Micro to nanostructural observations in neutron irradiated nuclear graphites PCEA and PCIB

H.M Freeman,B.E Mironov,W Windes,M.M Alnairi,A.J Scott,A.V.K Westwood,R.M.D Brydson

doi:10.1016/j.jnucmat.2017.05.011

Abstract

The neutron irradiation-induced structural changes in nuclear grade graphites PCEA and PCIB were investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), selected area electron diffraction (SAED) and electron energy loss spectroscopy (EELS). The graphite samples were irradiated at the Advanced Test Reactor at the Idaho National Laboratory. Received doses ranged from 1.5 to 6.8 displacements per atom and irradiation temperatures varied between 350 °C and 670 °C.XRD and Raman measurements provided evidence for irradiation induced crystallite fragmentation, with crystallite sizes reduced by 39–55%. Analysis of TEM images was used to quantify fringe length, tortuosity, and relative misorientation of planes, and indicated that neutron irradiation induced basal plane fragmentation and curvature. EELS was used to quantify the proportion of sp2 bonding and specimen density; a slight reduction in planar-sp2 content (due to the buckling basal planes and the introduction of non-six-membered rings) agreed with the observations from TEM.

Highlights

Graphite is a key component in some designs of current and future civil nuclear reactors whose lifetimes are heavily dependent on the physical and chemical performance of the graphite
Since the PCIB specimens investigated are at temperatures and, doses below this it is reasonable to assume none of the samples have undergone turnaround, i.e. they are in the shrinkage phase
A significant variation in pore size, shape and orientation can be observed between PCEA and PCIB; the shape and orientation of porosity can often indicate whether a pore is within the filler particles or binder matrix

Summary

Introduction

Graphite is a key component in some designs of current (e.g. advanced gas cooled) and future (e.g. high temperature gas-cooled) civil nuclear reactors whose lifetimes are heavily dependent on the physical and chemical performance of the graphite. After 3e7 dpa of neutron irradiation, crystallite fragmentation resulted in a 60% (La) and 70% (Lc) reduction in coherence lengths. Studies using electrons and ions to induce radiation damage have improved our understanding of dislocation structures and local bonding in irradiated graphites [12,17e21]. This is useful because the availability of data for neutron irradiated specimens is limited. Recent work by Karthik et al presented the microstructural changes occurring in graphite grades NBG-18 and IG-110 as a result of neutron irradiation [8]. The present work aims to extend this to the study of micro to nanostructural changes in neutron irradiated PCEA and PCIB nuclear graphites

Experimental

Polarized light microscopy

PLM and SEM

G FWHM cmÀ1

Conclusions