Modeling evolution of electronic structures of nuclear waste form within density functional theory.

摘要

In view of the importance of the storing issues concerning nuclear waste form, and the relative experimental difficulties, such as the extreme conditions, or the long experimental periods, it is of high necessary to provide reliable theoretical calculations of the evolution of structures and electronic properties during its storage. In the present work, we have performed electronic structure calculations for both ThxU 1--xO2 and Cs1-- xBaxCl compounds within Density Functional Theory approach, which are of long standing interest to nuclear industry.;In the chapter 2, we carried out theoretical descriptions of 2x1x1, 2x2x1 and 2x2x2 super cells for ThxU 1--xO2 compounds with and without the appearance of O interstitial. Based on our simulations, we could see that Th can cause a large volume swelling, whereas the O interstitial introduces a decrease in this tendency. And the doping amount of Th in our modeling systems has a positive effect on the volume shrink. Furthermore, the negative formation energy of the O interstitial implies that the O interstitial has a lower energy than the O2 molecule in our simulations. And we also noticed a gradual decrease in the values of formation energies with decreasing size of the super cell, which indicates that the better resistance to oxidation with the increasing amount of Th in ThxU 1--xO2 compounds. In order to explain this phenomena, we analyzed the effect of doped Th in the modeling systems as well. Although our results suggest that with the presence of one O interstitial, the constitution of valence band and conduction band, and the band gap of ThxU1-- xO2 compounds do not shift significantly, the modeling systems show an n-type semiconductor character, instead of the p-type semiconductor reported previously.;In the chapter 3, we performed electronic structure calculations of Cs-waste form. In terms of our simulations, we observed that the volume of Cs-waste form starts to swell since the begin of the decay reaction. However, due to phase transformation from cesium chloride (B2) to rocksalt (B1), the whole system has a significant volume shrink. And the following decay process causes a parabolic volume variation with the increasing amount of Ba+. We also studied the evolution of electronic properties of Cs-waster form. A series of Cs1--xBa xCl compounds and the unit cell of CsCl in B2 structure and the metastable compound BaCl in B1 structure were investigated. We noticed that the band gap of the Cs1--xBa xCl compound decreases with the appearance of Ba+. Corresponding to the phase transformation at x = 12:5%, the band gap would jump to a higher value. Then, with the increasing amount of Ba+ in the following decay process, the band gap would decrease until this decay system shows a metallic property. Our simulation demonstrates the complete evolution of Cs-waster form during the decay process.