Making use of a model based on the statistical theory, calculations were performed to obtain the mass yields, the most probable charges, the kinetic energies and the prompt neutron yields of fission fragments, and the mass yields of fission products from thermal-neutron-induced fission of ²³³U ²³⁵U ²³⁹Pu ²⁴¹Pu and from the spontaneous fission of ²⁵²Cf. The calculations are further extended to fast-neutron-induced fission. The scission-point distance is treated as a parameter varying with the mass number of the heavy fragments of fission. This proved successful in approaching the calculated curves closer to the observed values.
It is possible to predict unknown physical quantities in nuclear fission with use made of the method developed in the present work.

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This paper describes an LMFBR core design optimization procedure, where the sequential unconstrained minimization technique (SUMT) using the conjugate direction method for the unconstrained minimizer is adopted as the optimization algorithm. To minimize computing effort in estimating the equilibrium core condition, the procedure utilizes the regression func-tions of the burnup characteristics in a simplified method of calculation, which is called the "power balance method". As a numerical example, the procedure was applied to calculations on a 1, 000 MWe LMFBR core design, on which an evaluation is also made of the sensitivity shown by the results to changes in critical constraints.
It is shown that the optimization algorithm based on SUMT is well applicable to core design optimization, provided careful attention is paid in treating the penalty term of the transformed performance index in order to obviate a discontinuity occurring in the locus of the optimized solution at the constraint boundary. It is also demonstrated that the simplified method of burnup calculation is helpful in reducing computer time. Further, some consider-ations are presented on such aspects as the behavior of the design variables, computing effort required, and the main characteristics of the optimized core.

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A series of 30 runs were performed to gain information on the fragmentation of molten UO₂ when dropped into a bath of liquid sodium. In this experiment, hollow cylindrical UO₂ pellets were heated with electric current fed to a tungsten rod, passed through to center hole. The resulting molten UO₂ dripped down its fall into the sodium bath. High-speed cinema-tography (5002, 000 frames/sec) was utilized to observe the fragmentation of the UO₂ droplets upon its impact with the sodium surface. Records were taken of the changes in pressure and temperature induced in the sodium bath by the impact of the UO₂ droplets.
The physical characteristics of the solid UO₂ particles resulting from the quench were examined for particle size distribution, particle shape and surface condition, as well as micro-structure, to gain an insight into the mechanism of fragmentation. It is concluded from the study that, for the purpose of Fuel Coolant Interaction (FCI) analysis, the UO₂ particles can be assumed to be spherical in shape and to have log-normal particle-size distribution.
The density function in terms of particle weight, representing the finest particle-size distribution, can be expressed on the basis of presently available empirical data, by
f(logD)=58.1exp(-(logD-2.27)²/0.944), D: Diameter of UO₂ particle in microns.

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A method was developed for recovering the fission product ¹³³Xe from several kinds of reactor-irradiated U targets, including Al-U alloy, metallic U, and uranium oxides.
In order to observe the release of ¹³³Xe from U compounds at high temperatures, irradiated targets were heated at 5001, 000°C in vacuum or under gas flow. The liberated ¹³³Xe was trapped on charcoal beds, and the release rate of ¹³³Xe from the compounds was determined by measuring the activity accumulating in the traps.
More than 90% of the ¹³³Xe was liberated from the alloy upon melting and from metal and uranium oxide upon oxidation.
The isolated Xe was purified by a system embodying cold traps and cryogenic distillation.
The final products were sealed in ampoules. They proved to possess radiochemical purity exceeding 99.9 %, and less than 1μl/ampoule (1 ml) of non-radioactive gaseous contaminants.

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A numerical method was developed to calculate the distribution of concentration of ²³⁵UF₆ in the rotating cylinder of a centrifuge. Satisfactory convergence was obtained by employing the formulation of convective terms devised for the computation of high Reynolds number flow. Moreover, the pressure diffusion term was linearized and approximated by a constant value, which significantly reduced the number of cases to be run on a computer. The results of numerical analysis for a centrifuge of counter-flow type are shown, and the effect brought upon the separative efficiency by changes in various parameters is discussed. The results of the present analysis are compared with those obtained from the existing theory, and the reasons for the discrepancy between the two results are explained. Finally, it is shown that the results of the numerical analysis agree well with the experimental data obtained previously by Beams et al.

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A study was made of the effects brought upon the radiation-induced high-temperature embrittlement of AISI 316 austenitic steel by different conditions to the specimens to produce recrystallization.
Cold worked specimens were recrystallized at temperatures 950°1, 100°C held for periods ranging 260 min. The specimens were then irradiated to 1.7 ×10²¹nvt(>1 MeV) at 55°C after which they were subjected to tensile testing at 650°C.
In the specimens recrystallized at 950°C, carbide precipitation was observed to have occurred, and these specimens were found less liable to show radiation-induced embrittlement. On the other hand, specimens recrystallized at 1, 025° and 1, 100°C became completely solution treated, and exhibited severe radiation-induced embrittlement. It is surmisted that the carbides pre-cipitated on the grain-boundaries tend to inhibit propagation of grain-boundary cracks, and hence contribute to lessing radiation-induced embrittlement.

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A simple method of calibrating the cascade impactor has been developed by applying Hongo et al.'s two-stage method to evaluate the particle-size distribution. The constants ψ and σ-for a function representing the impaction efficiency were determined for the UNICO cascade impactor, using the test particles produced by a spinning-disc aerosol generator. The values were ψ=0.35, σ =1.46, respectively.
The validity of the method for the UNICO cascade impactor was confirmed from micro-photographic observation of a sample artificial aerosol containing different sizes of uranine methylene-blue particles. Both the mass median aerodynamic diameter and the geometric standard deviation obtained with the method agreed with the microphotographic measurements with relative errors not exceeding 10%.

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