A heat transfer experiment was performed on steam-water two-phase flow in an annular flow path with a uniformly heated rod under the conditions of the mass flow rates from 0.2×10⁶ to 1.0×10⁶ kg/m²•h, inlet qualities from 0.5 to 1.0, heat fluxes below 4.7×10⁵ W/m² and pressure of 31 bar. Dryout of the heater rod surface was observed resulting in the sharp rise of the heater rod surface temperature. Measured heat transfer coefficients were compared with the several empirical and semi-empirical correlations with the emphasis on the applicability of the correlations to the present test conditions being important in the analysis of the thermal hydraulic behavior during a LOCA of a nuclear reactor. The measured heat transfer coefficient in the pre-dryout region is lower than the existing correlations. The cooling of the heat transfer surface by the liquid phase in the post-dryout region is significant, which is neglected in the existing correlations. The heat transfer coefficients calculated for the post-dryout region by the Groeneveld correlation show good agreement with the presently measured results within the accuracy of 027%.

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Investigations into the flow pattern and the void fraction for countercurrent air-water flow in vertical tubes of diameter D=40 and 80 mm were reported. The flow maps were presented and showed slug flow regime occupied larger portion on them. The void frac-tion was measured by the quick-closing valve technique, in bubbly and slug flow regime.
The void fraction data available in the literatures as well as present work for counter-current flow in vertical tubes were correlated in terms of dimensionless groups. The experimental results of the present work were also compared with the drift flux model.

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With the view of obtaining basic data required for designing durable reflux vapor traps of high performance for use in sodium-cooled FBR's, experiments were conducted to (a) select a suitable packing, and (b) to examine the effect brought on trapping performance by changing the gas flow rate, packing material, packing density and trap outlet tempera-ture. The results indicated that:
(1) As trap packing, plane weave stainless steel mesh proved to ensure lower pressure drop through trap.
(2) Using the plane weave mesh packing, and with the trap outlet temperature kept at 130°C, the reflux vapor trap efficiency was found to exceed 99.6% in the range of vapor trap gas velocity below 1.3 m/s, and packing density below 0.07 g/cc. The efficiency decreased at outlet temperatures above 130°C.

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Microstructure effects on the strength of UO₂ pellets have been investigated. The strength was measured at room temperature using a biaxial flexure technique in which thin disk specimens, sliced from UO₂ pellets, were fractured by ring loading in the speci-men center. The strength of UO₂ was reduced, in all cases, by increase in porosity, grain size or pore size. Porosity and pore size had a significant influence, while grain size showed only a weak effect. The diameter of the maximun pore observed in the specimens was close to the critical flaw size calculated from the Griffith criterion. This indicates that the largest pore is the most important preexisting flaw which controls the pellet strength. The grain boundaries associated with the largest pore could play a role as a path for pore extension in the fracture process. The strength of UO₂ can be expressed by
σ_f=626×(PS+1/2GS)^-1/2exp(-0.057×P)
where σf is the fracture strength (MPa), PS the largest pore size (μm), GS the mean grain size (μm) and P the porosity (%). The equation demonstrated good predictive capabilities regarding strength of UO₂ pellets having wide variation in microstructure.

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The desorption rate of tritiated water from molecular sieve adsorbed HTO, by exchange with the environmental water vapor, was measured. The molecular sieve, packed in a column, was initially changed with tritiated water and then humidified Ar gas was made to flow through it and the tritium concentration of effluent gas was measured. The de-sorption rate of tritiated water increased linearly with the water vapor pressure in the gas at constant flow rate. In the case where both the flow rate and the vapor pressure were kept constant, the amount of tritium left adsorbed on the molecular sieve decreased exponentially with time. It should be noted that the desorption rate was rather rapid even at room temperature and nearly all the tritiated water adsorbed on the molecular sieve was recovered by the flowing humidified gas at room temperature within several hours.

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The neutronic properties of SENRI-I, a reference design of laser fusion reactor proposed by Institute of Engineering, Osaka University, are discussed on the basis of the one-dimen-sional neutron transport calculations in burning DT plasmas and blankets. The softening of the fusion neutron energy spectrum, the neutron heating and the neutron multiplication are studied and discussed for the compressed DT pellets with various thickness of fuel plasmas and lead or lead-polyethylene tampers. The neutronic and thermal features in the blanket of the SENRI-I design are also examined. The tritium breeding ratio is high enough (1.6), depending on the neutron energy spectrum from a pellet. The maximum temperature increase per 1, 000 MJ DT fusion reactions is 3°C in the inner liquid Li layer and 1.5°C in the stainless steel first wall. A parametric study is also presented on the effect of varying the thickness of the inner Li blanket ΔR_i to examine the thickness required for the enough tritium breeding ratio and energy deposition.

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Neutron nuclear data of ²³³U have been evaluated in the energy range from 10^-5 eV to 20 MeV. Evaluated quantities are the total, fission, capture, elastic and inelastic scattering, (n, 2n) and (n, 3n) reaction cross sections, and the average numbers of prompt and delayed neutrons emitted per fission. The thermal and resonance cross sections have been evaluated on the basis of the measured data. The resolved resonance parameters are given up to 100 eV and the unresolved resonance parameters between 100 eV and 30 keV. The total and fission cross sections have been evaluated in the higher energy region on the basis of the recently measured data, while the theoretical calculation with the optical, statistical and evaporation models has been used for evaluation of the other cross sections. The pre-sently adopted optical potential parameters have reproduced well the experimental total cross section in the entire energy range as well as the measured data of the s-wave strength function. The structure observed in the ν_p, values below 1 MeV is reproduced by the semi-empirical formula based on the fission fragment kinematics. The presently eval-uated fission cross section is considerably lower than that of ENDF/B-IV between 10 and 50 keV. This low fission cross section is expected to resolve the k_eff discrepancy pointed out from the benchmark tests in ²³³U critical assemblies.

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