Katsuichi Ikawa

Chemical vapor deposition of ZrC within a spouted bed by bromide process

Abstract ZrC coatings by chemical vapor deposition were applied to particles of ThO 2 , UO 2 and Al 2 O 3 at 1623–1873 K. The feed gas mixture consisted of ZrBr 4 , CH 4 , H 2 and Ar. The results were compared with the calculated chemical equilibria in the Zr-C-H-Br system. It was shown that the weight and composition of the deposit can be calculated by thermochemical analysis after correcting the methane flow rate for a pyrolysis efficiency. Predominant reaction presumably occurring were derived by a mass balance consideration on the calculated equilibrium species. A simplified model of the ZrC deposition was proposed.

High-temperature heating experiments on unirradiated ZrC-coated fuel particles

Abstract Triso-coated UO 2 particles with ZrC as the third layer were heated at 2173–2773 K. The particles withstood the heating at 2723 K for 1 h, though more than one half failed at 2773 K within 1 h. The results were compared with those on the conventional Triso-coated particles with SiC as the third layer. The thermodynamics of the system within ZrC-Triso particles are discussed and the internal CO pressure at high temperatures is estimated. The results of the impurity analysis on CVD ZrC is also presented.

Effect of gas composition on the deposition of ZrC-C mixtures: The bromide process

Mixtures of ZrC-C were chemically vapour deposited from gaseous mixtures of zirconium bromides, methane, hydrogen and argon. The effect of gas composition on the deposition behaviour was studied. The experiments have shown that the methane concentration in the feed gas mixture is a crucial factor in determining the deposition rate and the character of the deposit. Chemical equilibria in the Zr-C-H-Br system were calculated and compared with the experimental results.

Coating Microspheres with Zirconium Carbide-Carbon Alloy by Iodide Process

Simultaneous deposition of carbon and zirconium from vapor produced by the reaction between methyl iodide vapor and zirconium sponge was studied with the application of a spouted bed constituted by a funnel carrying a charge of alumina microspheres, which were blown upward and held in dynamic suspension by a jet of the vapor and gases spouting from the funnel. The purpose of the experiment was to determine the conditions favorable for obtaining a coat of zirconium carbide-carbon alloy on the microspheres. Deposition of the vapor on the microspheres, leading to the formation of the carbon alloy coating, was found to take place at temperatures exceeding 1,100°C. The C/Zr ratio of the deposited coat was found to increase with deposition temperature. The hydrogen concentration in the spouting gas affected both the deposition yield and the chemical composition of the deposit. Repeated use of the sponge was found to impair its performance due to deactivation by premature deposition of carbon.

Formation of carbon-excess SiC from pyrolysis of CH3SiCl3

Abstract Chemical vapor deposition of SiC from pyrolysis of CH 3 SiCl 3 was studied by a stationary substrate technique. A relatively large amount of excess carbon was co-deposited even in H 2 . This phenomenon was explaned on the basis of a competition between gas phase and solid surface formation of carbon in comparison with silicon.

Release of metal fission products from UO2 kernel of coated fuel particle

Abstract Release behaviors of Cs 137 , Cs 134 , Sb 125 , Ce 144 , Ru 106 and Ag 110m during irradiation from the high-density UO 2 kernel of Triso-coated fuel particle were studied. The intact coated fuel particles were carefully cracked after irradiation; the amounts of fission products remaining in the UO 2 and their partition between the UO 2 and the coating were measured hv gamma-ray spectrometry. The results are discussed in view of the high-temperature chemistry of the fission products within the coated fuel particles.

Fission product release from Triso-coated UO2 particles at 1940 to 2320°C

Abstract The fission product release from TRISO-coated UO 2 particles was measured by post-activation heating at 1940 to 2320°C for use in a safety analysis. The results are analyzed mathematically with effective diffusion coefficients in each medium. 103 Ru, 99 Mo and 95 Nb are released at 1940 to 2320°C and have high effective diffusion coefficients. Although 140 Ba and 137 Cs are retained in TRISO-coated particles at 2050°C, they are released rapidly at 2320°C. This is attributed to the transition of beta to alpha SiC at 2320°C. 141 Ce, 140 La and 95 Zr are released little if any at 2320°C. Rare gas nuclides, iodine and tellurium seem to be retained in coated particles at this high temperature.

Crushing strengths of SiC-Triso and ZrC-Triso coated fuel particles

Abstract The whole-particle and third-layer crushing strengths of SiC-Triso and ZrC-Triso coated fuel particles were measured. The third-layer crushing strengths did not differ for the two types of Triso-coated particles if their particle diameters and third-layer thicknesses were not different. The third-layer failure stresses calculated with the model by Delle et al. [1] were found still dependent on the particle radius. The failure stress divided by the particle radius was about constant for a given third-layer material. The whole-particle crushing strength of SiC-Triso particle was larger than that of ZrC-Triso particle in the as-coated condition, but the contrary was the case when those particles were annealed at 2073 K for 1 h. By the annealing the whole-particle strength of SiC-Triso particle decreased but that of ZrC-Triso particle increased. Both of these changes could not be ascribed to the corresponding changes in the third-layer strength.

Detection of Methyl Iodide upon Heating Irradiated Fuels

Laboratory scale experiments have been carried out to obtain information on the sources of CH3I, reported by various workers to have been detected under accidental conditions of fuels. Small pieces of slightly irradiated sintered UO2 and U metal turnings were heated externally up to 1,400° C in various carrier gases, and the carrier gases were analyzed for fission product iodine present in the form of CH3I. From the data obtained, it is concluded as follows: In the case of sintered UO2, some amount of CH3I appears to be released directly from the fuel below 600°C, whereas in case of U metal turnings, direct release does not apparently occur at any temperature. In oxidizing atmospheres, iodine released from either sintered UO2 or U metal is considered to change into CH3I only to a small extent, probably by gas phase formation.

In-pile release behavior of metallic fission products in graphite materials of an htgr fuel assembly

Abstract Distribution of metallic fission products in the graphite sleeve and block of the fifth OGL-1 fuel assembly was measured by gamma spectrometry with lathe sectioning. Considerably large release fractions of long-lived fission products with smooth axial profiles were observed in the sleeve due to a large failure fraction of coated fuel particles accompanied with failed silicon carbide layers. Nevertheless, a key nuclide110mAg, whose large release is suspected at increased burnups for low-enriched uranium fuels, was effectively retained within the graphite sleeve. The retention was also observed for125Sb, 154Eu and155Eu up to a burnup of 3.2% fission per initial metal atom, but was limited for134Cs and137Cs at high sleeve-temperatures above 900°C. In-pile diffusion coefficients in IG-110 graphite have been evaluated for Cs, Ag and Sb; those for Cs are in reasonable agreement with available in-pile data.