Computational and experimental studies on novel materials for fission gas capture

Abstract

Materials in nuclear power system can suffer from thermal/hydrothermal, radiation and chemical degradation due to the high-temperature, high-pressure operation condition along with the presence of water steam and radiation. One particular topic we are addressing is understanding and optimizing materials for fission gas capture. Computational modeling is an efficient tool to investigate materials behaviour in such extreme environment. Westudied a number of materials. One of these is mesoporous silica. We used a combination of Molecular Dynamics (MD) simulation and Monte Carlo (MC) simulation which were validated by detailed experiments. MD simulations reveal the porous structure transformation under high-temperature treatment up to 2885 K, suggesting the pore closure process is kinetically dependent. Based on this mechanism, we predict with the presence of water, the pore closure activation energy will be decreased due to the high reactivity between water and Si-O bond, and the materials become more susceptible to high temperature. A fundamental improvement of the material hydrothermal stability thus lies in bond strengthening. MC simulations then were used to study the the adsorption and selectivity for thermally treated MCM-41, for a variety o f gases in a large pressure range. Relative to pristine MCM-41, we observe that high temperature treated MCM 41 with its surface roughness and decreasing pore size amplifies the selectivity of gases. In particular, we find that adsorption of strongly interacting molecules can be enhanced in the low-pressure region while adsorption of weakly interacting molecules is inhibited. We have also investigated alumina as an example of a ceramic material that can be directly incorporated into the nuclear fuel itself. Unlike uranium oxide fuel, certain phases of alumina have appreciable capacity for gas absorption. The limited diffusion distance of helium and other fission product gases in the fuel may be addressed by coating micron-sized fuel particles with alumina, prior to sintering, using a unique atomic layer deposition process suitable for particles. We have investigated the feasibility of this approach using a combination of helium-focused experiments on fuel surrogate particles, together with analytical calculations of gas production rates and diffusion distances in uranium oxide. Additional studies of nanotubes of carbon and boronitride elucidated fundamental mechanisms of the influence of curvature on gas adsorption.