Science and Technology of Nuclear Installations

Volume 2017 (2017), Article ID 8296387, 8 pages

https://doi.org/10.1155/2017/8296387

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

^{1}Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China^{2}Collaborative Innovation Center of Advanced Nuclear Energy Technology, Beijing 100084, China^{3}The Key Laboratory of Advanced Reactor Engineering and Safety, Ministry of Education, Beijing 100084, China

Correspondence should be addressed to Chao Fang; nc.ude.auhgnist@oahcgnaf

Received 19 October 2016; Revised 13 February 2017; Accepted 12 March 2017; Published 20 March 2017

Academic Editor: Arkady Serikov

Copyright © 2017 Wenyi Wang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### 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.

#### 1. Introduction

Cobalt-60 (^{60}Co) is a kind of long half-life *γ*-ray radionuclide and it could be generated through activation reaction of impurities (^{59}Co and ^{60}Ni) in the material of fuel elements and metal/nonmetal reactor internals of High Temperature Reactor-Pebble-bed Modules (HTR-PM). The behavior of ^{60}Co is important for the safety analysis and radiation protection design in HTR-PM and has attracted a lot of attentions. In the fuel element of HTR-PM, ^{60}Co in porous pyrolytic carbon (buffer layer) and dense inner pyrolytic carbon (IPyC) of TRISO fuel particle will diffuse and interact with silicon carbide (SiC) of TRISO fuel particle [1], which is considered to possibly influence the performance of SiC. In the primary circuit of HTR-PM, ^{60}Co 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]. It is reported that, in the end of lifetime of HTR-PM, the radioactivity of ^{60}Co in the primary circuit will be accumulated to 8.6 10^{10} Bq [3], while the activity concentration of other radionuclides is at least two orders of magnitude lower than that of ^{60}Co.

It is known that the study of interaction between Co and reactor material is essential but it is difficult to obtain the experimental result. Fortunately, the first-principle calculation, a powerful tool to study on an atomic scale, provides a way to research the above issue [4–7]. 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. [9–11], which could be used to study the atom-material interaction in microlevel [12–14]. A number of theoretical studies about the adsorption behaviors using first-principle calculations, especially for the graphite [15–19] and SiC nanotubes [20–27], have been published. Ancilotto and Toigo have performed first-principles total-energy calculations to study the adsorption of potassium on graphite [15]. Electronic structure calculations based on spin-polarized DFT with the generalized gradient approximation (GGA) and ultrasoft pseudopotentials are used to investigate the interaction between H atoms and a graphite surface [16]. The single Co atom adsorbed on some graphite materials also is discussed [28–31]. Wehling et al. have researched the orbitally controlled Kondo effect of Co adatoms on graphene [28]. Rudenko et al. have researched the adsorption of Co on graphene and analysis of the electron correlation effects from a quantum chemical perspective [29].

However, there is few works on the adsorption/desorption of Co on graphite/SiC and the mechanism of interaction is also not clear. In this work, the behavior of Co adsorbed on graphite and SiC surface with DFT will be studied, including the charge density difference (CDD) and the density of states (DOS). Furthermore, the mechanism of adsorption will also be discussed. At last, the variation of adsorption ratios of Co will be given by a model of grand canonical ensemble, which is significant for understanding the adsorption of Co on graphite and SiC macroscopically.

#### 2. Method of Calculation

##### 2.1. The Construction of Graphite and SiC Micromodel

The first-principle calculation is also known as ab initio calculation; it has been performed based on DFT as implemented in the VASP code and employed the projector augmented wave (PAW) pseudo-potential [32] and GGA-PBE exchange-correlation functional [33] to describe the interaction of electron-ion. Since a large number of basis functions are usually required to describe the electronic wave functions appropriately, it is very demanding to use the first-principles pseudo-potential method to calculate the carbon properties [15]. To simplify the calculations, we choose a single, isolated graphite monolayer with 6 6 primitive cells (Figure 1(a)), by using a -point mesh of 1 1 1, as the substrate to calculate the adsorption energy. According to the anisotropic character of bonding in graphite, the coupling between adjacent graphite layers is much weaker than the in-plane coupling between carbon atoms [14, 34, 35]. This means that the electronic properties of a monolayer can usually provide a reasonable picture of the electronic properties of the infinite crystal [36]. As shown in Figure 1(b), graphite is here represented by graphene; there are three different adsorption sites on the graphene including Top (T), Bridge (B), and Hollow (H). SiC used in TRISO fuel particle of fuel element in HTR-PM is -SiC and the corresponding structure is diamond cubic crystal. Different from pure diamond, there are four silicon atoms surrounding carbon in each SiC crystal. Therefore, the interaction between adjacent atoms in a crystal is very strong and makes the structure of SiC especially stable, which is conducive to SiC to block the activation products diffuse from the fuel particle. The supersurface of SiC(001) lattice with 3 3 1 primitive cells (Figure 2(a)) and the -point mesh as 2 2 1 are employed to calculate the adsorption energy. The four kinds of adsorptive sites are including Top (T), Bridge (B), hcp-Hollow (hH), and fcc-Hollow (fH) as shown in Figure 2(b).