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Tensile strength of graphite is a crucial criterion in the design and structural integrity evaluation of graphite components in reactors. To investigate the tensile strength of domestic graphite, as well as the effect of specimen size, grain size and microstructure on tensile strength, three kinds of domestic graphites including coarse-grained, fine-grained and ultrafine-grained graphite that may be used in reactor were selected. The Brazilian disc tests of three graphites were performed on Φ6 mm×3 mm, Φ12.7 mm×6.35 mm, Φ24 mm×12 mm and Φ40 mm×20 mm Brazilian disc specimens in the WDW-50 tensile testing machine at a speed of 0.1 mm/min. Force sensor was used to measure the force in specimen. Photoelectric encoder controller was used to determine the displacement of specimen. The tensile failure process of specimens was observed by high speed camera test system. The failure morphologies were observed by SEM (scanning electron microscopy) to study crack propagation behaviors of specimens. The tensile strength and its distribution of graphites were compared, and the effects of specimen size, particle size, microstructure and other factors on tensile strength of graphite were studied. The results indicate that the splitting test of graphite meets the validity requirements of the Brazilian disc test. The larger the specimen size and particle size of graphite, the easier it is to induce secondary cracks and local compression failure. The tensile strength of graphite usually improves as specimen size increases, but the effect of specimen size and particle size should be considered comprehensively when specimen size is small. The specimen size effect of tensile strength becomes more conspicuous as graphite density decreases. Coarsegrained graphite’s tensile strength is the most sensitive to specimen size, finegrained graphite is the second and ultrafinegrained graphite is the weakest. The tensile strength of graphite decreases exponentially with the increase of particle size, showing significant particle size effect. The tensile strength of coarse-grained, fine graind and ultrafine-grained graphite increases sharply with the decrease of particle size. The dispersion of graphite’s tensile strength is noticeable, and it has an obvious correlation with graphite’s particle size. According to statistical analysis by using Weibull distribution model, the tensile strength dispersion of finegrained graphite is substantially better than that of coarsegrained graphite, and ultra-fine-grained graphite is better than fine-grained graphite. The microstructure of graphite has a significant effect on the tensile strength. There are many defects such as large primary pores in coarsegrained graphite, so its tensile strength is the smallest, followed by finegrained graphite, and ultrafinegrained graphite is the largest.

The high temperature gas-cooled reactor is one of the six types of the fourth-generation reactors recognized by the international nuclear energy community. It is a new reactor type supported by China’s major scientific and technological projects. The graphite core supporting structure is a discrete structure, and its structural dynamic response under seismic load is quite different from that of continuous structure, showing strong nonlinear and discrete characteristics of structural dynamics. Based on the seismic research of HTR-PM, this work designed and conducted a 1∶4 overall model seismic test of the HTR-PM graphite core supporting structure, and evaluated the seismic performance of the HTR-PM graphite core structure. In the seismic test of the 1∶4 overall model, the boundary conditions such as the graphite core structure and its external supporting were simulated and simplified as close to the actual core as possible. According to the experience of several previous seismic tests, this test also made some optimizations in the measurement of model acceleration, and improved the sampling frequency of the acceleration sensor. According to on-site observation, the control rod can fall smoothly under strong earthquake, which proves that its design strength is sufficient to cope with seismic excitation. When the model is dismantled, all graphite bricks remain structurally intact, and none of the graphite tenon-bonds are damaged. The bearing parts have obvious compression marks which are subjected to repeated alternating loads under seismic loads, but do not break, which prove that the design strength is sufficient to cope with seismic excitation. The Seismosignal software was used to process the test data, and the seismic performance of the graphite core was systematically studied. The dynamic characteristic data measured during the seismic test were analyzed, and the variation laws of the acceleration and displacement of the model were obtained: The acceleration value of the model increases with the height of the model; the higher the position of the brick, the larger the response amplitude; combined with the specific displacement data, it can be proved that both the T-direction and R-direction displacement values belong to the normal range, that is, it meets the requirements of the normal drop of the control rod. After studying various dynamic response characteristics and their distribution laws of the structure, it is concluded that the design of the high temperature gas-cooled reactor core supporting structure can meet the seismic design requirements.

Supercritical steam generator can further improve the power generation efficiency of high temperature gas-cooled reactor (HTGR). A supercritical steam generator design is proposed based on the experience of the subcritical once through steam generator of high temperature gas-cooled reactor-pebble bed module (HTR-PM) and the future development plan of HTGR. An one-dimensional steadystate thermalhydraulic code for the helical tube supercritical steam generator of HTGR was developed based on the thermal hydraulic code of HTRPM subcritical once through steam generator. The empirical correlations of convective heat transfer coefficients of supercritical fluids are different from that of subcritical fluids. Different empirical correlations of convective heat transfer coefficient for supercritical fluid were compared. Their influence on the thermal-hydraulic calculation results is found to be quite small due to the large thermal resistance of the primary side helium convection. The 100% load working condition and partial load working conditions were calculated using the developed code and the distributions of the parameters including temperature, velocity, convective heat transfer coefficient, etc., were obtained and analyzed. The secondary-side heat transfer coefficient reaches the highest value near the pseudo-critical point. The primary-side heat transfer coefficient increases with temperature because helium thermal conductivity increases with temperature. In the low temperature section with T22 as the tube material, the primary-side thermal resistance occupies more than 65%. In the high temperature section with 800H as the tube material, the thermal resistance of the primaryside and the tube wall are close. The thermal resistance of the secondaryside is always small. The secondaryside velocity varies intensely because of the severe change of density, while the primary-side velocity varies moderately. The hydraulic characteristic curves (pressure drop vs. flow rate) of different working conditions were calculated, and they all contain negative slopes which is not good for flow distribution and instability. The inlet throttling resistance coefficient of heat transfer tubes were added to avoid negative slopes of the curves. The critical throttling resistance coefficients of 100%-30% load working conditions are between 850-1600, and the value increases with decreasing power ratio.