Masters Theses

Keywords and Phrases

Gamma Tomography; Isotops; Void fraction


"Pebble Bed Reactors offer a future for new nuclear energy plants. They are small, inherently safe, and can be competitive with fossil fuels. The fuel forms a randomly stacked pebble with non-uniform fuel densities. The thermal-mechanical behavior of pebble bed reactor core is depends strongly on the spatial variation of packing fraction in the bed and in particular on the number of contacts between pebbles, and between the pebbles and the blanket walls. To investigate these effects, experimental data to characterize bed structure are needed along with other numerical simulation and computational tools for validation.

In this study, a powerful technique of high-energy gamma-ray computed tomography (CT scanner system) is employed for the first time for the quantification of the structure of pebble bed in term of the cross-sectional time-averaged void and distributions, it radial profiles and the statistical analysis. The alternative minimization (AM) iteration algorithm is used for image reconstruction. The spatial resolution of the CT scan is about 2 mm with 100 x 100 pixel used to reconstruct the cross-sectional image. Results of tomography with this advanced technique on three different pebble sizes at different axial levels are presented. The bed consisted of a glass spheres (Marbles) with a diameter d1= 1.27 cm, d2= 2.54 cm and d3= 5 cm in a Plexiglas cylinder with diameter D = 30.48 cm (D/d1 = 24, D/d2 = 12 and D/d3 = 6), and had an average void fraction ε̄1= 0.389, ε̄2= 0.40 and ε̄3 =0.43, respectively. The radial void fraction profile showed large oscillations with the bigger pebble diameters and the void fraction is higher on the wall with a minimum void fraction of 0.33 at 0.68 pebble diameter away from the wall. It was found that the void distribution in random packed bed depends strongly on the pebble diameter with respect to the bed diameter (D/dp) and the packing mode. The oscillation is quiet large with the smaller aspect ratio (D/dp) and decreases as the aspect ratio increases (D/dp). It has been shown that increasing the bed height has no influence on the radial void fraction at the three levels of the bed. It can be seen that there is an agreement between the experimental results and the exponential expression model at the smaller sphere diameter (D/dp=24). Comparison between the experimental and calculated methods were presented and discussed"--Abstract, page iii.


Al-Dahhan, Muthanna H.

Committee Member(s)

Mueller, Gary Edward, 1954-
Lee, Hyoung-Koo


Nuclear Engineering and Radiation Science

Degree Name

M.S. in Nuclear Engineering


Nuclear Energy Research Initiative (U. S. )


Missouri University of Science and Technology

Publication Date

Fall 2014


xiii, 84 pages

Note about bibliography

Includes bibliographical references (pages 78-83).


© 2014 Fadha Shakir Ahmed, All rights reserved.

Document Type

Thesis - Open Access

File Type




Subject Headings

Pebble bed reactors -- Computer simulation
Nuclear reactors -- Fluid dynamics -- Mathematical models

Thesis Number

T 10571

Electronic OCLC #