Kazumi Iwamoto

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.

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.

Fission product diffusion in ZrC coated fuel particles

Abstract The diffusion behavior of 137 Cs, 90 Sr and 144 Ce in CVD-ZrC was studied by measuring concentration profiles in ZrC and inner PyC layers of the coated particles, and the release from the particles during post-irradiation annealing. 90 Sr diffusion in ZrC is found to be controlled by a volume diffusion mechanism at 1600°C; this is found also for 137 Cs although it is influenced by micro-pores. The diffusion coefficient of 137 Cs obtained from the analysis of the concentration profiles in ZrC 1.3 is D = 2.3 × 10 −3 exp( −3.18 × 10 5 RT ) ( cm 2 s ) , where D is the diffusion coefficient, R the gas constant (8.314 J/mol · K) and T the temperature (K). In case of 144 Ce, the volume diffusion is not so remarkable as 90 Sr.

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.

Diffusion behavior of fission product in pyrolytic silicon carbide

Abstract The concentration distributions of 133 Xe, 140 Ba, 89 Sr, 141 Ce, 103 Ru and 95 Zr- 95 Nb in a SiC layer and releases of these fission products from SiC coated fuel particles were measured in a temperature range from 1650° to 1850°C, to obtain the diffusion parameters and to investigate the diffusion behaviors in SiC. Temperature dependences of the diffusion coefficients of these fission products except 95 Zr- 95 Nb were obtained. In diffusion behaviors of the alkali earth fission product, volume diffusion occured considerably besides grain boundary above 1650°C. 141 Ce and 103 Ru diffusions were almost through grain boundaries in the temperature range. 95 Zr- 95 Nb showed volume diffusion besides boundaries diffusion in their distribution annealed at 1850°C for 35 h, but those for the shorter time or at the lower temperature would be due to boundary diffusion. In comparison of the diffusion coefficients in SiC and PyC, it was proved that SiC was effective for retention of the solid fission products.

Concentration profiles of fission products in the coating layers of irradiated fuel particles

Abstract The distributions of 137 Cs, 90 Sr and 144 Ce in the coating layers of several kinds of TRISO coated fuel particles for HTGR were determined by method of removing the coating layers stepwise after irradiation. The distributions in SiC coating layer are influenced by its density and irradiation temperature. High dense SiC coating layer showed good retention of the fission products at such high irradiation temperature as 1530°C, while the retention of low dense one would change at 1300°C; above the temperature, it would be lost. There are several patterns of the distribution in outer PyC coating layer such as high accumulation of the fission products in a certain position of the layer, outerward rises in the concentrations and high concentrations in the layer than those in some positions of SiC coating layer. The post irradiation annealing was also carried out for one kind of the particle.

Deposition of dense isotropic carbon from acetylene in fluidized bed

Abstract Pyrolytic carbon was deposited from acetylene in a fluidized bed at 1100°C on alumina microspheres. In case when only argon was used as the diluent gas, density and BAF of the deposit decreased and deposition yield increased with increasing acetylene concentration. Hydrogen sypply suppressed carbon formation and, consequently, caused an increase in density and Lc. BAF was relatively independent of hydrogen concentration. It was found that high density, isotropic carbon can be deposited from acetylene at 1100°C, if an appropriate proportion of hydrogen is added.

The Behavior of Iodine in Adsorption and Desorption by Graphite

Using tracer technique, studies were undertaken on the adsorption of iodine on natural graphite at temperatures ranging from 27° to 900°C under pressures from 0.03 to 1.4 mmHg, and on the desorption of adsorbed iodine from the same specimens into either vacuum or Ar stream. The specimens were heated up either isothermally, stepwise, or at constant rate of temperature rise, up to 1,100°C. The resulting data accorded well with a theoretical treatment presuming desorption from heterogeneous surface, considering the desorption as an n-th order irreversible reaction having distributed activation energies. The iodine is considered to be held on the graphite by physical adsorption below about 450° C, and above that temperature, by chemisorption through bond formation between iodine and carbon atoms. The heat of adsorption and the activation energy of desorption obtained was found to be a function of surface coverage. The presence of Ar gas was observed to inhibit chemisorption of iodine and to decrease the activ...