Page 71

1

, ! i

! !

47

I

j

i i j

I

j

6.1.3 Examination of Surface Samples

by X-Ray Diffraction Previous attempts to determine the chemical form of fission products deposited on graphite surveillance specimens by x-ray reflection from flat surfaces failed to detect any element except graphitic carbon. A sampling method which concentrated surface impurities was tried at the suggestion of Harris DUM of the Analytical Chemistry Division. This method involved lightly brushing the surface of the graphite stringer with a fine Swiss pattern file which had a curved surface. The grooves in the file picked up a small amount of surface material which was transferred into a glass bottle by tapping the file on the lip of the bottle. In this way samples were taken at the top, middle, and bottom of the graphite stringer from the fuel channel surface, from the surface in contact with graphite, and from the curved surface adjacent to the control rod thimble. Samples were also taken of the “dirty” graphite and salt on the top end surface of the stringer. Although each sample contained only about a dozen tiny particles and weighed less than 0.1 mg, the various samples read 1 to 4 R/hr at a distance of 6 in. with a soft-shelled “Cutie Pie” outside the glass bottle wall. This radiation field made transfer of the particles into glass capillaries for powder x-ray diffraction examination a difficult operation. However, three (0.2 m) capillaries were packed and mounted in holders which fitted into the x-ray camera. Short-exposure x-ray photographs were taken with precautions to minimize blackening of the film by the radioactivity. In spite of the precautions, samples from the fuel channel surfaces yielded very dark films which were difficult to read. Many weak lines were observed in the x-ray patterns. Since other analyses had shown Mo; Te, Ru, Tc, Ni, Fe, and Cr to be present in significant concentrations on the graphite surface, these elements and their carbides and tellurides were searched for by careful comparison with the observed patterns. In all three of the graphite surface samples so far analyzed, most of the lines for Mo2C and Ru metal were certainly present. For one sample, most of the lines for Cr7C3 were seen. (The expected chromium carbide in equilibrivm with excess graphite is Cr3C2, but nearly half the diffraction lines for this compound were missing, including the two strongest lines.) Five of the six strongest lines for NiTe, were observed. Mo metal, Te metal, Tc metal, Cr metal, CrTe, and MoTe, were excluded by comparison of their known pattern with the observed spectrum. These observations (except for that of Cr7C3) are in gratifying accord with

expected chemical behavior and are significant in that they represent the first experimental identification of the chemical form of fission products known to be deposited on the graphite surface. Several hot cell techniques were tried for mounting the x-ray diffraction samples in glass capillaries or on quartz fibers, but they proved unsuccessful. To avoid undue radiation exposure to analytical chemistry personnel, the remaining samples were not analyzed. 6.1.4 Examination with a Gamma Spectrometer

In order to provide an inexpensive guide for later radiochemical work with milled stringer samples, a simple technique was developed (with the help of E. I. Wyatt and H. A. Parker of the Analytical Chemistry Division) for taking gamma scans of a number of locations on the cross-section samples of the graphite stringer. The sample, approximately 2 X 2 X 0.050 in., was placed over a 6 -in.diam hole through the center of a large face of a 4 X 4 X 8 in. lead brick. The hole was positioned over the center of the NaI crystal of a gamma spectrometer. The sample was shielded above by a 3/4-in.-thick sheet of plate glass with a scratched cross on the bottom surface positioned above the hole in the lead brick. By means of a 12-in. plastic ruler taped to the specimen, any desired region of the graphite cross section could be positioned over the hole in the lead brick for gamma scanning. With this relatively rapid technique, 18 to 20 positions were gamma scanned on each of the cross-section samples from the top, middle, and bottom of the graphite stringer. The distribution of the four main activities observed (106R~,’12SSb, 134Cs,and I3’Cs) in the three samples is shown graphically in Fig. 6.1. The nhmbers on each map are counts per minute above background and are proportional to the radioactivity at each position for a given isotope. (For comparison between isotopes, counts per minute are not proportional to disintegrations per minute because of different counting efficiencies.) In broad outline, the results were as expected. Heavy surface depositions of lo6Ru and lZsSb were observed with little penetration to the interior. The Cs isotopes were found at deeper locations since they possessed gaseous precursors (5.27day Xe and 4.2-min 37Xe). The volume of graphite scanned by this technique was quite small 6 in. diam X 0.050 in.); this permitted a more fine-grained look at fission product distribution than was possible by the milling techniques previously used for sampling graphite surveillance specimens. Correspondingly, the surface concentrations of all

ORNL-4728  

http://www.energyfromthorium.com/pdf/ORNL-4728.pdf

ORNL-4728  

http://www.energyfromthorium.com/pdf/ORNL-4728.pdf

Advertisement