Designation: C968 − 12

Standard Test Methods for

Analysis of Sintered Gadolinium Oxide-Uranium Dioxide
Pellets1
This standard is issued under the fixed designation C968; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.

1. Scope Section
Ceramographic Determination of Free Gd2O3 and Free UO2 to 17 to 24
1.1 These test methods cover procedures for the analysis of Estimate the Homogeneity of (U,Gd)O2 Pellets
sintered gadolinium oxide-uranium dioxide pellets to deter- Ceramographic Determination of Average Grain Size by Linear 25 to 32
mine compliance with specifications. Intercept after Chemical Etching
1.3 The values stated in SI units are to be regarded as the
1.2 The analytical procedures appear in the following order:
standard.
Section
Carbon (Total) by Direct Combustion—Thermal Conductivity Method 2 1.4 This standard does not purport to address all of the
3
C1408 Test Method for Carbon (Total) in Uranium Oxide Powders safety concerns, if any, associated with its use. It is the
and Pellets By Direct Combustion-Infrared Detection Method
Chlorine and Fluorine by Pyrohydrolysis Ion-Selective Electrode 4 responsibility of the user of this standard to establish appro-
Method priate safety and health practices and determine the applica-
3
C1502 Test Method for Determination of Total Chlorine and Fluorine bility of regulatory limitations prior to use.
in Uranium Dioxide and Gadolinium Oxide
4
Gadolinia Content by Energy-Dispersive X-Ray Spectrometry
C1456 Test Method for Determination of Uranium or Gadolinium, or 3 2. Referenced Documents
Both, in Gadolinium Oxide-Uranium Oxide Pellets or by X-Ray 2.1 ASTM Standards:3
Fluorescence (XRF)
Hydrogen by Inert Gas Fusion 4 C922 Specification for Sintered Gadolinium Oxide-Uranium
C1457 Test Method for Determination of Total Hydrogen Content of 3
Dioxide Pellets
Uranium Oxide Powders and Pellets by Carrier Gas Extraction C1347 Practice for Preparation and Dissolution of Uranium
2
Isotopic Uranium Composition by Multiple-Filament Surface-
Ionization Mass Spectrometric Method Materials for Analysis
C1413 Test Method for Isotopic Analysis of Hydrolysed Uranium 3
C1408 Test Method for Carbon (Total) in Uranium Oxide
Hexafluoride And Uranyl Nitrate Solutions By Thermal Ionization
Mass Spectrometry
Powders and Pellets By Direct Combustion-Infrared De-
C1347 Practice for Preparation and Dissolution of Uranium Materials 3 tection Method
for Analysis C1413 Test Method for Isotopic Analysis of Hydrolyzed
Nitrogen by Distillation—Nessler Reagent (Photometric) Method 6 to 16
Oxygen-to-Metal Ratio of Sintered Gadolinium Oxide-Uranium Diox- 4 Uranium Hexafluoride and Uranyl Nitrate Solutions by
ide Pellets Thermal Ionization Mass Spectrometry
3
C1430 Test Method for Determination of Uranium, Oxygen to C1430 Test Method for Determination of Uranium, Oxygen
Uranium, and Oxygen to Metal (O/M) in Sintered Uranium Dioxide
and Gadolinia-Uranium Dioxide Pellets by Atmospheric Equilibra-
to Uranium (O/U), and Oxygen to Metal (O/M) in
tion Sintered Uranium Dioxide and Gadolinia-Uranium Diox-
4
Spectrochemical Determination of Trace Impurity Elements
3
ide Pellets by Atmospheric Equilibration
C1517 Test Method for Determination of Metallic Impurities in Ura-
nium Metal or Compounds by DC-Arc Emission Spectroscopy
C1456 Test Method for Determination of Uranium or Gado-
Total Gas by Hot Vacuum Extraction 2 linium (or both) in Gadolinium Oxide-Uranium Oxide
Pellets or by X-Ray Fluorescence (XRF)
C1457 Test Method for Determination of Total Hydrogen
1
These test methods are under the jurisdiction of ASTM C26 on Nuclear Fuel Content of Uranium Oxide Powders and Pellets by Carrier
Cycle and are the direct responsibility of C26.05 on Methods of Test. Gas Extraction
Current edition approved Jan. 1, 2012. Published February 2012. Originally C1502 Test Method for Determination of Total Chlorine and
approved in 1981. Last previous edition approved in 2006 as C968 – 06. DOI:
10.1520/C0968-12. Fluorine in Uranium Dioxide and Gadolinium Oxide
2
Discontinued 1999. See C968 – 94. C1517 Test Method for Determination of Metallic Impurities
3
For referenced ASTM standards, visit the ASTM website, www.astm.org, or in Uranium Metal or Compounds by DC-Arc Emission
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on
Spectroscopy
the ASTM website. D1193 Specification for Reagent Water
4
Discontinued 2005. See C968 – 99. E112 Test Methods for Determining Average Grain Size

Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States

1
 C968 − 12
E146 Methods of Chemical Analysis of Zirconium and CARBON (TOTAL) BY DIRECT COMBUSTION—
Zirconium Alloys (Silicon, Hydrogen, and Copper) (With- THERMAL CONDUCTIVITY METHOD
drawn 1989)5 This Test Method was discontinued in January 1999 and
replaced by Test Method C1408
3. Significance and Use
CHLORINE AND FLUORINE BY PYROHYDROLYSIS
3.1 The test methods in this method are designed to show ION-SELECTIVE ELECTRODE METHOD
whether a given material is in accordance with Specification This Test Method was discontinued in March 2005 and
C922. replaced by Test Method C1502
GADOLINIA CONTENT BY ENERGY-DISPERSIVE
4. Reagents
X-RAY SPECTROMETRY
4.1 Purity of Reagents—Reagent grade chemicals shall be This Test Method was discontinued in March 2005 and
used in all tests. Unless otherwise indicated, it is intended that replaced by Test Method C1456
all reagents shall conform to the specifications of the commit-
HYDROGEN BY INERT GAS FUSION
tee on Analytical Reagent of the American Chemical Society,
This Test Method was discontinued in March 2005 and
where such specifications are available.6 Other grades may be
replaced by Test Method C1457
used, provided it is first ascertained that the reagent is of
sufficiently high purity to permit its use without lessening the ISOTOPIC URANIUM COMPOSITION BY
accuracy of the determination. MULTIPLE-FILAMENT SURFACE-IONIZATION
MASS SPECTROMETRIC METHOD
4.2 Purity of Water—Unless otherwise indicated, references
This Test Method was discontinued in January 1999 and
to water shall be understood to mean reagent water conforming replaced with C1413
to Type IV of Specification D1193. Samples can be dissolved using the appropriate dissolu-
tion techniques described in Practice C1347
5. Safety Precautions
NITROGEN BY DISTILLATION—NESSLER
5.1 Proper precautions should be taken to prevent inhalation REAGENT (PHOTOMETRIC) METHOD
or ingestion of gadolinium oxide or uranium dioxide dust
during grinding or handling operations. 6. Scope
6.1 This test method describes the determination of nitrogen
in gadolinium oxide-uranium dioxide pellets (Gd2O3/UO2).
5
The last approved version of this historical standard is referenced on With a 2 to 5-g sample, concentrations from 5 to 100 µg of
www.astm.org.
6
Reagent Chemicals, American Chemical Society Specifications, American
nitrogen are determined without interference.
Chemical Society, Washington, DC. For suggestions on the testing of reagents not
listed by the American Chemical Society, see Analar Standards for Laboratory 7. Summary of Test Method
Chemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeia
and National Formulary, U.S. Pharmacopeial Convention, Inc. (USPC), Rockville, 7.1 Pellet samples of gadolinium oxide-uranium dioxide are
MD. crushed, then dissolved in phosphoric acid. Hydrochloric acid

