gamma - ray source reaching the detector in the absence of the material. µ = Linear absorption coefficient (mm -1 ) The transmitted intensity of gamma ray flux through any material changes with thickness. Presence of any imperfection would mean reduction in net thickness of the shielding material and it increases transmitted gamma ray flux. The transmitted gamma ray flux from the material under inspection can be used to ascertain the shielding thickness if the transmitted gamma ray flux from a known thickness of shielding material is measured.

3 Instrumentation and test parameters selection: Radioactive source, gamma detector and count rate meter are the primary test components required. Since thickness of lead is in the order of 150mm, a high energy radioactive source would be required to achieve full penetration. Hence, in this case, radioactive Cobalt-60, with strength of 2.2 Curie has been employed. NaI (Tl) crystal based scintillation detector has been employed for detection of gamma radiation passing through the material, given its high light yield and efficiency compared to other detectors. The scintillation detector generally consists of crystal coupled with a photomultiplier tube ( PMT) which is then connected to dynodes where electrical pulse is generated. The detector is coupled with a Ludlum made count rate meter to measure the counts obtained due to transmitted radiation. The count rate meter is equipped with a single channel analyzer system which enables selective acquisition of gamma rays based on the energy of radiation. Such a selective acquisition of the required energy photons is advantageous in decreasing the background counts. Both the source and detector are heavily collimated with lead to avoid any scatter radiation which may affect the counts obtained from the area of interest. To take care of the positional accuracy of the source and detector w.r.t cask, a robust automated fixture has been developed which ensures the total source-detector distance remains the same. Counting statistics: Since radiation emitted from nucleus is a statistically random process, there is always an inherent uncertainty in measurement of counts.it implies, if counts are measured N number of times at a same location, different count values would be obtained each time. The counts measured are known to follow Gaussian distribution wherein the individual counts measured oscillate about the mean value of the total counts measured and the oscillation value is quantified by standard deviation(sd). Hence the inherent uncertainty in measurement of count can be represented as a factor of Standard deviation (SD). For a single measurement, the standard deviation (SD) is known to be the square root of the count. SD =, where N is the count measured. From above equation it is clear that, to maintain the percentage uncertainty in measurement in terms of measured count ( 100) as low as possible, one would need to increase the total count (N) as high as possible. High values of 100 % can potentially mask a defective

4 portion in the material and therefore overall sensitivity of system is directly linked to the N value measured during scan. Fig: Gaussian distribution curve In our case, the inherent uncertainty in measurement due to Gaussian distribution has been minimized by tactically selecting the parameters thereby increasing the sensitivity. Total counts of 10,000 was targeted in each measurement. By targeting so, the expected SD was 100 counts, thereby maintaining the 100 % at 1%. To achieve the desired count of 10,000 counts, enough scan duration was provided for each measurement and scan duration was established during calibration itself. When correlated with lead thickness using exponential intensity equation, the 100 counts (SD) correlates to 0.3 mm of lead. Hence by selecting the above parameters, the statistical accuracy and sensitivity of 0.3 mm of lead has been achieved for the scanning. The sensitivity/uncertainty in measurement can be adjusted Calibration: Since gamma scanning is a relative measurement of thickness, it requires calibration with respect to a known standard. Hence a mock up block was made with same lead and steel thickness as that of cask. The counts from the known thickness of lead was then established by measuring counts from mock up block. The mockup block itself has been tested to be free of defects using radiography with a high energy X-ray source (12 MeV Linear accelerator). On the mockup, reference count for the given design thickness of lead has been established for the particular scan duration. The reference count is then used to compare the count values obtained during the scanning of the cask. Any increase in count on the cask can be correlated to loss of thickness as per the exponential intensity formula in above sections.

5 Testing: Grid wise scanning of the entire cask was then carried out using an automated fixture which manipulates movement of both source and detector in line with each other. The fixture is designed in such a way that the source and detector stops at a grid for required period of time and proceed automatically to the next grid in vertical direction. The single channel analyzer is set to acquire the counts only from the Co-60 twin peak range (1.17 MeV, 1.33 MeV) thereby greatly reducing the background. Fig: Schematic of setup Safety: ALARA principles have been thoroughly implemented throughout the testing. The automation of movement of radioactive source and detector has minimized the time spent by the personnel near to the source. Source is placed in a collimator which directed the rays in one specific direction. Also, lead blocks (2 TVT s thick) were manufactured and placed around the calibration block during calibration to reduce radiation leakage to surroundings. The total accumulated dose for all personnel involved in the gamma scanning was less than 0.1 msv.

6 Results of testing: Entire cask has been divided into 1.5 X 1.5 grids and the count measurements were taken from each and every grid. The count values are then compared against the values obtained on the calibration block. From the count values obtained on the cask, thickness of lead at the particular location was resolved by using the exponential intensity equation mentioned in the above sections of this paper. The thickness obtained on the cask is then compared with the minimum shielding thickness as per the design of the cask and disposition is taken. The plots of lead thickness vs grid location in one of the elevations (Elevation 1) is shown below. Thickness in mm Grid Location Few areas in the cask were found to be having shielding thickness less than that of the required design thickness and hence they are repaired by excavating and re-pouring the lead. During repair, while excavating the lead, the macro cavities partially filled with impurities were evident. Further gamma scan is planned on the cask after repair work is complete. Fig : Cavities revealed in excavation

7 Conclusion The gamma scanning technique is a unique, fast and precise tool to measure and quantify the thickness of lead shielding adequacy. In scenario where most of conventional NDE techniques fail, gamma scanning has demonstrated accuracy of shielding thickness measurement in the order of +/- 0.3mm by selecting optimum parameters. The criticality of selection of parameters like SD, count rate, scan duration are discussed in detail. Sensitivity of 0.3 mm was possible by selecting the critical parameters such as count rate and SD of the scan accordingly. Another important parameter, source to detector distance was kept constant throughout scanning by designing and developing a fixture which is capable of manipulating source and detector in line with each other using remote control. ALARA principle employed throughout the scan, keeping in view the radiation safety, was also discussed in the paper. References: 1. ASNT, Nondestructive Testing Handbook, Second Edition, Volume 3, Radiography & Radiation Testing, Columbus, Ohio, American Society or Non-destructive Testing. 2. Terry M. Mitchell Terry, Handbook on Nondestructive Testing of Concrete, Chapter 12. Radioactive/Nuclear Methods. 3. Radiation Detection and Measurement by Glenn Knoll. 4. Radiography testing Level -2 course book, BARC