A relationship between the Doppler and the self-shielding effects has been experimentally studied by measuring the effective average total cross sections of the metal and sintered dioxide of ²³²Th for 24keV Fe-filtered neutrons. The relationship is a theoretical prediction that the apparent value of the total cross section measured with the neutrons of an adequate energy spread is expressed by a single parameter N/√θ in the high temperature limit, N being the thickness and θ the effective temperature of the samples. The temperature has been varied from room-temperature to 1, 173K and the thickness from about 0.3cm to about 3.8cm. The results of the measurements support the theoretical prediction for both cases of the metal and dioxide samples. The Doppler and the self-shielding effects for the metal and dioxide agree within the experimental errors though some dependence on atomic binding was expected.

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Pattern recognition techniques for loose parts location in nuclear power reactors are in. vestigated. The location method is based on determining a reference impact position closest to the unknown impact from a library in which data for reference impacts are listed. The values of sound amplitude and arrival time as obtained from accelerometers are used for the pattern information which is inherent in impact position. The distance between unknown and reference impact positions is estimated from the similarity of each pattern. The pattern distance between two sound sources is defined as an index of the pattern similarity. Relationships between the pattern distance and the actual distance of the impacts are studied using impact data from a commercial boiling water reactor. It is found that the pattern distance and the actual distance have a very close correlation and the false probability that the zero impact distance is determined to be a distance of more than 0.6 m is 0.15. These results suggest that the pattern recognition techniques can be applied to loose parts detection and location on pressure vessels.

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Experiments were performed in order to clarify the effect of the grid spacers on reflood heat transfer in PWR-LOCA. The flow pattern, the thermal responses and the water accumulation near the grid spacer were investigated by shifting the grid spacer at the midplane of the simulated core.
The heat transfer coefficient before the quenching was about 20 to 50% higher just above the grid spacer than just below the grid spacer. The decrease of the droplet diameter due to the grid spacer was observed in the droplet dispersed flow regime. In the slug flow regime, the grid spacer was rewetted early in the reflood transient and the increased water accumulation near the grid spacer was observed. Hence, the heat transfer enhancement due to the grid spacer is mainly attributed to the increased interfacial surface area of droplets in the dispersed flow and also to the increased film boiling heat transfer in the slug flow. The thermo-hydraulic behavior near the grid spacer was well predicted with the model developed based on the present experiment.

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The heat transfer regime just above the quench front during the reflood phase in PWR-LOCAs is the film boiling. A two-region model is developed for the description of saturated film boiling in this region. It is considered that the violent two-phase flow surrounds the vapor film and promotes the transition of laminar to turbulent vapor film in the reflood phase. In the model, the vapor film surrounded with the two-phase mixture core is treated with the turbulent boundary layer theory. The higher void fraction of the mixture core gives the lower heat transfer in the model. The results obtained by the model are compared with the data of the Slab Core Reflood Tests which cannot be explained with the previous laminar vapor film theory.
The data fall between the curves predicted by the models assuming zero interfacial shear stress (τ_i=0) and zero interfacial velocity (υ_i=0) at the different elevations in the core, different core inlet velocities and different system pressures. These show the applicability of the two-region model for the reflood phase.

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Diffusion coefficients of Ce, Sr and Ba in the matrix graphite for VHTR fuel compact have been determined by measuring the radial profiles of ¹⁴¹Ce, ⁸⁹Sr and ¹⁴⁰Ba concentrations in slightly irradiated graphite spheres which have UO₂ kernels at the center. Temperature dependences of the diffusion coefficients of Sr and Ba have been etermined between 1, 175 and 1, 375°C, and they are expressed as, where T is the absolute temperature and R the gas constant (8.314 J/mol•K).
The diffusion coefficient of Ce in the above temperature range was rather higher than those of Sr or Ba, and the magnitudes of these coefficients were expressed in following order ; D_Ce>D_Sr>D_Ba.
The effect of matrix graphite density on diffusion of the fission products was studied, but difference was not found clearly in the diffusion coefficients of Ba which had been ob-tained in both of low and high density matrices at 1, 375°C.

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Single stage separation factors a have been determined for ⁶Li and ⁷Li between lithium ions in methanol and complexed ions with a cryptand (2_B, 2, 1) polymer. The ⁶Li was con-centrated in the cryptand. The separation factors were compared with the values of other chemical exchange systems. The maximum enrichment factor obtained was ε =0.047±0.002. The figure is one of the greatest in the chemical exchange reactions without valence change and almost 10 times larger than the values of ion exchangers. The variation in a depend-ing on the chemical species was small in the non-aqueous system. High enrichment of lithium isotopes was expected to be achievable by means of the chromatographic application of the cryptand (2_B, 2, 1).

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For the safety assessment of geologic disposal of high-level radioactive wastes, an analy-tical solution was obtained for one-dimensional migration of radionuclide through two-layered geologic media without dispersion. By applying it to geologic media composed of granite and soil layers, the effect of interlayer boundary on the discharge profile of radionuclides in decay chains into biological environment is examined.
The time-space profiles of radionuclides in the vicinity of interlayer boundary are much complicated as shown in the results of calculation. Those profiles in case that the ground-water flows through granite followed by soil are quite different from those in case that the groundwater flows through soil followed by granite. Each of complicated dependence of profiles on time and space can be physically explained. The characteristic profiles in the vicinity of interlayer boundary have not been discussed previously.
Recently, numerical computer codes has been developed to apply to much more realistic geologic situations. However, the numerical accuracies of the codes are necessary to be confirmed. This is achieved by comparing computational results with results from analytical solutions. The analytical solution presented will serve as a bench-mark for numerical accuracy.

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