Irradiation resistance study of binderless nanopore-isotropic graphite for use in molten salt nuclear reactors

Morphological and chemical changes in nuclear graphite target under vacuum and high-temperature conditions

Nuclear-grade graphite is a high-efficiency material, widely used for vacuum applications in nuclear reactors and accelerators as targets facing particle beams. In these contexts, graphite is often exposed to extreme thermal stresses altering its physical and chemical properties. The thermal-induced release of volatile contaminants from targets and the damage of structural components are critical issues that can affect the safety and operation efficiency of beamline facilities. Here, we provide for the first time a detailed picture of the chemical and morphological changes occurring in a nuclear-grade graphite target, obtained through Electrical Discharge Machining (EDM), when exposed in vacuum to high temperatures. The radial temperature gradient induced by the impact of a pulsed energetic (MeV- GeV range) focused particle beams was reproduced by cyclically heating, in the 1300-1800 K temperature range, a disc-shaped graphite target in a vacuum setup. An accurate surface and in-depth chemical analysis of the graphite target was obtained thanks to the high sensitivity (ppm/ppb) of the Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) technique. The chemical maps clearly show the presence of several metal oxides and impurities in the surface and subsurface regions of the untreated sample. Such contaminants were removed because of the thermal treatment in vacuum more or less efficiently, as demonstrated by Thermogravimetric analysis (TGA), X-ray Photoelectron Spectroscopy (XPS), and ToF-SIMS. However, Raman spectroscopy and SEM-EDS revealed that the high-temperature treatment induces a decrease in the crystallite size of the graphite as well as changes in the target surface porosity with the appearance of microvoids, leading the graphite target to be more prone to the breakage.

Real-Time Observation of Nanoscale Kink Band Mediated Plasticity in Ion-Irradiated Graphite: An In Situ TEM Study

Graphite IG-110 is a synthetic polycrystalline material used as a neutron moderator in reactors. Graphite is inherently brittle and is known to exhibit a further increase in brittleness due to radiation damage at room temperature. To understand the irradiation effects on pre-existing defects and their overall influence on external load, micropillar compression tests were performed using in situ nanoindentation in the Transmission Electron Microscopy (TEM) for both pristine and ion-irradiated samples. While pristine specimens showed brittle and subsequent catastrophic failure, the 2.8 MeV Au2+ ion (fluence of 4.378 × 1014 cm-2) irradiated specimens sustained extensive plasticity at room temperature without failure. In situ TEM characterization showed nucleation of nanoscale kink band structures at numerous sites, where the localized plasticity appeared to close the defects and cracks while allowing large average strain. We propose that compressive mechanical stress due to dimensional change during ion irradiation transforms buckled basal layers in graphite into kink bands. The externally applied load during the micropillar tests proliferates the nucleation and motion of kink bands to accommodate the large plastic strain. The inherent non-uniformity of graphite microstructure promotes such strain localization, making kink bands the predominant mechanism behind unprecedented toughness in an otherwise brittle material.

Direct characterization of ion implanted nanopore pyrolytic graphite coatings for molten salt nuclear reactors

Nanopore pyrolytic graphite coatings (PyC, average pore size ∼64 nm) were prepared on graphite to inhibit liquid fluoride salt and Xe¹³⁵ penetration. The samples were irradiated with 7 MeV Xe²⁶⁺ to a total peak dose of 0.1, 0.5, 2.5 and 5.0 displacements per atom at room temperature to study the irradiation resistance of the PyC. The effect of irradiation on the properties of the graphite was evaluated. With the increase of irradiation dose, the surface morphology of the coatings tends to be smoother. At the total peak dose of 2.5 dpa, peeling and spalling on the surface of the samples have been identified, indicating the surface microstructure of the graphite has been damaged by Xe²⁶⁺ bombardment. Raman results indicated the increase in the degree of disorder and decrease of in-plane crystallite size with the irradiation dose, and the new PyC was more sensitive to irradiation than IG-110 graphite. The nanohardness at peak dose increased with the irradiation dose, but decreased at 2.5 dpa. The results of a hardness test also show PyC has a higher irradiation sensitivity.

Nanopore pyrolytic graphite coatings (PyC, average pore size ∼64 nm) were prepared on graphite to inhibit liquid fluoride salt and Xe¹³⁵ penetration.