QUANTITATIVE ANALYSIS OF URANIUM DISTRIBUTION IN PLATEFUEL ELEMENT USING REAL TIME DIGITAL X-RAY RADIOGRAPHY

V. P. Sinha, R. Rakesh, P. V. Hegde, G. J. Prasad, S. Pal, and G. P. Mishra

Metallic Fuels Division, Bhabha Atomic Research Centre, Mumbai, India

Dispersion type plate fuel elements were fabricated with U3Si2 dispersoids in aluminiummatrix and clad in Al-alloy for the modified core of APSARA reactor by standard pictureframing technique followed by hot roll bonding operation. In general around 85% reductionin thickness was carried out in fabricating the finished fuel plates. These fuel plates werethen characterized by digital X-ray radiography technique to outline fuel meat boundaryand also for the quantitative analysis of surface uranium metal density in the fuel meat.In order to evaluate the surface uranium density a non destructive technique was developedin which X-ray radiography image was digitized and gray value was calibrated in terms ofuranium metal density. In addition, fuel meat and clad thickness of fuel plate wasdetermined along longitudinal section by optical microscopy.

Keywords: digital radiography, dispersoid, fuel meat, gray value, matrix, powder

1. INTRODUCTION

X-ray digital radiography offers the possibility of increased accuracy forassessing fuel loading values of research reactor fuel plates in minimum time[1,2]. It also offers a much quicker assessment of the fuel zone and platequality for a faster turnaround time on designs and modifications. Digitalradiography eliminates the need for any chemical processing and storageof film radiographs, which in turn reduces the space requirement and oper-ational cost. It also provides important three-dimensional data that would bevery difficult and expensive to obtain using the traditional methods. Theinformation generated by digital radiography can be easily integrated intocomputer analysis and modeling programs [3,4].

A number of detector technologies have been developed to support digi-tal radiography system based upon amorphous silicon thin film transistor(TFT) arrays, but the most successful and widely used detectors are calledindirect detectors. These detectors are based on amorphous silicon TFT=photodiode arrays coupled to X-ray scintillators. The array consists of a sheetof glass covered with a thin layer of silicon that is in an amorphous or dis-ordered state. At a microscopic scale, the silicon has been imprinted with

Address correspondence to V. P. Sinha, Metallic Fuels Division, Bhabha Atomic Research Centre,Mumbai 400085, India. E-mail: vedsinha@barc.gov.in

Research in Nondestructive Evaluation, 24: 223–234, 2013

Copyright # American Society for Nondestructive Testing

ISSN: 0934-9847 print=1432-2110 online

DOI: 10.1080/09349847.2013.800620

223

millions of transistors arranged in a highly ordered array, like the grid on asheet of graph paper. Each of these TFTs is attached to a light-absorbingphotodiode making up an individual pixel (picture element). Although sili-con has outstanding electronic properties, it is not particularly good absorberof X-ray photons. For this reason, X-rays first impinge upon scintillatorsmade from either gadolinium oxy-sulfide or cesium iodide. The scintillatorabsorbs the X-rays and converts them into visible light photons which arethen passed onto the photodiode array. Photons striking the photodiodeare converted into two carriers of electrical charge, called electron-holepairs. The number of charge carriers produced will vary with the intensityof incoming light photons, which in turn depends on the incoming X-rayintensity. The signals from the photodiodes are amplified and encoded byadditional electronics positioned at the edges or behind the sensor arrayin order to produce an accurate and sensitive digital representation of theX-ray image [5,6]. The image thus produced by digital X-ray radiographyis stored in gray scale.

A grayscale digital image can be defined as an image in which thevalue of each pixel is a single sample which carries only the intensityinformation. In a way, gray scale image is distinct from bi-tonal blackand white images, since they have many shades of gray in between.The grayscale images are often the result of measuring the intensity ofradiation in a single band of electromagnetic spectrum at each pixeland the intensity of a pixel is expressed within a given range of minimumand maximum. In an abstract manner the range is expressed from ‘0’ftotal absence, blackg to ‘1’ ftotal presence, whiteg with any fractionalvalues in between them. Although in computing grayscale values it canbe computed in any rational number, but the image pixels are stored inbinary format. The images are then stored in either 8 bit or 16 bit grayscale. In an 8 bit grayscale each pixel value is represented by 8 bitsrepresenting 256 shades f28¼ 256g while in a 16 bit grayscale eachpixel value is represented by 16 bits representing 65,536 shadesf216¼ 65,536g. For a 16 bit gray scale a complete black shade is explainedas f0000000000000000g and a complete white shade is explained asf1111111111111111g.