2
 C968 − 12
with hydrogen peroxide can also be used. The resulting 10. Reagents and Materials
solution is made alkaline with sodium hydroxide, and the 10.1 Nessler Reagent—Dissolve 50 g of potassium iodide
nitrogen is separated as ammonia by steam distillation (see (KI) in a minimum of cold water (approximately 35 mL). Add
Method E146). Nessler reagent is added to the distillate to form a saturated solution of mercuric chloride (HgCl2) slowly until
the yellow ammonium complex, and the absorbance of the the first slight precipitate of red mercuric iodide persists. Add
solution is measured at approximately 430 nm, using a cell 400 mL of potassium or sodium hydroxide solution (505 g of
depth of 2 cm (1, 2).7 KOH or 360 g of NaOH/L). Dilute the solution to 1 L with
NOTE 1—This procedure has been written for a cell having a 2-cm light
path. The range of the method can be extended by suitably varying sample
ammonia-free water, mix, and allow the solution to stand
mass, aliquot size, amounts of reagents, and cell depth. overnight. Decant the supernatant liquid and store it in a brown
bottle. This reagent is stable indefinitely.
8. Interferences 10.2 Ammonium Chloride (NH4 Cl)—Dry the ammonium
8.1 There are no known interfering elements. chloride at 110 to 120°C for 2 h.
10.3 Nitrogen Reference Solution (1 mL = 10 µg N)—
9. Apparatus Dissolve 3.819 g of dried NH4Cl in water and dilute the
9.1 Nitrogen Distillation Apparatus, with 100-mL flask, Fig. solution to 1 L. Transfer 10 mL of this solution to a 1-L
1; micro-Kjeldahl apparatus. volumetric flask and dilute it to volume with water.
9.2 Photometer—A filter photometer with a narrow-band 10.4 Hydrochloric Acid (6 N)—Dilute 6 volumes of con-
filter; or a spectrophotometer equipped with 2-cm cells. centrated hydrochloric acid (HCl) to 12 volumes.
9.3 Heater, 750-W, electric, full-control. 10.5 Hydrogen Peroxide (30 %).

11. Precautions
11.1 The use of ammonia or other volatile nitrogenous
compounds in the vicinity of the experiment can lead to serious
7
The boldface numbers in parentheses refer to the list of references at the end of
this standard.

FIG. 1 Micro-Kjeldahl Apparatus

3
 C968 − 12
errors. To ensure freedom from contamination, take the fol- 14.1.6 Connect the distillation flask containing the sample
lowing precautionary measures: solution to the distillation unit. Add 25 mL of NaOH solution
11.1.1 Steam clean all glassware immediately prior to use. (37.5 %) to the sample solution through the thistle tube so as to
11.1.2 Use ammonia-free water in all cases. form two layers in the flask. Close the thistle tube stopcock and
distill until 40 mL of distillate is collected. (The NaOH solution
12. Purity of Water must be added slowly to avoid a violent reaction which may
12.1 Unless otherwise indicated, all references to water in lead to a loss of sample.)
this method shall be understood to mean ammonia-free water, 14.1.7 Pipet 1.0 mL of Nessler reagent to the distillate,
prepared as follows: Pass distilled water or other water of dilute to the mark with water, stopper, and mix well.
equivalent purity through a mixed-bed resin demineralizer. 14.2 Reference Solution—Carry a reagent blank through the
Prepare all solutions in an ammonia-free atmosphere and store entire procedure, using the same amount of all reagents.
them in tightly stoppered chemical-resistant glass bottles. Boil
14.3 Photometry—Take the photometric reading of the
all rubber stoppers used for 30 min in sodium hydroxide
samples as described in 13.3.
solution (100 g NaOH/L) and rinse them with ammonia-free
water.
15. Calculation
13. Preparation of Calibration Curve 15.1 Correct for the blank and convert the photometric
reading of the sample to micrograms of nitrogen by means of
13.1 Calibration Solutions—Pipet 5, 10, 25, 50, 100, and
the calibration curve.
150 µg of the reference nitrogen solution (1 mL = 10 µg N) into
50-mL volumetric flasks containing 25 mL of water. Pipet 1.0 15.2 Calculate the nitrogen content, N, in micrograms per
mL of Nessler reagent into each flask and dilute to the mark gram of sample as follows:
with water. Stopper the flask and mix well. N 5 A/W (1)
13.2 Reference Solution—Pipet 1.0 mL of Nessler reagent where:
into a small volume of water in a 50-mL volumetric flask.
A = micrograms of nitrogen found, and
Dilute to volume with water. Stopper and mix well.
W = sample mass, grams of Gd2O3/UO2.
13.3 Photometry—Transfer a suitable portion of water to a
2-cm absorption cell and adjust the photometer to the initial 16. Precision and Bias
setting, using a light band centered at approximately 430 nm.
16.1 The relative standard deviation for the measurement of
While maintaining this photometer adjustment, take the pho-
nitrogen at the 100 µg/g level is 3 %.
tometric readings of the reference solution and the calibration
solutions. OXYGEN-TO-METAL RATIO OF SINTERED
13.4 Calibration Curve—Correct for the blank (reference GADOLINIUM OXIDE-URANIUM DIOXIDE
solution) reading and plot the photometric readings of the PELLETS
calibration solutions against micrograms of nitrogen per 50 mL This Test Method was discontinued in March 2005 and
of solution. replaced by Test Method C1430

