Microstructural Evolution and Radiation Effects of Uranium-Bearing Diffusion Couples

Abstract

Metallic nuclear fuels, as potential candidates in advanced reactors development, have been extensively studied for over five decades. The main challenges are the fuel performance designed for higher burn-up and higher temperatures. Fuel-cladding chemical interaction, or FCCI, is one of the primary material problems during reactor operations. A series of tests using uranium-bearing fuel alloys and various cladding materials were performed to assess the diffusional interactions. However, the knowledge of thermally-activated multicomponent diffusion followed by irradiation tests is quite limited. Combined, both experimental and theoretical investigation are essential to predict the feasibility of fuel designs. The overall objective of this dissertation is to unravel the radiation effects in microstructural and kinetic data. In order to achieve this, two major diffusion systems are chosen.

For the first system, we start with two uranium-free diffusion couples to study Microstructural evolution. Primary experiments assembled zirconium/molybdenum With iron. Zr and Mo are the major constituents in fuel alloys and Fe is the surrogate for ferritic cladding materials. Both diffusion couples were annealed at 850°C for 15 days and irradiated with 3.5 MeV Fe^++ at 600°C. Post irradiation examination Involved scanning electron microscope (SEM) and transmission electron microscope (TEM). Through this work, we identified the phases formed in the interaction layers and showed enhanced diffusion in the ion bombarded regions. Additionally, the mechanism of intermetallics formation (e.g.Fe23Zr6) and radiation stability were discussed.

Second, a matrix of uranium-bearing couples is established. 1) Depleted uranium (DU) was bonded with polycrystalline iron to form binary diffusion couples followed by 2 MeV He^+ irradiation. Our intent is to understand microstructural information and thermokinetic data. 2) DU vs. single crystalline iron/nickel couples were assembled and annealed for various temperatures/time. Interdiffusion coefficients and activation energy are calculated for each phase formed in the interaction layers 3) In addition to solid-solid diffusion experiments, Fe/(Fe+Cr)/(Fe+Cr+Ni) were deposited on the DU substrates to form thin-film diffusion couples with concurrent 3.5MeV Fe++ ion irradiation. Rutherford backscattering spectrometry (RBS) was used to extrapolate interdiffusion profiles and intermetallic phase formation. These results provide strong evidence to support multiscale modeling of FCCI. In particular, goals of the modeling are to provide detailed analysis in fuel performance development.