الفهرس 
IntroductionOnly 14 pages are availabe for public view
 |  | from | 171 |  |  |
| | | from | 171 | | |
Abstract
Both a and y- spectroscopic measurements are used to determine the isotopic composition of uranium in several test samples of natural isotopic composition and others enriched in uranium-235, including the nuclear fuel material of the research reactor at Inshas EG-RR-1. This thesis is subdivided into three main chapters including, the introduction, the experimental and the results and related discussion.
Chapter-1, the introduction, includes a concise review about nuclear reactors including both the first and second Egyptian research reactors at Inshas, indicating in particular the mostly used fuel and clad materials, moderators and coolants. This chapter also includes an overview about the radiochemistry of uranium, the natural occurring decay series of both uranium-238 and uranium-235 as well as a number of other man-made uranium isotopes.
The third part of this chapter, deals with the different methods used for separation and purification of uranium and a concise summary of its different oxidation states, with special reference to the chemical behaviour of related compounds. Several publications are also reviewed about the different methods commonly used for separation of uranium by precipitation reactions, solvent extraction, ion exchange, volatilization and those based on electrochemical separation.
The last part of this chapter includes a literature review about alpha spectrometry and related applications for the determination of the radioactivity levels of uranium and its different radioisotopes in various types of environmental samples. It also includes a literature survey about non-destructive gamma- spectrometry and its use for determination of uranium in different matrix materials and in the determination of its
isotopic composition.
Chapter-2, the experimental, includes in its first part the standard
materials and test samples used, chemicals and reagents with related specifications as well as a full description of the nuclear fuel material of (EG-RR-1). It also includes the method used for preparation of uranium sources for a-spectroscopic measurements and the methods carried out to study the effect of different factors on electrodeposition of uranium, particularly: uranium concentration, time, voltage, pH and stirring rate. The second part of this chapter includes detailed description of a- ray spectroscopic system and the procedures carried out for energy calibration and for determination of detector’s efficiency. The methods used to determine the effect of geometry and counting time on count errors in spectroscopic measurements are also given. The third part of this chapter includes a brief description of y- ray spectroscopic system and the procedures performed for energy calibration and for determination of the photopeak detection efficiency of used detector and those used to study the effect of geometry and counting time on count errors in nuclear measurements. The last part deals with spectrophotometric determination of uranium through its arsenazo complexes from alkaline media. This will be referred to for determination of the separation yield of uranium in different test sources.
Chapter-3, results and discussion, includes in its first part the different variables which might affect the reliability of a-spectroscopic measurements of the absolute radioactivity levels of uranium-238, uranium-235 and uranium-234 in test samples. The measuring system was standardized through energy calibration using different standards with well known energy lines and the counting efficiency of used Si(Li) detector determined and proved to be 9.25 % at 1.5 cm from its surface. Several other factors including the counting geometry, system electronics and other operating conditions were kept the same for all test samples and used standard sources. Corrections for self absorption and internal layer emission effects were also considered. This part also includes a number of alternatives for the calculation of the absolute radioactivity levels of
different uranium isotopes based on respective measurements according to the following routes:
a)for determination of the absolute radioactivity levels of uranium-238 in different test samples, a mean correction factor (Fcm, = 1/ Eseif Ebeck = 2.67 ± 0.6) needs to be referred to, to correct for self absorption and back scattering, particularly when dealing with geometrical sources with concentrations ranging from 237 to 1896 lig uranium in each source. The mean value of Featt was experimentally determined by successive measurements of several uranium sources with different uranium sources with different uranium concentrations under same geometrical conditions.
b)for determination of the absolute radioactivity levels of uranium-234, the mean adjustment factor Wadi = F / F2 = 1.79 ± 0.33) needs to be applied to match the calculated radioactivity levels of uranium-234 comparable to the corresponding uranium-238 levels in each source. The radioactivity levels of uranium-234 will be further corrected for both self absorption and back scattering by using the mean value of F.,.= 2.61 ± 0.7. Both mean values of Fadj and F. for uranium-234 were experimentally determined with sources of different uranium concentrations, ranging from 237 to 1896 mg in each source.
The use of cc-spectroscopic measurements to determine the yield of uranium separated by electrodeposition in each source was experimentally verified, by calculating uranium concentrations from respective measured absolute radioactivity levels of uranium isotopes comparable to the original concentration in each source. The mean value of the deposition yield in the series of the experiments carried out proved to be z 94.8 ± 0.76 %, which assures negligible losses during preparation of test sources by electrodeposition, comparable to the mean value 95.3 %) obtained by spectrophotometric determinations. This further supports previous postulates and validates the applicable routes for the correction of measured radioactivity levels and subsequent quantitative measurements by cc-spectroscopic analysis.
material (being about 0.06%).
The following part of this chapter deals with isotopic composition determination by y-spectroscopy. In this part, the optimum experimental conditions for y-spectroscopic measurements were defined, through energy calibration of the measuring set up, using the same geometrical conditions for counting, determination of the photopeak detection efficiency of HPGe measuring system for a wide spectrum of y-energies using suitable counting periods that might lead to minimal count errors and by selection of certain y-ray energy lines with high relative intensities and/or those emitted from the parent uranium isotopes or from one of its different decay products which is not affected by disequilibrium effects. The results and related discussion are based on determination of the absolute radioactivity levels of different uranium isotopes in a sample with natural isotopic composition and in another sample from the nuclear fuel material used in EG-RR-1.
For natural uranium, the absolute radioactivity of uranium-238 determined through its daughter thorium-234, and based on the count rate and the photopeak detection efficiency determined at the 92.6 KeV y-energy line, considering its relative percent intensity; gave a mean radioactivity level 129 E2 Bq of uranium-238. For uranium-235, the absolute radioactivity of uranium-235 is determined through the count rate of the y- energy lines 163 KeV, 186 KeV and 205 KeV for uranium-235, the respective detection efficiencies of used detector and their relative percent intensities. The mean value of the absolute radioactivity level of uranium-235 proved to be within 5.6 E2 ± 0.2 E2 Bq. Based on these data and by considering that in natural uranium the radioactivity level of uranium-238 is almost equal to that of uranium-234; the isotopic composition of natural uranium gave uranium-238 = 99.325 %, uranium-235 = 0.679 % and uranium-234 = 0.006 %; which is in good agreement with theoretical values. For enriched uranium in the nuclear fuel material of EG-RR-1, the absolute radioactivity level corresponding to uranium
238, as determined through the 92.6 KeV y-energy line of thorium-234, is equivalent to 169.6 E2 Bq. Similar measurements based on the y-energy lines at 163, 186 and 205 KeV gave a mean value of the absolute radioactivity level of uranium-235 within 97.5 E2 f 15 E2 Bq. In the light of these experimental findings, the mean calculated isotopic composition of the enriched uranium in the nuclear fuel material of EG-RR-1, lies within 91.644 % for uranium-238, uranium-235 = 8.296 % and uranium-234 = 0.060 %.
In conclusion, table (1) presents a summary of the mean percent isotopic composition of uranium in naturally occurring samples and in the enriched uranium of EG-RR-1 by both a- and y-nuclear spectroscopic measurements. This obviously illustrates the high reliability of a-spectroscopic measurements elaborated in this thesis, after necessary corrections for self absorption and internal layer emission effects in geometrical sources; giving percent errors within s 0.01 - 0.082 % for uranium-238 and s 0.4 - 0.8 % for uranium-235 for natural and 10 % enriched uranium, respectively.