Atomistic and mesoscale simulations to determine effective diffusion coefficient of fission products in SiC

Abstract

The silicon carbide (SiC) layer in tristructural isotropic (TRISO) particles serves as the barrier to prevent escape of fission products produced in the fuel kernel. Knowing the diffusion coefficient of fission products through SiC is critical to determining whether fission gas can escape from the particle. It has been observed in experiments that Ag accumulated in grain boundaries and triple junctions in SiC. It is hypothesized that grain boundary diffusion is the primary pathway by which fission products penetrate the SiC layer. In this report, the effective diffusion coefficient of the fission product Ag through the grain boundary network is calculated using a combination of atomistic and phase-field methods. The grain boundary diffusion coefficient is calculated using molecular dynamics simulations. The bulk diffusion coefficient is determined using a combination of density functional theory and nudged elastic band methods. An effective diffusion coefficient is calculated, accounting for the grain structure using a phase-field method. The effective diffusion coefficient will be incorporated into Bison and fission product release calculations are compared to available experimental data.

Mechanistic FCCI model for BISON

“…In Ref. [5], initial calculations were performed to obtain the effective diffusivity of Nd. Although these calculations were useful to develop the methodology, the effective diffusivity function was fit to the data only for a temperature of 1000 K, and subsequent work showed that the polynomial that was used to fit this data was not readily extensible to other temperatures.…”

“…The microstructure is represented with a single order parameter 𝑐. The diffusivity of the bulk phase, represented by 𝑐 ≤ 0.1, was set to 𝐷 𝑏𝑢𝑙𝑘 = 1.58 × 10 −2 𝜇m 2 /s based on Nd diffusivity through 𝛼-U at 1000 K [5].…”

The Effective Diffusion Coefficient of Hydrogen in Tungsten: Effects of Microstructures From Phase-Field Simulations

“…One is that these two slabs are parallel to each other (Figure 1A), while the other one is that they are perpendicular to each other (Figure 1B). For the first structure shown in Figure 1A, the effective diffusion coefficient along the X and Y direction can be described analytically as (Hart, 1957;Jiang et al, 2021)…”

“…Despite atomistic simulations can give the microscopic mechanism of H interacting with defects, they are hard to predict the effective diffusion coefficient at the microstructural scale due to the limited spatial and time scales. Several analytical models have been developed to estimate the effective diffusion coefficient of the diffusion component in microstructures (Maxwell, 1881;Hart, 1957;Bakker et al, 1995;Chen and Schuh, 2007;Moradi, 2015;Jiang et al, 2021). Maxwell-Garnett et al (Maxwell, 1881;Moradi, 2015) developed a model to predict the effect of the volume fraction of the second phase on the effective diffusion coefficient in a two-phase system.…”

“…direction are the same because of the symmetric structure. Two different solutions derived by Jiang et al based on Eqs 28, 29 are respectively expressed as(Jiang et al, 2021) …”