Libin Sun

Microstructure and molten salt impregnation characteristics of a micro-fine grain graphite for use in molten salt reactors

Abstract The microstructure and molten salt impregnation characteristics of a micro-fine grain isotropic graphite ZXF-5Q from Poco Inc. was investigated. The microstructural characteristics of the pores caused by gas evolution, calcination cracks, Mrozowski cracks, and the crystal structure were characterized by optical microscopy, mercury porosimetry, helium pycnometry, transmission electron microscopy, X-ray diffraction and Raman spectroscopy. Results show that the ZXF-5Q has uniformly-distributed pores caused by gas evolution with very small entrance diameters (∼0.4 μm), and numerous lenticular Mrozowski cracks. Molten salt impregnation with a molten eutectic fluoride salt at 650 °C and 1, 3 and 5 atm, indicate that ZXF-5Q could not be infiltrated even at 5 atm due to its very small pore entrance diameter. Some scattered global salt particles found inside the ZXF-5Q are possibly formed by condensation of the fluoride salt steam during cooling.

A New Method to Measure Crack Extension in Nuclear Graphite Based on Digital Image Correlation

Graphite components, used as moderators, reflectors, and core-support structures in a High-Temperature Gas-Cooled Reactor, play an important role in the safety of the reactor. Specifically, they provide channels for the fuel elements, control rods, and coolant flow. Fracture is the main failure mode for graphite, and breaching of the above channels by crack extension will seriously threaten the safety of a reactor. In this paper, a new method based on digital image correlation (DIC) is introduced for measuring crack extension in brittle materials. Cross-correlation of the displacements measured by DIC with a step function was employed to identify the advancing crack tip in a graphite beam specimen under three-point bending. The load-crack extension curve, which is required for analyzing the R-curve and tension softening behaviors, was obtained for this material. Furthermore, a sensitivity analysis of the threshold value employed for the cross-correlation parameter in the crack identification process was conducted. Finally, the results were verified using the finite element method.

Seismic Test on Double-Layer Model of HTR-PM Graphite Structure

The HTR-PM (High Temperature Reactor Pebble bed Module) reactor core consists of assemblies that include graphite bricks, carbon bricks and keys/dowels. The double-layer structure, which is composed of two layers of bricks connected by keys and dowels, is the basic load-bearing unit of the graphite internals. A series of seismic experiments have been carried out to study the dynamic characteristic of this multi-body structure. As a part of them, the present study aimed to evaluate the nonlinear dynamic response of a full-size graphite/carbon unit and investigate the integrity of this assembly.Dynamic excitations were applied at the bottom of the double-layer structure. The experimental random-motion excitation ranged from 0.2g to 0.75g. The sine-sweep excitation frequency band ranged from 5–100Hz, sweep rate was 1oct/min, with acceleration magnitudes of 0.3g and 0.5g. The sine beat wave excitation frequencies were 5Hz, 10Hz, 20Hz and 40Hz with acceleration of 0.5g. Response data of acceleration and displacement at certain points on bricks were measured. PSD curves and displacement time-histories were acquired. Results indicated that the fundamental frequency of the assembly is 42∼49Hz and the damping rate is 4.9%. The dynamic response of the assembly exhibited as an integrated component only when the excitation is lower than 0.35g.Copyright