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Axial, circumferential and radial distributions of fission products in the graphite sleeve, inner-tube and block of irradiated high temperature gas-cooled reactor (HTGR) fuel assemblies were measured by gamma spectrometry, lathe sectioning and beta counting with ion-exchange separation. Some distinctive peaks of 144Ce and 125Sb in their axial profiles, together with the very high activity level of fission products are ascribed to the failure of coated fuel particles. The effective retention capability of the graphite sleeve was observed for 90Sr, 106Ru, 125Sb, 144Ce and 155Eu; whereas not for 134Cs and 137Cs. Silver-110m was detected in graphite materials of the fourth OGL-1 fuel assembly with an increased burnup of 1.96% fissions per initial metal atom (FIMA). Effective in-pile diffusion coefficients of 90Sr, 125Sb and 144Ce in the graphite sleeves have been estimated using the Fickian diffusion theory.

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.

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.

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.

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.

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.