At Metallic Fuels Division, BARC dispersion type plate fuel elementsare being fabricated for the modified core of APSARA reactor withU3Si2 dispersoids in aluminium metal matrix and clad in Al-alloy. The fin-ished plate fuel elements are characterized by X-ray radiography for out-lining the fuel meat boundary and also for the quantitative analysis ofsurface uranium density. In the present text, these two different types ofexposures are termed ‘‘location exposure’’ and ‘‘density exposure,’’respectively. The ‘‘location’’ exposure forms a shadowgraph which is usedfor qualitative analysis of the location of fuel meat=core, drawing theoutline boundary of the fuel meat, and detecting any fuel in the nonfuelzone areas like the edges, corners, and end region of the plate. The

224 V. P. SINHA ET AL.

shadowgraph is then used for trimming the rolled fuel plate to the requireddimension. During trimming necessary care is taken in the case of fuelplates with slight bow so that a minimum of 2.5mm clad region on bothsides of the plate is maintained. In addition, density exposure is used todetermine the surface uranium density in the fuel plate directly bymeasuring only the gray value of digital image. The X-ray beam para-meters set for the shadowgraph and for the determination of the uraniumdensity have been finalized after detailed experimentation to suit theheavy metal loading in the fuel meat and fuel plate thickness used inour ongoing fuel fabrication program.

2. EXPERIMENTAL WORK

2.1. Fabrication of Fuel Plate Standards for Calibration

To calibrate the gray value to surface uranium density of the fuel plate,five fuel plate standards were prepared (see Table 1) under the identicalfabrication conditions used for the production of plate fuel elements forthe modified core of APSARA reactor. To start with U3Si2 compound(see Fig. 1(a)) fabricated by powder metallurgy route [7] was mixed withaluminium metal powder (see Fig. 1(b)) in the required proportion usingcubical blender. The mixture was then compacted in a hydraulic pressto fabricate fuel meat. The fuel meat was then picture framed (see Fig. 2)followed by welding along edges to prepare a fuel-clad sandwich (seeFig. 3). A gap of around 20mm was kept open at one side during edgewelding of sandwich to allow gases to escape out during hot rolling. Upto this stage. the entire operation was carried out inside a glove box main-tained under once through flow of high purity argon gas. The sandwichwas then heated inside a resistance furnace at 450�C for 4 hours undervacuum, and then hot rolling was carried out in multiple passes with inter-mittent soaking. Final plate dimension was achieved by cold rollingoperation. These standards were then subjected to blister test operationin which they were heated at 500�C for 30 minutes under vacuum. Fivedifferent compositions of fuel plate standards were prepared by the abovementioned technique and were carefully stored with proper identifications(see Fig. 4).

TABLE 1 Fuel Plate Standard Sample Details

Sample Type Sample ‘A’ Sample ‘B’ Sample ‘C’ Sample ‘D’ Sample ‘E’

Uranium requirement per plate (gm) 118.82 118.82 118.82 118.82 118.82U3Si2requirement per plate (gm) 127.77 127.77 127.77 127.77 127.77U3Si2vol%:Al vol% 46:54 44:56 42:58 40:60 38:62

QUATITATIVE ANALISIS OF URANIUM DISTRIBUTION 225

FIGURE 2. Sketch of disassembled view of fuel-clad sandwich showing picture frame, fuel meat, andcover plates.

FIGURE 1. Photograph of (a) U3Si2 granules and (b) aluminium metal powder. (Figure appears in coloronline.)

FIGURE 3. Photograph of fuel-clad sandwich. (Figure appears in color online.)

226 V. P. SINHA ET AL.

2.2. Characterization of Fuel Plate Standards by X-Ray Radiography andOptical Microscopy

The fuel plate standards were then characterized by X-ray radiographyfor outlining fuel meat boundary=core location and for quantitative uraniumdensity analysis and are termed ‘‘location exposure’’ and ‘‘densityexposure,’’ respectively. The location exposure was carried out at 32.0 KV,5.0mA, and at a focal spot size of 1.0mm, while the density exposure wascarried out at 96.0 KV, 5.0mA, and at a focal spot size of 0.4mm. In bothcases images were grabbed with 16 frame integrations which took 10 sec-onds. The location exposure and density exposures in the fuel plate standardswere grabbed in segments because of the limit imposed by the size of thedetector. These image segments were then stitched together to generate asingle complete radiography image (see Fig. 5).

The fuel meat and clad thickness for each fuel plate standard wasdetermined along longitudinal section by optical microscopy for which

FIGURE 4. Photograph of fuel plate standards.

FIGURE 5. Digital radiography image of fuel plate segments from (a) to (d), and (e) stitched radiographyimage.

QUATITATIVE ANALISIS OF URANIUM DISTRIBUTION 227

cutting was performed very carefully so that flow of aluminium from thecladding is avoided.

3. RESULTS AND DISCUSSION

3.1. Location Exposure

The fuel plate standards were first given the location exposure and theirshadowgraphs are shown in Fig. 6. These shadowgraphs were used to locatethe fuel meat outer boundary so that the plates can be trimmed in requireddimension. The shadowgraph shows three different shades in the imagewhich are due to detector region, clad region (aluminium portion) and fuelmeat region (see Fig. 6).

