Ishio Takahashi

Fission product palladium-silicon carbide interaction in htgr fuel particles

Abstract Interaction of fission product palladium (Pd) with the silicon carbide (SiC) layer was observed in irradiated Triso-coated uranium dioxide particles for high temperature gas-cooled reactors (HTGR) with an optical microscope and electron probe microanalyzers. The SiC layers were attacked locally or the reaction product formed nodules at the attack site. Although the main element concerned with the reaction was palladium, rhodium and ruthenium were also detected at the corroded areas in some particles. Palladium was detected on both the hot and cold sides of the particles, but the corroded areas and the palladium accumulations were distributed particularly on the cold side of the particles. The observed Pd-SiC reaction depths were analyzed on the assumption that the release of palladium from the fuel kernel controls the whole Pd-SiC reaction.

Fission product release from ZrC-coated fuel particles during postirradiation heating at 1600°C

Abstract Release behavior of fission products from ZrC-coated UO 2 particles was studied by a postirradiation heating test at 1600°C (1873 K) for 4500 h and subsequent postheating examinations. The fission gas release monitoring and the postheating examinations revealed that no pressure vessel failure occurred in the test. Ceramographic observations showed no palladium attack and thermal degradation of ZrC. Fission products of 137 Cs 134 Cs, 106 Ru, 144 Ce, 154 Eu and 155 Eu were released from the coated particles through the coating layers during the postirradiation heating. Diffusion coefficients of 137 Cs and 106 Ru in the ZrC coating layer were evaluated from the release curves based on a diffusion model. 137 Cs retentiveness of the ZrC coating layer was much better than that of the SiC coating layer.

Release behavior of metallic fission products from HTGR fuel particles at 1600 to 1900°C

Abstract Release behavior of metallic fission products from the Triso-coated UO 2 particles was studied by postirradiation heating tests in the temperature range 1600 to 1900°C (1873 to 2173 K) and subsequent postheating examinations. The fission gas release monitoring and the postheating examinations revealed that no pressure vessel failure occurred in the tests. Ceramographic observations showed no palladium attack and thermal decomposition of SiC, 137 Cs, 134 Cs, 110m Ag, 154 Eu and 155 Eu were released from the coated particles through the coating layers during postirradiation heating. The diffusion coefficient of 137 Cs in the SiC layer was evaluated from the release curves based on a simple diffusion model assuming a one-layer coated particle. Fractional release measurements suggested that the diffusion coefficient of 110m Ag in SiC be larger than that of 137 Cs.

Fission product behavior in Triso-coated UO2 fuel particles

Abstract The behavior of fission products in irradiated Triso-coated UO 2 fuel particles was examined by electron probe microscopy, and the thermodynamic analysis was carried out on the fission products-UO 2 -C system to understand the fission product behavior in the coated particles. In the UO 2 kernels the precipitates of Mo, Pd-Te and Pd-Mo-Sn were observed besides the Mo-Tc-Ru-Rh-Pd alloy. In the coating layers palladium, tellurium, cerium and barium were often observed. The chemical form of barium and cerium was oxide, while the probable form of tellurium was elemental tellurium. The calculated vapor pressure of CeO 2 was the highest of the species containing the rare earth elements, and the calculated main Ba-containing gaseous species was BaO. The intercalation compound C n Cs was thermodynamically predicted to exist as a dominant chemical form of cesium at high temperatures instead of Cs 2 MoO 4 .

Evaluation of toughness degradation by small punch (SP) tests for neutron-irradiated 214Cr-1Mo steel

Mechanical properties correlations between the small punch (SP) test and conventional tensile, Charpy impact and fracture toughness tests were investigated on neutron irradiated 214Cr-1Mo ferritic steel. Estimation of radiation induced changes on tensile strength, elastic-plastic fracture toughness (JIC) and ductile-to-brittle transition temperature (DBTT) are thought to be basically possible by mechanical properties correlations when based on sufficient pre-irradiation data.

Carbon monoxide-silicon carbide interaction in HTGR fuel particles

The corrosion of the coating-layers of silicon carbide (SiC) by carbon monoxide (CO) was observed in irradiated Triso-coated uranium dioxide particles, used in high-temperature gas-cooled reactors, by optical microscopy and electron probe micro-analysis. The mechanical failure of the coating-layer of inner dense pyrolytic carbon (IPyC) was often observed beside the area of the SiC corrosion. The grain boundaries of the SiC seemed to be selectively corroded during early stages of corrosion. Silicon dioxide, or more stable (Si, Ce, Ba) oxide, was accumulated at the buffer-IPyC and IPyC-SiC interfaces on the cold side of the particles and the formation of (Pd, Rh, Ru, Tc, Mo) silicides was observed in the fuel kernels, which probably resulted from the vapour transport of silicon monoxide from the corroded areas.

A model to predict the ultimate failure of coated fuel particles during core heatup events

In this paper a model to predict the ultimate failure of TRISO-coated fuel particles under hypothetical core heatup events is proposed. Features of the model include the ability to treat the statistical variation of the number of coated fuel particles and to make a thermodynamic estimation of the stoichiometry of irradiated UO{sub 2} kernels and the equilibrium CO pressures. The model predictions agree well with the results of postirradiation heating tests. The thermal creep of pyrolytic carbon, however, must be taken into account to further improve the accuracy of the prediction.

Gibbs free energies of formation of molybdenum carbide and tungsten carbide from 1173 to 1573 K

The gas equilibrium method of CH4/H2 has been widely used for measuring carbon potential. However, it has been reported that this method is not applicable at high temperatures since the equilibrium between CH4 and H2 is disturbed by the reaction of CH4 with moisture in the system. Nevertheless, this method should be applicable theoretically at high temperatures below which CH4 decomposition can be neglected because the equilibrium between CH4 and H2 reaches constant ratio in spite of the reaction. Since the role of moisture is to oxidize the sample during the measurements under the oxygen potential determined byPh2o/ph2 ratio, the Gibbs free energies of formation of Mo2C and WC were successfully measured from 1173 to 1573 K by keeping the moisture level in the system low enough not to oxidize the sample. The experimental results are expressed by the following equations which were derived by least squares treatments of the data: Mo2C:ΔG = -68270 + 8.23T J mol-1 WC:ΔG = -52330 + 14.06T J mol-1 These values were in good agreement with those measured by M. Gleiseret al. for narrow tempareture ranges using the CO/CO2 gas equilibrium method.

The phase reaction in the UC-W system

Abstract The phase reaction in the UC-W system has been investigated by metallography, X-ray diffraction, microhardness measurement and electron-microprobe analysis. The results show that in the system a peritectic four-phase reaction takes place: UC + W / ag η - UWC 2 + liquid ( at 2150 ± 20 ° C ), where η-UWC 2 denotes a low carbon form of UWC 2 . The compositions of the peritectic liquid and of the peritectic point are determined to be near 48 U/34 C/ 18 W (at%) and 40 U/40 C/20 W (at%), respectively. The phase diagrams are presented for the U-C-W ternary and UC-W quasibinary systems.

A new gas equilibration method for the measurement of carbon potential: Carbon potential in austenitic 316 stainless steel at 1000°C

Abstract A new method for the determination and control of the carbon potential of a sample material using CH 4 /H 2 gas equilibration was developed. The carbon potential of the sample could be controlled in a wide range with an accuracy of about ±0.1 kcal/mol by adopting several kinds of reference mixtures, in which the temperature of the reference material, metal/metal-carbide mixture set, was changed. The carbon potential in austenitic 316 stainless steel at 1000°C was measured successfully by this method.