The most probable charge Zp is obtained from Myers & Swiatecki mass formula by analytical method and the Zp is expressed in terms of the part of simple liquid drop model, the shell and deformation effects. The deviation of the predicted Zp-value from the Zpucp based on the unchanged charge distribution can be well interpreted by these terms. The shell effect shows the saw-tooth curve and the deformation effect shows a hump on the range of mass numbers where the shell function S(N, Z) exceeds the criti-cal value Scrit introduced by Myers & Swiatecki. The predicted Zp-values fairly well agree with the semi-empirical result obtained by Wahl and the fine structure is well reproduced. The magnitudes of the shell and deformation effects in the thermal neutron fission of ²³⁵U are about 0.7 and 0.25 charge units, respectively and both effects act so as to cancell each other.

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A formula for neutron cross sections of heavy nuclei is derived on the basis of the Wigner-Eisenbud theory. The derived formula is the same as the single-level one intro-duced by Breit and Wigner except for the inclusion of additional interference parameters (u, v) which represent the contribution of the interference effect between resonances. The present formula is therefore applicable to reactor calculations without much modifications of the existing resonance integral codes.
The present formula has been applied to the analyses of the ²³⁵U and ²³³U fission cross sections and the ²³⁸U total cross sections in the resolved resonance region. By the use of least squares fits of the experimental data, the interference parameters (u, v) are obtained for resonance levels of these nuclei in their typical resonance regions. It is shown that the present formula well represents the experimental data.
In the unresolved resonance region, both the single-level resonance parameters and the present interference parameters are generated by using a random sampling method. The contributions of the interference between the resonances.to the Doppler effect are also evaluated for the fission cross sections of ²³⁵U and ²³⁹Pu in the unresolved resonance energy region of 1 to 2 keV.

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A method for estimating the multiple control rod worth in large fast reactors has been developed using the higher order perturbation technique. With this method, physical significance of the interaction effect between multiple control rods can be understood by decomposing it into independent components with different physical processes. Further-more, a new technique is proposed, whereby the interaction effect between multiple control rods can be estimated from a combination of the interaction effects between two or three control rods depending on the strength of the effect. The validity of the method is ex-amined with the experimental data of PHENIX and numerically for MONJU. The results show that the interaction effect between multiple control rods in prototype fast reactors can be estimated from a combination of the effects between 2 control rods with a good accuracy. The numerical examinations for a 1, 000 MWe fast reactor showed that the interaction effect is too large to apply the present method for large fast reactors. To overcome this difficulty, a new concept of "quasi-control-rod" is introduced in the method. With this improved method, the worths of 19 and 18 control rods in the 1, 000 MWe fast reactor are estimated with accuracies of ±2%.

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A study, both theoretical and experimental, was undertaken on the hydrogen dissolu-tion ratio in liquid sodium, with the view of estimating the quantity of hydrogen that dissolves into sodium before reaching the free surface, following the occurrence of sodium-water reaction in a steam generator. Superheated steam was injected into the sodium in a reaction vessel of configuration simulating the steam generator. The hy-drogen dissolution ratio was determined from analysis of the responses obtained on a hydrogen detector installed downstream of the reaction vessel.
The hydrogen dissolution ratio was found to vary from 5 to 15% over a leak rate ranging of 310 g/sec, as determined both from the transient response data given by the hydrogen detector and from the corresponding data obtained from tests on changes in hydrogen concentration upon complete mixture with the entire sodium inventory.
A theoretical model was assumed for estimating the hydrogen dissolution ratio, which yield results in good agreement with the experimental data, in respect of the rising tendency of the dissolution ratio with decreasing leak rate and with rising sodium tem-perature.

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The behaviors of CH₃He⁺ and C₂H₅He+ formed by the decay of CH₃T and C₂H₅T were studied theoretically using the STO-3G molecular orbital method and was compared with that of HHe⁺ in the decay of HT. It was clearly shown that the ground state daughter ions CH₃He⁺ and C₂H₅He>⁺ dissociate instantly to give CH₃⁺ and C₂H₅⁺ because their poten-tial energy curves are repulsive, whereas the daughter ion HHe+ in the ground state does not dissociate. The transition probability to the ground state ions of CH₃He+ and of C₂H₅He⁺ are computed to be 66.5 and 64.8%, respectively. These values are in fairly good accordance with those obtained experimentally.

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The nucleation and growth of voids have been observed successively in differentthickness specimens of Type 316 stainless steel electron-irradiated at 550°C in a high voltage electron microscope. In a bulk representative 1.5μm thick specimen, the void number density increases rapidly and saturates during the initial stage of irradiation and then decreases with following dose by void coalescence. The swelling increases proportionally with (dpa) 1.5 up to about 30 dpa. In a thin specimen, of 0.4μm thickness, on the other hand, the void number density increases continuously with dose up to about 25 dpa. The swelling of the thin specimen showed a tendency to saturation due to the disappearance of voids at the specimen surfaces. The difference in swelling behavior between the 1.5 and 0.4μm thick specimens can be ascribed to the different effects of the specimen surfaces, which serve as a dominant sink for both radiation-produced point defects and dislocations.

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A study was performed on the effect produced by radiation on the gas permeation and gas separation performances of cellulose acetate and silicone rubber membranes, to be used in gas separation cells. When cellulose acetate membranes are irradiated, the resulting temperature rise causes a reduction in the gas permeability and improves the gas separation factor. By producing an approximately equal decrease in gas permeability, a dose rate of 2.0×10⁵ rad/sec was found equivalent to annealing at 140°C, which is in substantial agreement with the membrane surface temperature measured during irradiation. The results obtained with silicone rubber membrane were similar to those reported by Robb and Nakagawa.

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