3.2. Density Exposure

The fuel plate standards were then exposed under X-ray for densityexposure and the radiographs are shown in Fig. 7. These radiographs clearlyreveal that at 96.0 KV and 5.0mA the clad region (aluminium portion)becomes transparent, and hence two different shades in the image were

FIGURE 6. X-ray shadowgraph of fuel plate standards: (a) Sample ‘A,’ (b) Sample ‘B,’ (c) Sample ‘C,’ (d)Sample ‘D,’ and (e) Sample ‘E.’.

228 V. P. SINHA ET AL.

FIGURE 7. X-ray radiograph for density exposure of fuel plate standards: (a) Sample ‘A,’ (b) Sample ‘B,’ (c)Sample ‘C,’ (d) Sample ‘D,’ and (e) Sample ‘E.’.

FIGURE 8. Matrix showing segment wise determination of gray values in X-ray digital radiograph. (Figureappears in color online.)

QUATITATIVE ANALISIS OF URANIUM DISTRIBUTION 229

observed. The image information for each segment was then converted togray value by drawing the matrix as shown in Fig. 8. The gray valueinformation for each segment was then stitched together to get the complete

FIGURE 9. Distribution of gray value along the length of fuel meat.

FIGURE 10. Data plot between uranium density (in gm=cc) and gray value. (Figure appears in coloronline.)

230 V. P. SINHA ET AL.

gray value distribution all along the length of the fuel plate (see Fig. 9). Asimilar technique was adopted to calculate the gray value distribution alongfuel plate length for all the other fuel plate standards, and a plot was gener-ated between the absolute gray value and estimated uranium density (seeFig. 10). The points were then fitted using linear and polynomial functions(see Table 2). It was found that the best fit for the data was linear as is shownin Eq. (1). Similarly, the absolute gray value data was plotted against surfaceuranium density (see Fig. 11) and the point were fitted by linear function andis shown in Eq. (2):

Uranium density ðgm=ccÞ ¼ 9:63318� 2:15902E� 4� Gray Value ð1Þ

TABLE 2 Different Regression for Data Fitting

Linear Fir (Y¼A þB � X) Polynomial Fit (Y¼A þB�X þC�X^2)

Parameter Value Error Parameter Value Error

A 9.63318 0.07114 A 7.81988 1.22171B �2.15902E-4 2.95916E-6 B �6.42192E-5 1.02103E-4

C �3.16105E-9 2.12717E-9

FIGURE 11. Data plot between surface uranium density (in gm=cm2) and gray value. (Figure appears incolor online.)

QUATITATIVE ANALISIS OF URANIUM DISTRIBUTION 231

Surface Uranium Density ðgm=cm2Þ ¼ 0:67432�1:51131E�5� Gray Value:

ð2Þ

3.3. Optical Microscopy

The fuel plate standards were then cut in longitudinal section to therolling direction. The microstructures were developed for all the stan-dard fuel plates and are shown in Fig. 12. These microstructures alsoreveal that fragmentation of fuel particles took place during rolling oper-ation. Additionally, clad and meat thickness was also determined byoptical microscopy for each fuel plate standard in longitudinal direction(see Fig. 13).

FIGURE 12. Microstructure of fuel plate standards: (a) Sample ‘A,’ (b) Sample ‘B,’ (c) Sample ‘C,’ (d) Sample‘D,’ and (e) Sample ‘E.’.

232 V. P. SINHA ET AL.

4. CONCLUSION

The quantitative analysis of uranium distribution in plate fuel elementusing real time digital X-ray radiography was developed as quality checkfor hot roll bonded fuel plates of modified APSARA core [8]. This techniqueis also suitable to address quick and real time estimation of fissile materialdistribution along the fuel meat length in a fuel plate which complementsvery well for the production requirements. The estimation of uranium distri-bution by this technique is highly dependent upon thickness and radiography

FIGURE 13. Cross-section of fuel plate standard Sample ‘A’: (a) clad, (b) fuel; Sample ‘B’: (c) clad, (d) fuel;Sample ‘C’: (e) clad, (f) fuel; Sample ‘D’: (g) clad, (h) fuel; and Sample ‘E’: (i) clad, (j) fuel. (Figure appearsin color online.)

QUATITATIVE ANALISIS OF URANIUM DISTRIBUTION 233

exposure parameters. Therefore, it is very important to take exposures of fuelplates under similar parameters at which standards were exposed. It is alsoimportant to note that the fuel plate standards used for the calibration arerequired to be rolled to almost same thickness as that of actual fuel plates(i.e., clad: 400m, meat: 700m, and clad: 400m).

ACKNOWLEDGMENT

The authors gratefully acknowledge with thanks the team effort andhard work put up by all the scientific and technical staff members of FuelDevelopment Section, MFD.

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