On May 11, 2015, at the invitation of Professor Wu Hongchun of The School of Nuclear Science and Technology affiliated to the College of Energy and Power Engineering, Professor Barry D Ganapol from the University of Arizona came to Xi'an Jiao Tong University for a three-week teaching and academic visit. On the morning of May 19th, he delivered an academic report entitled "Nuclear Power and Apple Pie". On The morning of May 26, he gave an academic report entitled "The Analytical Solution to the 1D Multigroup Diffusion Equation in Heterogeneous Media".

The lecture focused on the Big Bang theory, and students of nuclear reactor physics was able to understand the source of neutrons and neutron-nucleon interaction. Professor Ganapol held students' attention from the very beginning, detailing what happened at various stages in the very first moments of the beginning of the universe. From 10^-44s to the present day, in the intervening years, the universe expanded, particles formed, fused, and evolved into the present universe. You can imagine how much fun it was to talk about the beginning of the universe in tens of billions of years from now. Then, the professor introduced the first reactor designed by human beings to achieve criticality. When the students marveled at the wisdom of human beings, the professor told them an interesting thing: The first critical reactor on the earth is not made by human. long before humans, somewhere in Africa, a natural reactor formed. The natural uranium enrichment degree can reach 5%, so rely on natural water moderated reactor critically run for thousands of years. Now the local natural uranium enrichment degree is only 0.5% - 0.6%, Lower than normal natural uranium, precisely because it has already been "burned up". After two hours' lecture, the students heard a lot of information they had never heard before, which greatly broadened their horizons.

The presentation on 26th was on the analytical solution of one-dimensional diffusion equation, which needs to be studied in a single course in reactor physics. For the one-dimensional diffusion equation, Professor Ganapol pointed out that we can solve the equation by thinking of analytical solution, and proved the existence of analytical solution in the following discussion. The professor first analyzed the simplest problem of one-dimensional single group single medium. For this kind of problem, only one diffusion equation can be obtained by adding boundary conditions on both sides of the medium. Then the professor analyzed the problem of one-dimensional single group two media. For this kind of problem, there is an equation in each medium, and the final result can be obtained by adding the boundary conditions on both sides and the condition that the neutron current and neutron flux at the interface of the two media are equal. In this process, the general expressions of one-dimensional single group single medium and one-dimensional single group double medium solutions are derived. Subsequently, the professor extended the problem to one-dimensional multi-group multi-media, and derived solutions with the same general formula. Finally, the professor designed a series of benchmark problems to prove the correctness of the method, and proved that the general solution can be applied to all one-dimensional models.

After the two reports, the teachers and students had a heated discussion with the professor on some issues, and the in-depth communication benefited everyone a lot. At the same time, the way of teaching step by step, focusing on thinking rather than formula, is also worth learning.