Neutron-induced fission cross sections of 24 actinide nuclei were analyzed in terms of the double-humped fission barrier model to deduce the barrier heights. Good fits were obtained by assuming that the first barrier is mass-symmetric and axially asymmetric, while the second barrier is mass-asymmetric and axially symmetric. Systematic trends were observed in the barrier heights of the actinide nuclei ; the first barrier height as a function of neutron number tends to be peaked at N ?? 147, whereas the second barrier height increases linearly as a function of (1-χ)³ A ^2/3, where χ is the fissility. By decomposing the barrier heights into liquid-drop and shell correction parts, the surface energy coefficient was deduced to be 17.55 MeV. This value is consistent with existing values obtained from nuclear mass system-atics. This fact corroborates the theoretical conjecture that the shell correction is damped at larger deformation corresponding to the second barrier. Near constancy of the fission barrier heights for actinides (fission barrier anomaly) was interpreted in terms of the three-component analysis.

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Thermal interaction which is caused by explosive boiling of water droplet in contact with hot molten tin was investigated, paying attention on the influences of the tin tempera-ture T_tin and the water temperature T_water together with the impact Weber number We. The results were summarized as follows. (1) Ttin, of lower boundary of thermal interaction zone (TIZ) is almost constant at about 290°C for a larger We than about 70, independent of T_water. (2) The corresponding contact interface temperature is around 275°C and almost agrees with (27/32) T_crit, or the spontaneous nucleation temperature, where T_crit=647 K (=374°C). (3) T_tin of upper TIZ boundary increases from around Tcrit to above 500°C as Twater decreases. (4) Twater of upper TIZ boundary is about 80°C. (5) For the woodsmetal-water system, in spite of a very low melting point of 78°C, the woodsmetal temperature of lower TIZ boundary rises up to about 350°C because of a low thermal conductivity of woodsmetal. (6) The Leidenfrost temperature is about 195°C for water droplet on solid tin, which has a melting point of 232°C, and appreciably lower than Ttin of lower TIZ boundary.
In conclusion, the present results support the spontaneous nucleation temperature concept by Fauske & Henry, as far as extremely energetic thermal interaction is concerned.

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The swelling of the two-phase mixture in the lower plenum governs the Emergency Core Cooling (ECC) water bypass and the refill of the lower plenum at the end of blowdown phase of a PWR-LOCA. In order to develop an analytical model of the swell behavior, a visual air-water experiment was performed by simulating the flashing in the lower plenum. In the experiment, two layers, i.e. the froth at the lower portion and the liquid deficient layer at the upper portion were observed in the lower plenum. And it was observed that the liquid drops were ejected from the froth surface and carried to the downcomer by the ascending air flow. Based on these results, a new correlation was developed for the overall averaged void fraction of the lower plenum, This correlation was applied to the CREARE flashing experiments and good comparisons were obtained between the calculated and the measured mass transients in the lower plenum.

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To study the thermodynamic aspects of blowdown, the depressurization rate equation has been numerically solved. The equation, derived from macroscopic mass and energy balances in the pressure vessel, consisted of the energy and volumetric discharges terms multiplied by the decrease rate of residual coolant. By applying a dimensional analysis, dimensionless equations were obtained together with dimensionless parameters of blowdown. Blowdown calculations starting at typical BWR operating conditions indicated that the de-crease rate of coolant increased for the liquid and two-phase mixture, and decreased for the vapor discharge. Further, the energy discharge term made a larger contribution to the depressurization rate in the case of vapor escape, while the volumetric discharge term did so in the case of liquid and two-phase mixture escape blowdowns. In the lumped model analyses, the averaged specific enthalpy and entropy of the residual coolant increased for the liquid discharge, remained almost constant for the two-phase mixture discharge, and decreased for the vapor discharge blowdown.

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A position sensitive proportional counter of a charge division type with a single resistive anode wire was constructed for the measurement of the movement of ³H labelled gas which is flowing or diffusing in a pipe. The introduction of resistors between the anode wire and pre-amplifiers brought a uniform detection efficiency for ³Hβ-rays throughout the counter. The position resolution was 3.1 mm FWHM. Detection efficiency was almost 100% uniformly over about 700 mm in the total anode length of 740 mm. The movement of ³H labelled gas could be measured effectively.

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The equilibrium distributions of several lanthanide and actinide elements between molten LiF-BeF₂ salt and liquid bismuth were determined over a wide range of salt composition at some temperatures from 600 to 800°C. The distribution behavior of each element was ap-parently affected by salt composition and temperature. The effect of salt composition on the distribution behavior was well explained by considering the formation of complex in the salt phase. Also, the excess free energies of the solutes in the bismuth phase, calculated from the equilibrium distribution data, were considerably negative, which suggests the for-mation of intermetallic compound in the bismuth phase. Taking into account the formation of these compounds, the equilibrium distribution can be expressed as
Li_(m-n)MF_m+nLi+x Bi=MBi_x+m LiF.

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