[en] Effect of surrounding total gas pressure on oxidation rate of nuclear grade graphite with water vapor is an important factor in evaluating corrosion behaviors of graphite structural components of high temperature gas cooled reactor. The rate of oxidation with water vapor at high temperature is controlled by diffusion of gas through fine pore in graphite and by rate of chemical reaction. The oxidation rates of a nuclear grade graphite, SM1-24, with water vapor were measured at 1000degC within a range of 1 to 5 atm of helium. Diffusion of water vapor in pore was treated as counter diffusion of water/helium system in a uniform single capillary at a constant total gas pressure. An equation of oxidation rate under the condition of inpore diffusion control was derived as a function of wide range of total gas pressure. The calculated results were in good agreement with the experimental data of SM1-24 and also data of a graphite used in a literature. A flow parameter, β(p), is defined as a ratio of molecular diffusion resistance to Knudsen diffusion resistance of water vapor diffusion through pore and this value can be determined from the total gas pressure dependence of oxidation rate. Degree of contribution of the Knudsen diffusion to water vapor diffusion was evaluated on both SM1-24 and A3-27 graphites. (author)

[en] Oxidation reactions of candidate graphites for core support post with atmospheric air were studied in a temperature range between 550₀C and 1000₀C. The reaction rates, temperature dependence of the rates and distribution of bulk density in the oxidized graphites were measured and the characters obtained were compared between the brand of graphites. On the basis of the experimental results, dimension and strength of the post after corrosion with air, which might be introduced in rupture accident of primary coolant tube, were discussed. In the case of IG-11 graphite, it was proved that the strength of post is still sufficient even 100 hours after the beginning of the accident and that, however, it is necessary to insert more deeply the post against graphite blocks. (author)

[en] Oxidation experiments have been carried out mainly on a fine-grained isotropic graphite, IG-110, at temperatures between 1173 and 1473 K in a water vapor/helium mixture. In most cases water vapor concentration was 0.65 vol% and helium pressure, 1 atm. Reaction rate and burn-off profile were measured using cylindrical specimens. On the basis of the experimental data the oxidation behavior of fuel block and core support post under the condition of the VHTR operation was estimated using the first-order or Langmuir-Hinshelwood equation with regard to water vapor concentration. Strength and stress-strain relationship of the graphite components with burn-off profiles estimated above were analyzed on the basis of the model for stress-strain relationship and strength of graphite specimens with density gradients. The estimation indicated that the integrity of the components would be maintained during normal reactor operation. (author)

[en] The present study was carried out to improve the oxidation resistance of carbon materials for thermal barrier of the experimental VHTR. The reactivity of the six kinds of carbons against air was measured as a function of heat-treatment temperature in a range of 1100 ⁰ - 2000 ⁰C, as well as effect of acid leachings with hydrochloric acid and with nitric acid on the reactivity. The treated carbons were reacted in the atmospheric air at 450 ⁰C, and the reactivity change of carbon was compared with the reaction time needed to oxidize up to a fixed burn-off. It was found that the corrosion resistance of carbon was greatly improved by the heat-treatment, while there was no effect of acid leaching on the reactivity of carbon. It was considered that the improvement of corrosion resistance was based on vaporization of impurities contained in the carbon which act as catalyzer in the oxidation. The best condition for the improvement of carbon materials was discussed by taking into account the changes in the other properties by the heat-treatment. (author)

[en] The effect of hydrogen on the rate of reaction with water vapor was studied for four kinds of graphites including IG-11 which is a candidate material for use in the core of the Experimental High Temperature Gas-cooled Reactor. The experiment was carried out at 800 - 1000sup( o)C in the helium flow which contained 0.65 - 1.0 % water vapor and 0 - 1.5 % hydrogen. The rate of reaction with water vapor decreased by addition of hydrogen and the effect of hydrogen was much larger for highly purified graphite than for contaminated one. Based on the experimental results, the effect of hydrogen in the reactor condition was estimated and it was shown to be negligible small at such impurity level as the coolant of the reactor. (author)

[en] The effect of mechanical prestress on the chemical reactivity of graphite to air was studied in the temperature range 430 - 590⁰C both on anisotropic and isotropic nuclear graphites. Compressive prestress was applied to the graphite specimens and the reaction rates were compared with those of unprestressed specimens. The reaction rate of the prestressed specimen increased much more for the anisotropic than for the isotropic and the stress induced reactivity change for the anisotropic graphite depended on the direction of applied prestress with respect to the extrusion direction. However, the effect of the prestress on the reaction rate gradually diminished with increase of the reaction temperature. The enhanced reactivity introduced by prestress almost disappeared by heat-treatment of the specimen at 2000⁰C, although annealing at 1000⁰C was not effective. It was considered that the acceleration of reaction rate of the prestressed graphite was not due to formation of cracks in the graphite but increase of the chemical reactivity of carbon crystallite itself which has been resulted from lattice strain and deformation. The less effect observed on the isotropic graphite would be attributed to the less damage in the crystallite which might be brought from the smaller size of coke grain used in the graphite. Study was also carried out to clarify the prestress effect on powder of the graphite. (author)

