PGX Graphite Research Articles

Change in Electrical Resistivity of Nuclear Graphite and Carbon Materials during Low Cycle Fatigue

Resistivity change during room temperature fatigue test was measured for IG-11 and PGX graphites, and ASR-ORB carbon. Loading was carried out in the tension-compression mode at about 2.2Hz. The maximum applied stresseswere equal to each other on the tensile and compressive sides. Electrical resistivity was measured by four point contactmethod using copper needles stuck to the gage section of a specimen. It was found that for IG-11 and PGX the resistivityduring a fatigue cycle became maximum at the maximum compressive stress, being minimum at the maximum tensilestress. The trend was vice versa for ASR-ORB. This behavior was explained from thedifference in the resisitivitychange during tensile or compressive test between graphite and carbon. Increase inthe resistivity was observed withincreasing number of cycles, which was primarily attributed to the resistivity increase caused by tensile stresses.Curves for the resistivity increase as a function of number of cycles showed inflection points at which the rate ofresistivity increase started to be higher well before the actual fatigue failure. This implied a possibility that an in-situresisitivity measurement is to be applicable to prediction of fatigue failure.

Biaxial Strength and Fracture Criterion for HTGR Graphites

The strength and fracture criterion at the biaxial stress state were examined for two grades of HTGR graphites, a fine-grained isotropic IG-11 and a medium-grained semi-isotropic PGX. Two types of specimens different in size were tested. The biaxial stress state was realized using a servohydraulic testing machine in combination with an apparatus for applying internal pressure or torque force to a tubular specimen. Main results are: (1) On the basis of the statistical analysis of the data on the smaller IG-11 specimens, the maximum strain energy criterion gave the best fit to the data points both in the first and fourth quadrants of the failure surface. (2) The data on the larger specimens obtained at ORNL fell on the scatter band of the JAERI data, which indicated that no appreciable difference in the biaxial strength of IG-11 graphite was found despite the difference in the test fixture and specimen dimensions. (3) The maximum strain energy criterion was also believed to be most appropriate for PGX graphite for the first quadrant of the failure surface.

Biaxial Strength and Fracture Criterion for HTGR Graphites.

The strength and fracture criterion at the biaxial stress state were examined for two grades of HTGR graphites, a fine-grained isotropic IG-11 and a medium-grained semi-isotropic PGX. Two types of specimens different in size were tested. The biaxial stress state was realized using a servohydraulic testing machine in combination with an apparatus for applying internal pressure or torque force to a tubular specimen. Main results are: (1) On the basis of the statistical analysis of the data on the smaller IG-11 specimens, the maximum strain energy criterion gave the best fit to the data points both in the first and fourth quadrants of the failure surface. (2) The data on the larger specimens obtained at ORNL fell on the scatter band of the JAERI data, which indicated that no appreciable difference in the biaxial strength of IG-11 graphite was found despite the difference in the test fixture and specimen dimensions. (3) The maximum strain energy criterion was also believed to be most appropriate for PGX graphite for the first quadrant of the failure surface.

Burn-off and Production of CO and CO2in the Oxidation of Nuclear Reactor-Grade Graphites in a Flow System

The oxidation behaviors of IG-110 and PGX graphites were experimentally investigated in the temperature range from 500 to 1,500°C for the oxygen concentration of 10 and 30 wt% (1.37 and 5.09 mol%) and the reacting gas velocities of 6.63 and 19.9 m/s (estimated at 900°C). The graphite specimens were prepared to simulate the annular coolant channel in the core of the HTGR. It has been found that the upstream surface is more oxidized for both IG-110 and PGX graphites, and that the oxidized surface of PGX is rougher than that of IG-110. The oxidation of IG-110 starts at about 700°C and reaches the maximum above 800 to 900°C accompanied by the maximum production of CO2 around 850°C and a monotonic increase in CO with increasing temperature. The oxidation behavior of PGX is similar to that for IG-110, except that the oxidation starts at lower temperature and produces more CO2 over 750 to 950°C while the concentration of CO becomes lower in this temperature range. The present result for the product ratio of CO t...

Burn-off and Production of CO and CO2 in the Oxidation of Nuclear Reactor-Grade Graphites in a Flow System.

The oxidation behaviors of IG-110 and PGX graphites were experimentally investigated in the temperature range from 500 to 1,500°C for the oxygen concentration of 10 and 30 wt% (1.37 and 5.09 mol%) and the reacting gas velocities of 6.63 and 19.9 m/s (estimated at 900°C). The graphite specimens were prepared to simulate the annular coolant channel in the core of the HTGR. It has been found that the upstream surface is more oxidized for both IG-110 and PGX graphites, and that the oxidized surface of PGX is rougher than that of IG-110. The oxidation of IG-110 starts at about 700°C and reaches the maximum above 800 to 900°C accompanied by the maximum production of CO2 around 850°C and a monotonic increase in CO with increasing temperature. The oxidation behavior of PGX is similar to that for IG-110, except that the oxidation starts at lower temperature and produces more CO2 over 750 to 950°C while the concentration of CO becomes lower in this temperature range. The present result for the product ratio of CO to CO2 deviates from the linear line in the Arrhenius plot in the region where the production of CO2 increases. However, it agrees best with the Rossberg correlation among the previous empirical correlations. New empirical correlations for the product ratio of CO to CO2 have been obtained from the present data which exclude those in the region of CO2 increase.

