Studies On Irradiation Damage And Molten Salt Penetration In Pyrolytic Carbon Coating On Nuclear Graphite

The Molten Salt Reactor (MSR) is one of the six reactors proposed by the Generation IVNuclear Energy System, with advantages of inherent safety, effective utilization of nuclearpower and nuclear nonproliferation, unique fuel cycle capabilities, economics, and so on.Nuclear graphite has been used as neutron moderator and structural material in MSR due to itslow Z, high moderating ratio, high thermal conductivity, mechanical strength and etc. Nucleargraphite is porous material. In MSR, molten salt flows through the nuclear graphite channels.Molten salt and gas fission product (primary, Xe-135) maybe diffuse into nuclear graphite, whichmaybe effect mechanical, thermal and irradiation properties of nuclear graphite, and so on.Therefore, we cooperated with the Institute of Metal Research (IMR), Chinese Academy ofSciences (CAS) to prepare pyrolytic carbon (PyC) coating to seal nuclear graphite. In this thesis,the structures and properties of PyC coating have been studied before and after ion irradiationand static FLiNaK molten salt experiments.In this thesis, synchrotron-based grazing incidence X-ray diffraction (GI-XRD),field-emission scanning electron microscopy (FE-SEM), polarized light microscopy(PLM) andhigh resolution transmission electron microscopy (HRTEM) were used to study themicrotopography, degree of graphitization, distribution of pore diameter in PyC coating.Meanwhile, static FLiNaK molten salt experiments and helium gas permeability were performed.Results show that PyC coating prepared by thermal chemical vapor deposition (CVD) exhibitsanisotropic with nano pores. There is no obvious crack in PyC coating, which make the gaspermeability of IG-110nuclear graphite decreased to less than10-8cm2/s. Therefore, it’s possiblefor PyC coating to decrease the diffusion of gases into nuclear graphite.In the core of MSR, nuclear graphite with PyC coating is irradiated by neutron and fissionproducts in molten salt. In this thesis, ion beam irradiation was used to induce defects in PyCcoating. FE-SEM, synchrotron-based GI-XRD, Raman, XPS and TEM were employed to studythe microstructure of PyC before and after ion irradiation. Results reveal that300keV Ar+ionirradiation leads to the structural deterioration along c axis and the increase of interlayerspacing of graphite. The damage depth in PyC coating increases with the increase of irradiationdose and reaches the maximum at the peak damage dose of0.26DPA. The surface crystalstructure in the irradiation damage zone of PyC coating tends to disorder with the increase ofirradiation dose and amorphous at the peak damage dose of1.58DPA. Static FLiNaK molten salt experiments, FE-SEM, Raman spectroscopy, near-edge X-rayabsorption fine structure (NEXAFS) spectroscopy and XPS were performed to study the effect ofirradiation defect on the compatibility of PyC coating with FLiNaK salt. Results reveal that thereare C-F bond forming in the PyC coating after FLiNaK experiment. And results suggest thatirradiation defects maybe increase the fluorination of PyC coating in FLiNaK salt.