Adsorption Behaviors of Cobalt on the Graphite and SiC Surface: A First-Principles Study

Wenyi Wang,Chao Fang,Chuan Li,Jianzhu Cao

doi:10.1155/2017/8296387

Abstract

Graphite and silicon carbide (SiC) are important materials of fuel elements in High Temperature Reactor-Pebble-bed Modules (HTR-PM) and it is essential to analyze the source term about the radioactive products adsorbed on graphite and SiC surface in HTR-PM. In this article, the adsorption behaviors of activation product Cobalt (Co) on graphite and SiC surface have been studied with the first-principle calculation, including the adsorption energy, charge density difference, density of states, and adsorption ratios. It shows that the adsorption behaviors of Co on graphite and SiC both belong to chemisorption, with an adsorption energy 2.971 eV located at the Hollow site and 6.677 eV located at the hcp-Hollow site, respectively. Combining the charge density difference and density of states, it indicates that the interaction of Co-SiC system is stronger than Co-graphite system. Furthermore, the variation of adsorption ratios of Co on different substrate is obtained by a model of grand canonical ensemble, and it is found that when the temperature is close to 650 K and 1700 K for graphite surface and SiC surface, respectively, the Co adatom on the substrate will desorb dramatically. These results show that SiC layer in fuel element could obstruct the diffusion of Co effectively in normal and accidental operation conditions, but the graphite may become a carrier of Co radioactivity nuclide in the primary circuit of HTR-PM.

Highlights

Cobalt-60 (60Co) is a kind of long half-life γ-ray radionuclide and it could be generated through activation reaction of impurities (59Co and 60Ni) in the material of fuel elements and metal/nonmetal reactor internals of High Temperature Reactor-Pebble-bed Modules (HTR-PM)
In the primary circuit of HTR-PM, 60Co on the surfaces of fuel elements, graphite reflectors, graphite reactor internals, and metal reactor internals will adsorb on the graphite dust, which is generated through abrasion or corrosion effect when the fuel elements flow in the primary circuit and is playing a significant role in contributing to the source term of HTR-PM [2]
The first-principle calculation is known as ab initio calculation; it has been performed based on density functional theory (DFT) as implemented in the VASP code and employed the projector augmented wave (PAW) pseudo-potential [32] and generalized gradient approximation (GGA)-PBE exchangecorrelation functional [33] to describe the interaction of electron-ion

Summary

Introduction

Cobalt-60 (60Co) is a kind of long half-life γ-ray radionuclide and it could be generated through activation reaction of impurities (59Co and 60Ni) in the material of fuel elements and metal/nonmetal reactor internals of High Temperature Reactor-Pebble-bed Modules (HTR-PM). The first-principle calculation, a powerful tool to study on an atomic scale, provides a way to research the above issue [4,5,6,7]. Based on the density functional theory (DFT) [8], the first-principle calculation is implemented in the Vienna Ab initio Simulation Package (VSAP) by the group of Kresse et al [9,10,11], which could be used to study the atom-material interaction in microlevel [12,13,14]. A number of theoretical studies about the adsorption behaviors using first-principle calculations, especially

Methods

Results

Conclusion