[en] The reaction of four kinds of nuclear graphites with oxygen under Co-60 gamma-ray irradiation were examined in the gas flow at 1atm, at temperatures 25 - 204⁰C and gamma dose rates 3.8 - 12.5 x 10⁵R/hr. Air and oxygen diluted with helium were also used as the atmosphere. Radiation induced oxidation rates of the graphites in pure oxygen at dose rate 12.5 x 10⁵R/hr were found to be 6.6 - 7.5 x 10⁸g/g.hr, showing little differences between the graphites. Over all oxidation rate of the graphites increased with temperature, but the radiation induced rate kept constant with temperature. The increase of the over all rate at high temperatures was concluded to be due to the increase in thermally induced oxidation rate. The radiation induced oxidation rate was expressed as R = 8.3 x 10^-12(D)sup(0.64), where R is the radiation induced oxidation rate in g/g.hr and D is the dose rate in R/hr. Oxidation rate of the graphite in air was about 40% of that in pure oxygen, and that in the oxygen diluted with helium was constant 15% in 0.2 - 1.0 vol.% oxygen. It was shown that the gamma-rays in a reactor would little influence oxidation of the structural graphite at high temperatures. (author)

[en] Nuclear graphites more than 10 brands were oxidized with water vapor in helium and then some selected graphites were irradiated with fast neutron in the Japan Materials Testing Reactor to clarify the effect of radiation damage of graphite on their reaction behaviors. The reaction was carried out under a well defined condition in the temperature range 800 -- 1000⁰C at concentrations of water vapor 0.38 -- 1.30 volume percent in helium flow of total pressure of 1 atm. The chemical reactivity of graphite irradiated at 1000 +- 50⁰C increased linearly with neutron fluence until irradiation of 3.2 x 10²¹ n/cm². The activation energy for the reaction was found to decrease with neutron fluence for almost all the graphites, except for a few ones. The order of reaction increased from 0.5 for the unirradiated graphite to 1.0 for the graphite irradiated up to 6.0 x 10²⁰ n/cm². Experiment was also performed to study a superposed effect between the influence of radiation damage of graphite and the catalytic action of barium on the reaction rate, as well as the effect of catalyser of barium. It was shown that these effects were not superposed upon each other, although barium had a strong catalytic action on the reaction. (author)

[en] Graphites more than 10 brands were oxidized with water vapor in helium in order to select some favourable ones for the multi purpose experimental VHTR as well as to provide the basic corrosion data for its design. The reactions were carried out under well-defined conditions in a helium stream of total pressure of 1 atm. The corrosion behavior of irradiated graphite was studied in temperature range 800 - 1000⁰C at concentrations of water vapor between 3800 - 13000 vpm and the effect of high temperature irradiation was estimated on the reaction rate, its dependence upon temperature and also the order of the reaction. The chemical reactivity of graphite irradiated at 1000 +- 50⁰C increased linearly with neutron fluence up to 3.2 x 10²¹ n/cm². The activation energy for the reaction was found to decrease with neutron fluence for almost all the graphite, while that for a few graphites increased. The orders of the reaction were 1.0 for the irradiated graphite and 0.5 for the unirradiated one. Experiment was also performed under irradiation of Co-60 gamma ray to obtain the effect of irradiation on corrosion gas, which led a formation of active species of gas. The corrosion rates were found to be in the order of 10^-9 g/g.hr at 1.2 v/o water vapor concentration at 30⁰C under the exposure rate of 1.25 x 10⁶ R/hr of gamma ray. The activation energy for the reaction was found to be about 1 kcal/mole in the temperature range 30 - 300⁰C. The influence of water vapor concentration for the reaction was scarcely observed in the range 0.6 - 2.3 v/o. (author)

[en] Changes in thermal conductivity, coefficient of thermal expansion (CTE), electrical resistivity, and Young's modulus were measured for the two nuclear grade graphites IG-11 and H451 oxidized thermally in air at 723 K and by water vapor at 1123 K. Thermal conductivity, CTE and Young's modulus decreased and electrical resistivity increased owing to the oxidation. The changes were dependent on the graphites as well as the oxidation condition. The results were analyzed and discussed. Furthermore the changes in thermal shock resistance were discussed for the thermally oxidized nuclear grade graphites. (orig.)