14. Procedure SPECTROCHEMICAL DETERMINATION OF TRACE,

IMPURITY ELEMENTS
14.1 Test Solution: This Test Method was discontinued in March 2005 and
14.1.1 Weigh and transfer to a 500-mL Erlenmeyer flask 2 replaced by Test Method C1517
to 3 g of the crushed gadolinium oxide-uranium dioxide pellet
sample. (The pellets should be crushed to a fine powder with a TOTAL GAS BY HOT VACUUM EXTRACTION
stainless steel mortar and pestle.) This procedure was discontinued in January 1999
14.1.2 Add to the distillation flask 60 mL of 6 N hydrochlo-
ric acid and 20 mL of 30 % hydrogen peroxide. CERAMOGRAPHIC DETERMINATION OF FREE
14.1.3 Boil for 10 min. While the solution is boiling, Gd2O3 AND FREE UO2 TO ESTIMATE THE
carefully add additional 30 % hydrogen peroxide dropwise HOMOGENEITY OF (U,Gd)O2 PELLETS
until the solids are dissolved and a clear yellow solution is
obtained; cool the solution. 17. Scope
14.1.4 While the samples are dissolving, fill the steam- 17.1 The homogeneity of Gd2O3 in UO2 has been cited in
generating flask of the distillation unit with water. Apply heat Specification C922 as an important requirement for this fuel
and pass steam through the distillation flask and into the form. The uniform distribution of gadolinia in urania will result
condenser. Collect 50-mL portions of the distillate and add 1.0 in up to three components in the pellet: free Gd2O3, free UO2,
mL of Nessler reagent to each. When the absorbance of these and a Gd2O3-UO2 solid solution. There are a number of ways
solutions shows the apparatus to be free of ammonia, the for assessing uniformity of which the ceramographic method
distillation unit is ready for use with the samples. described here may not be the most definitive. This technique
14.1.5 Transfer quantitatively the dissolved gadolinia- has been used over the gadolinia concentration range from 1 to
urania solution into the 100-mL distillation flask. 10 weight %.

