Ion-irradiation of Nuclear Grade NBG-18 and Highly Ordered Pyrolytic (HOPG) Graphites

Principal Investigator
Name:
Korukonda Murty
Email:
[email protected]
Phone:
(208) 526-6918
Team Members:
Name: Institution: Expertise: Status:
Dr Jacob Eapen North Carolina State University Science-based modeling and simulation of nuclear fuel for advanced reactors, nano-structured materials, plasma-material interactions, energy transport at nano-scales and interfaces, modeling of diffusion and viscosity in glass matrices for radioactive waste management. Faculty
Dr Ram Krishna North Carolina State University Characterization techniques such as E-SEM, FEG-SEM, TEM & HRTEM, AFM, focused ion beam (FIB), electron backscattered diffraction (EBSD), X-ray diffraction (XRD), Raman Spectroscopy and modeling tools such as Thermo-Calc, and JMatPro Post Doc
Experiment Details:
Experiment Title:
Ion-irradiation of Nuclear Grade NBG-18 and Highly Ordered Pyrolytic (HOPG) Graphites)
Work Description:
The primary aim of this study is to irradiate NBG-18 and HOPG samples (3 mm disks) with 2MeV C+ ions at the NSUF Tandem Accelerator Ion Beam Facility at the University of Wisconsin, Madison. In the proposed set of experiments, a total of ten samples of NBG-18 and HOPG (5 samples each), are planned to be irradiated at temperatures of 300K, 600K and 900K at two doses, 1 and 25 dpa, in the accelerator facility. The main objectives of the experiments are: 1) Determine the critical dpa for the turnaround behavior in the dimensional change at high temperatures (600K and 900K) with C+ ion irradiation on NBG-18/HOPG samples. 2) Compare the low fluence (1 dpa) dimensional change behavior at three different temperatures (300K, 600K and 900K) with C+ ion irradiation on NGB18/HOPG samples. The proposed tasks are listed in the test matrix shown below in Table 1. Table 1: C+ Ion Irradiation Test Matrix for Graphite Samples Run ID Ions Ion Energy (MeV) Temp (K) Dose (dpa) Specimens 1 C+ 2 300 1 NGB-18, HOPG 2 C+ 2 600 1 NGB-18, HOPG 3 C+ 2 900 1 NGB-18, HOPG 4 C+ 2 600 25 NGB-18, HOPG 5 C+ 2 900 25 NGB-18, HOPG
Abstract
Graphite is currently proposed for neutron moderation and structural support for the next generation high temperature reactors. Radiation creep is arguably the most important of the radiation effects that occur in graphite. A reactor grade graphite shows shrinkage followed by a positive dimensional change beyond a critical neutron fluence (dpa) which generally decreases with increasing irradiation temperature. At room temperature (300K) significant swelling is observed along the c-axis with smaller swelling rate along the a-axis possibly due to plane destruction and/or vacancy line/loop collapse. At higher temperatures pores and cracks that are formed during the processing stage collapse thus accommodating the enhanced swelling along the c-axis resulting in an overall shrinkage of volume. Beyond a critical dpa, a ‘turnaround’ behavior is observed indicating a leveling behavior for the shrinkage of pores and cracks. Funded by NEUP, neutron irradiation and creep investigations are being performed on NGB-18, a nuclear grade graphite that has been down selected for the next generation, very high temperature, gas cooled reactors. Given the irradiation time that spans several months for neutron irradiation, a plausible alternative for faster testing is by conducting experiments with ion irradiation that can speed up the damage rate by orders of magnitude. In the past neutron and ion damage has been reasonably correlated for several metallic structural materials, however, not for graphite. As the microstructure and irradiation damage mechanisms are significantly different from those in metals, a correlation between neutron and ion damage needs to be first established for proper interpretation of ion irradiation results. If a strong correlation is established, damage results from future ion irradiation tests that can be completed in days in a university setting can be correlated to neutron damage with reasonable confidence.
Book / Journal Publications

"Early Damage Mechanisms in Nuclear Grade Graphite under Irradiation" Jacob Eapen, Ram Krishna, Timothy Burchell, Korukonda Murty, Materials Research Letters 2 2013 43-50 Link

"ATR National Scientific User Facility 2013 Annual Report" Sarah Robertson, Julie Ulrich, INL/EXT-14-32057 2013 Link