高温ガス炉用黒鉛の衝撃引張り強度特性

Impact tensile strength test was performed with two kinds of HTTR graphites, fine grained isotropic graphite, IG-11 and coarse grained near isotropic graphite, PGX and deformation and fracture behavior under the strain rate of over 100s-1 was measured and the following results were derived:(1) Tensile strength for IG-11 graphite does not depend on the strain rate less than 1s-1, but over 1s-1, tensile strength for IG-11 graphite increase larger than that measured under 1s-1. At the strain rate more than 100s-1, remarkable decrease of tensile strength for IG-11 graphite was found. Tensile strength of PGX graphite does not depend on the strain rate less than 1s-1, but beyond this value, the sharp tensile strength decrease occurs.(2) Under 100s-1, fracture strain for both graphites increase with increase of strain rate and over 100s-1, drastic increase of fracture strain for IG-11 graphite was found. (3) At the part of gage length, volume of specimen increase with increase of tensile loading level and strain rate.(4) Poisson's ratio for both graphites decrease with increase of tensile loading level and strain rate.(5) Remarkable change of stress-strain curve for both graphites under 100s-1 was not found, but over 100s-1, the slope of these curve for IG-11 graphite decrease drastically.

円管内混合気体層流における黒鉛酸化時の物質伝達

Experiments and numerical analyses on mass transfer in a gas mixure laminar flow through a circular graphite tube accompanied with graghite oxidation were performed to clarify characteristics of graphite corrosion in connection with the primary cooling pipe rupture accident in a high temperature gas cooled reactor. In the experiment, inlet Reynolds numbers ranged from 16 to 320, graphite temperatures from 600 to 1, 050°C and inlet oxygen mass fractions from 10 to 50%. IG-110 and PGX graphites were used. The mass transfer coefficients in the experiment at a high temperature were less than those obtained on the basis of analogy between heat and mass transfer. It was found from the numerical analysis that the reduction of mass transfer coefficient resulted from the injection flow from the wall. The mass fractions of carbon monoxide and carbon dioxide in the experiment were well predicted on the basis of the one-dimensional numerical analysis in which graphite oxidation and carbon monoxide combustion were taken into consideration.

高温ガス炉用黒鉛材料の破壊じん性特性

Fracture toughness tests were carried out on nuclear graphite, IG-110 and PGX, and the effects of width, ligament ratio of compact tension specimen and cross-head speed on fracture toughness were examined. Crack extension processes in graphite specimen were observed by acoustic emission technique and optical observation method. From the results, it is concluded that:(1) A knee-point on the electrical potential-load displacement curve indicates macro-cracking in compact tension specimens.(2) Cracking occurred in the middle part of the notched surface of compact tension specimen and extended from the middle to the surface of specimen.(3) The value of JIC was constant within the range of ligament ratio from 0.4 to 0.55.(4) The value of JIC was constant within the range of cross-head speed from 0.005mm/min to 0.05mm/min.(5) The value of JIC was constant when the specimen width was more than 10mm for PGX graphite and 20mm for IG-110 graphite, respectively. However, JIC decreased as the specimen width decreased below the above-mentioned values. It is considered that this phenomenon is due to the formation of process-zone at the crack front of CT specimen.

198. Relationships between strength, electrical conductivity and density for oxidized PGX graphitet

The core of a High Temperature Gas-Cooled Reactor (HTGR) rests on massive graphite core support blocks; which, in turn, are supported by core support posts. PGX graphite was used for the core support blocks of the Fort St. Vrain HTGR (the only operating HTGR); and, evidently, is the leading candidate material for use in advanced HTGRs. Therefore, PGX was chosen for the initial tests on the use of eddy current techniques to monitor strength changes as a result of oxidation. The results of these initial tests showed that both compressive strength and electrical conductivity correlated very well with density. However, only a single log of PGX was used for the initial tests; therefore, it was necessary to determine if the correlations could be extended to other logs of PGX.

Effect of oxidizing environment on the strength of H451, PGX and IG-11 graphites

The effect of gaseous environment on the compressive and tensile strengths of H451, PGX and IG-11 graphites has been examined. Oxidation was carried out in O 2-, CO 2- and H 2O-containing environments. The strength losses incurred by H451 and IG-11 graphite did not appear to be affected by the nature of the oxidizing gas. However, the strength loss incurred by PGX graphite appeared to be greater in O 2 than in CO 2 or H 2O. A mechanism consistent with the observed behavior of PGX graphite is proposed: the iron impurity, which is locally distributed in the binder, may be catalytically active during oxidation in CO 2 or H 2O, leading to formation of a few large pits, whereas oxidation in O 2 may lead to the more detrimental condition of a high density of small pits. No effect of reaction temperature on the strengths of the graphites was observed. Attempts to fit strength-burnoff data to a standard expression were only partially successful. Much improved fits were obtained when this expression was modified to account for the development of the micro/mesopore network during oxidation.