4
 C968 − 12
18. Summary of Test Method 23. Evaluation
18.1 This ceramographic test, similar to those reported by 23.1 Measure the weight percent of free gadolinia by
Rooney (3) and by Hammerschmidt and Saiger (4), consists of scanning the entire micrograph surface for gadolinia inclusions
making micrographs of polished pellet surfaces that have been (white-colored) and using the following calculation:
chemically etched with a hydrogen peroxide-sulfuric acid A ~ 0.72! 10 000
solution. Oxidation makes it possible to differentiate between Free Gd2 O 3 wt% 5 (2)
%Gd2 O 3 ~ A c !
components.
where:
19. Apparatus A = total observed surface of all Gd2O3 inclusions, cm2,
19.1 Diamond Saw, low-speed. Ac = total pellet surface examined without Gd2O3, cm3
nominal weight percent of Gd2O3, and
19.2 Ultrasonic Cleaner. 0.72 = factor from the density ratio Gd2O3/(U,Gd)O2.
19.3 Polisher/Grinder Table. 23.2 Measure the weight percent of free uranium dioxide by
19.4 Metallurgical Microscope, with camera attachment. means of a blue spot analysis on a representative color photo
using a calibrated grid, counting the number of uranium
20. Reagents and Materials particles covered by crosses and converting this count to
20.1 Hydrogen Peroxide (30 %). weight percent of free UO2. (Estimate the average particle size
20.2 Hydrogen Peroxide–Sulfuric Acid Etch Solution—10 and assume sphericity to assign a particle weight.)
parts 30 % hydrogen peroxide mixed with 1 part concentrated 23.3 Calculate the weight percent of solid (U,Gd)O2 solu-
sulfuric acid. tion by difference:
20.3 Silicon Carbide Grinding Disks, 240, 320, 400, and w/o ~ U,Gd! O 2 5 100 2 wt% free UO2
600-grit.
20.4 Diamond Paste, 7, 3, and 1-µm grains.
2 wt% free Gd2 O 3 S wt% Gd2 O 3
100 D (3)

20.5 Color Film. 24. Precision and Bias

21. Sample Preparation 24.1 This test method is subjective, and inadequate data are
available for a statistical evaluation.
21.1 Cut the fuel pellet in the longitudinal axis, somewhat
off center, using a low-speed diamond saw.
21.2 Cast the larger pellet part in cold-setting resin. CERAMOGRAPHIC DETERMINATION OF
AVERAGE GRAIN SIZE BY LINEAR INTERCEPT
21.3 Grind the cast specimen on a polisher/grinder table AFTER CHEMICAL ETCHING
using successively, silicon carbide in four stages from 240 to
600 grit; then polish with diamond pastes in stages with 7, 3, 25. Scope
and 1-µm grains.
25.1 This test method covers the average grain size deter-
21.4 Clean the specimen in an ultrasonic cleaner after each mination of (U,Gd)O2 pellets after ceramographic preparation
stage of grinding and polishing. and chemical etching. The evaluation of the average grain size
is done by a linear intercept method.
22. Sample Analysis
22.1 Generate a weak grain surface by applying a 1-min 26. Summary of Test Method
wiping etch to the polished surface with hydrogen peroxide- 26.1 A (U,Gd)O2 pellet is cut, ground and polished to
sulfuric acid solution. obtain a smooth pellet surface. After these ceramographic
22.2 Flush the sample with water, then alcohol, and dry with preparation steps, the revelation of the grain borders is ob-
warm air. tained by chemical etching using a hydrogen peroxide-sulfuric
acid solution. Finally, the average grain size is determined
22.3 Dip the specimen into a 30 % hydrogen peroxide
using a linear intercept method.
solution. (Eventually heat the specimen to 45°C and dip it into
the etch solution maintained at 45°C). 27. Apparatus
NOTE 2—Over-etched specimens must be reground and polished.
27.1 Diamond Saw, low-speed, with water cooling system.
22.4 Allow the etch to develop for 3 min: 45 s. Touch the
top of the peroxide solution every 30 to 40 s to dispel bubbles 27.2 Ultrasonic Cleaner.
formed on the specimen surface. 27.3 Polisher/Grinder Table.
22.5 Flush the sample with water, clean ultrasonically in 27.4 Metallurgical Microscope, with camera attachment,
alcohol, and dry with warm air. eventually equipped with image analysis system.
22.6 Examine and photograph the samples under a metal-
lurgical microscope at a magnification of 100× in white light 28. Reagents and Materials
(bright-field illumination with no filters). 28.1 Hydrogen Peroxide (30 %).

