Fission spectrum averaged cross sections of twenty one threshold reactions were measured in the core center of YAYOI which was a fast neutron source reactor. Fast neutron spectrum in the core was experimentally determined by using a set of activation foils and micro-fission counters, prior to the cross section measurement. It was found that the shape of the fast neutron spectrum was approximately the same as that of fission neutrons above about 2 MeV. This fact was also supported by theoretical calculation.
Since this neutron field has scarce thermal and epithermal neutrons, measurement of nuclei produced by threshold reactions is not affected by (n, γ) reactions which are induced by thermal and epithermal neutrons. Moreover, considerably high fast neutron flux (about 5 × 10¹¹ n/cm² sec) enables to measure cross sections of small values.
The results in general agreed with the previous values obtained in a reactor core or with a fission plate within an experimental error, while they were systematically smaller by about 10% than those recommended by Fabry. The measured values are also compared with the results calculated by Pearlstein based on a statistical model.

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A modified one-group diffusion scheme and a coarse-mesh diffusion method were newly developed independently, and are combined together to make an effective brief scheme for the solution of few-group diffusion equations in a 3-dimensional BWR core. In the above two methods, the local spatial behavior of the neutron spectrumthe ratio of thermal neutron flux to the fast neutron flux -and of the fast neutron flux within a volume element are approximately taken into account by using analytical procedures, therefore the resultant numerical equation, which is slightly more complicated than those of the conventional brief schemes, gives sufficiently accurate results.
The special behavior of the spectrum in the neighborhood of the core-reflector interface is also treated in the appendix of this paper. An analytical expression for the spectrum on the core-reflector boundary of various shapes is derived and some numerical examples are presented.
The results obtained from a series of test calculations indicate that the present method assures an accuracy comparable to the standard few-group fine-mesh finite-difference calculation without requiring much more computing time than those of the conventional brief calculation methods. Thus the present method is particularly suited for improving the computational accuracy of the BWR simulation codes.

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Neutron pulse die-away experiments for small graphite assemblies by using a pulsed "cold" neutron source technique were carried out for the purpose to measure the pseudo-decay-constants of trapped neutrons at the lowest Bragg-peak energy E_B of coherent scattering in graphite, and also to estimate the temperature dependence of inelastic. scattering cross section of neutrons at E_B. Experiments were carried out for 77 and 194°K graphite systems with dimensions from 40 x 40 x 40 cm³ to 30 x 30 x 20 cm³.
The experimentally determined pseudo-decay-constants showed distinct temperature dependence and good agreement with the theoretical inelastic scattering cross sections by Young-Koppel model of phonon frequency distribution of graphite at these low temperatures.

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The resonance neutron self-shielding factors for cylindrical samples of various thicknesses of gold and silver have been measured in the narrow energy groups with a lead slowing-down time spectrometer as a neutron source.
Calculation by Wigner's rational approximation gives 1020% smaller self shielding factors than the experimental ones except for the limiting cases of sample thickness.
The collision probability theory, on the other hand, was found to give rather good agreement with the experiments in all the measured cases.

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An advanced transport equation containing only three reduced terms H, K_C and K_d has been examined. The well-known, classical Furry-Jones-Onsager theory is ameliorated by introducing three factors such as a circulation constant of natural convection, new definitions of mean temperature between hot and cold wall and of mean composition from top to bottom in a thermal diffusion column.
The theory makes it possible to estimate with sufficient accuracy the separation performance for two component mixture under total reflux operation as a function of pressure. A correlation and analysis are made of steady state data obtained in the experiments of a hot-wire type thermal diffusion column using three kinds of gas mixture such as Ar-Kr, Ar-Xe and Kr-Xe.
Here are shown the reasons why the Furry-Jones theory could hardly predict the experimental separation performance in a steady state, though it has been sometimes considered that big difference between the theory and the experiments should be caused by "the parasitic remixing effects".

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Zircaloy-2 tubes were hydrided up to a nominal content of 200 ppm and irradiated as fuel claddings in HBWR. Post-irradiation ring-tensile testing revealed that hydrogen enhances the irradiation-induced decrease of elongation and wall thickness reduction at room temperature. On the other hand, no effect of hydrogen was observed on ultimate tensile strength. With testings at 300°C, the effect was negligible on elongation too. From the evaluation of the test results including metallographic observation of ring specimens after fracture, it was concluded that segregation of hydrides due to thermal diffusion of hydrogen during irradiation was at least a part responsible to the above effect of hydrogen enhancing embrittlement.

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The separation of ⁹⁹Mo from other fission products has been successfully attempted by isolating S in a HNO₃ solution of neutron-irradiated UO₂ and collecting ⁹⁹Mo on it. Influence of various factors on the separation of ⁹⁹Mo was examined. When 5% aqueous solution of K₂S was added to the solution of fission products of high HNO₃ concentration and the mixture of S stood for 10 min, over 80% of the ⁹⁹Mo could be separated with the precipitate. Besides ⁹⁹Mo, ¹³²Te, ¹⁰³Ru and other nuclides also accompanied the precipitate. The presence of Mo carrier and U affected greatly the separation of ⁹⁹Mo, but that of Zr or Te carrier did not.

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In simulation of partial failure affecting a fast breeder core, experiments on sodium boiling were performed in a single vertical channel of annular cross section (15 mm I.D.), indirectly heated by high flux heater pin (6.5 mm 0.D., 60 cm heating length) and provided with a blockage disk (11 mm 0.D., 1 mm thick, obstructing 42% of the channel area). The experimental conditions were ; Pressure of cover gas : 1.0 kg/cm² abs., Heat flux : 080 W/cm², Flow rate of sodium : 14 l/min.
The results revealed that the degree of incipient boiling superheat is reduced to a fraction of the value obtained previously for direct joule heating under similar conditions, and that, consequently, the intensity of pressure pulses is correspondingly reduced. On the other hand, the incipient boiling superheat increases, together with the extent of its scattering, with rising intensity of the heat flux.
Local boiling was observed to precede the onset of bulk boiling, causing small pressure spikes to be detected by all three pressure sensors installed along the test channel. The pressures evaluated from the measured velocities of the liquid sodium column based on single-bubble model were compared with the measured pressure signals, and a fairly good agreement was obtained.

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