5
 C968 − 12
28.2 Hydrogen Peroxide–Sulfuric Acid Etch Solution—One 30.2 Flush the sample with water, then alcohol, and dry with
part 30 % hydrogen peroxide mixed with nine parts concen- warm or pressurized air.
trated sulfuric acid. 30.3 Examine the samples under a metallurgical microscope
28.3 Silicon Carbide Grinding Disks, 120, 320, and 600-grit at a magnification of 100 to 1000× in white light (bright-field
or Resin bounded diamond discs with similar abrasive effect. illumination with no filters) depending on the grain size. Take
28.4 Polishing Cloth, for fine polishing. photographs if requested for the project or for the measure-
ment.
28.5 Diamond Paste, 3-µm grains or similar.
31. Evaluation
28.6 Cold setting resin, two or three components, polyester,
epoxy, or acrylic based. 31.1 Draw an arbitrary, straight line on the analysed area of
the pellet surface and count the number of intersections N
28.7 Alcohol, industrial grade. where the straight line cuts the grain boundaries. Adopt the
magnification of the microscope or length L of the linear line
29. Sample Preparation in order to obtain at least 50 intersections.
29.1 Cut the fuel pellet along the longitudinal axis, some- 31.2 Calculate the average grain size at this area by dividing
what off center, using a low-speed diamond saw. the length of the straight line for exactly N intersections (in
29.2 Cast the larger pellet part in cold-setting resin. µm) by the number of intersections N.
29.3 Grind the cast specimen on a polisher/grinder table in 31.3 Perform this calculation in at least three different areas
three or more stages from 120 to 600 grit. After every stage, the (at the border, at 1/4th distance, and in the middle) of the pellet
scratches from the previous stage must be disappeared. surface to take into account a possible heterogeneity in grain
29.4 Polish in one or more stages with diamond paste size at different areas in the pellet.
having 7 to 1 µm grains on a short fibre polishing cloth until a 31.4 Calculate the average grain size of the pellet by
smooth pellet surface is obtained. averaging all results of average grain size at the different
29.5 Clean the specimen, eventually in an ultrasonic measured areas in the pellet.
cleaner, after each stage of grinding and polishing. 31.5 Besides the linear intercept method as explained in this
method, also any other evaluation method described in Test
29.6 After preparation, check the polished surface under the
Methods E112 may be used.
microscope. If scratches are present, disturbing the
measurement, the grinding/polishing steps can be repeated. 32. Precision and Bias
32.1 In order to get an estimation of the precision of the
30. Sample Analysis method, repetitive grain size measurements were performed at
30.1 Immerge the pellets in an etching bath containing the the same area of a pellet surface with homogeneous grain size
hydrogen peroxide-sulfuric acid solution for about 5 min.Other distribution. The relative standard deviation of a single deter-
etching solutions and etching times may also be effective to mination can be estimated at 15 % relative at 95 % confidence
reveal the grain borders. level for grain sizes from 5 to 35 µm.

REFERENCES

(1) Rodden, C. J., “Analysis of Essential Nuclear Reactor Materials,” NEDO-12024, March 1969.
USAEC, 1964, p. 749. (4) Hammerschmidt, H., and Saiger, S., “Determining Homogeneity of
(2) Lathouse, S., et al, Analytical Chemistry, Vol 31, 1959, p. 1606. (U,Gd)O2 Mixed Oxide Pellets,” Conference on Characterization and
(3) Rooney, D. M., “Ceramographic Technique for Revealing Inhomoge- Quality Control of Nuclear Fuels, Karlesruhe, Germany, June 13–15,
neity in UO2 Specimens with Small Additions of Selected Oxides,” 1978.

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