The fission cross section ratio of ²⁴³Am to ²³⁵U has been measured in the energy range of 1.16.8 MeV with monoenergetic neutrons. An ionization fission chamber was used to detect fission events. The quantitative analyses of the fission samples were made with a low geometry counter and a 2π counter. Uncertainties of the measured data were analyzed considering correlations between error elements. The present result is very close to that of Fursov et al. and lower by about 20% than the values reported by Behrens & Browne.

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Reactivity decrease due to temperature rise of a single fuel rod sample was measured in the SHE-14 core using a sample heating device with purpose to verify the calculation accuracy of the Doppler effect for resonance neutron absorption in a Very High Temperature Reactor. The fuel rod sample was a stuck of coated particle fuel compacts containing 4% enriched UO₂ kernels, and it was heated up to about 700°C in a sample heating tube which was placed along the axis of the core.
The difference of reactivity decrease between the two same size samples of fuel rod and graphite rod due to temperature rise can be interpreted as the increased resonance neutron capture of ²³⁸U, i.e. Doppler effect. The SRAC code system was applied to the Doppler effect calculation where the collision probability method was used in the cell calculation and the one-dimensional, multi-group diffusion approximations were adopted in the core calculation. The results gave a ratio of the calculated to the measured Doppler effect of 0.93.

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Natural convection tests were conducted using Pb-Bi alloy, with the view to evaluating the effects brought by thermal stratification in liquid metal on the natural convection heat transfer along an immersed vertical metal surface. The vertical metal surface was represented by a stainless steel plate 300 mm high, placed in a cylindrical vessel 400 mm in diameter filled with molten Pb-Bi. The experiment was performed with the heat flux of the plate surface maintained constant and uniform. The temperature distribution through the liquid metal filling the vessel was controlled by regulating a flow of air blown onto the vessel surface; temperature measurements were made by means of a traversing thermocouple.
For the case of unstratified bulk fluid, the values obtained for heat transfer rate agreed well with corresponding data given by Sheriff for Na. With increasing degree of stratification, the thickness of the boundary layer decreased in keeping. The resulting increase of heat transfer rate indicated a definite dependence on the stratification parameter; this dependence was determined and expressed in terms of nondimensionalized boundary layer equations. Calculation of heat transfer rate based on numerical method yielded values agreeing fairly well with the experimentally determined data, but that based in integral approximation proved to give underestimated values.

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This paper proposes a new method of failure diagnosis of nuclear power plant (NPP). Transient behavior of the NPP includes ample failure information even for a limited period of time from the failure onset. We tried to develop a diagnosis system with high capability of identifying the failure cause and of estimating failure severeness. The Walsh function transformation of transient time series data and the reduction of the Walsh coefficients into ternary valued amplitude indicators were utilized to extract the essential characteristics of failure. The correspondences of the transient characteristics and causes were summarized in a failure symptom database. A method of ternary tree search using an information measure as a heuristic strategy was adopted to conduct the efficient retrieval of failure causes in the database. Through numerical experiments using a simulation model of a NPP, the diagnostic capability of the system was proved to be satisfactory.

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Ion-exchange property and thermal stability were studied on four synthetic hydrous metal oxides from the viewpoint of applicability for the processing of liquid radioactive waste. Hydrous titanium (IV) oxide and hydrous niobium (V) oxide, consisting of metal ions having larger ionic potential, possessed large cation-exchange capacities, and showed high affinity for both the transition metal ions and the representative element ions. Hydrous tin (IV) oxide and hydrous zirconium (IV) oxide, consisting of metal ions having smaller ionic potential, were amphoteric ion exchangers possessing large anionexchange capacities, and showed much higher affinity for the transition metal ions than for the representative element ions. The selectivity series are understood in terms of the formation of a bond partially covalent in character for the transition metals, and the purely electrostatic interaction between anionic exchange sites and unhydrated or hydrated cations for the representative elements. An examination of the effects of heat treatment in air on the ion-exchange properties indicated that the hydrous niobium (V) oxide was the most stable among the four hydrous metal oxides. Even this material, however, was not stable enough to be used under hydrothermal conditions.

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In order to evaluate separative performance of a thermal diffusion column, a simplification is usually made in which the temperature dependence of the relevant properties such as thermal diffusion constant is ignored and some proper mean values evaluated at a specific "mean" temperature are used. Adoption of weighted average of temperature distribution is common for the "mean" temperature, but there exists no definite way of determining mean temperature. The present paper proposes a new reference mean temperature determined by the equation governing the free convection. It is based on the fact that the multiplication effect of free convection is essential to separation by thermal diffusion column. The reference mean temperature is related to pressure difference between top and bottom of column and is higher than a mass-averaged temperature (due to gravitational force) by a contribution of viscous force. The reference mean temperature was calculated, as a reference, for an Ar isotope separating column with an inner hot radius of 0.2 mm and an outer cold radius of 5 mm. The results confirmed the validity of an approximate formula expressing effects of temperature difference and ratio of inner and outer radii of column explicitly for the temperature. The reference mean temperature calculated from pressure difference given by axisymmetric solution of equations of change was in good agreement with the analytical solution.

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As an initial part of development of Pt-Mo alloy thermocouples for applying them to in-core outlet-gas temperature measurement in a very high temperature gas-cooled reactor (VHTR), bare element wires of Pt-5%Mo/Pt-0.1%Mo thermocouple were tested in a high-temperature Ar atmosphere of 1, 0001, 400°C for 30 h and in vacuum at 1, 200°C for 3, 000 h, and stabilities of their electromotive force and mechanical strength were examined as well as observations of their metallographical changes. In the high-temperature Ar atmosphere, selective reaction of Mo with impurity gases contained in the Ar took place very actively in the element of Pt-5%Mo, and its mechanical strength decreased considerably with losing its luster. In the high-temperature vacuum, however, the element wires showed very stable electrical, mechanical and metallographical characteristics for a long-term of 3, 000 h. This suggested the importance of clean environmental atmosphere around the element wires. Based on the result, Ta-sheathed thermocouples were fabricated in trial and tested, expecting that the Ta sheath would work not only as a protector but also as a getter for impurity gases existing around the element wires. They showed very stable characteristics under a high-temperature condition for a long term with encouraging the future work in development of highlyreliable, long-life thermocouples for actual in-core services in VHTR.

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Intensity of the thermal neutrons emitted from the moderator with a reflector was calculated to study the effects on the intensity caused by a macroscopic total neutron cross section and an average logarithmic energy loss of the reflector materials.
A reflector with a large macroscopic total neutron cross section produced higher thermal neutron intensity than that with a small cross section if they had the same average logarithmic energy loss. Among the reflectors with the same total macroscopic neutron cross section, the thermal neutron intensity was not changed by decreasing the average logarithmic energy loss to a range less than about 0.1 but above this value the intensity was weakened. From this result it was found that a large macroscopic total cross section and a small average logarithmic energy loss are preferable characteristics for reflector materials.
As actual reflector materials, three reflector materials were examined, namely beryllium, graphite and lead, which are now considered to be candidates. The lead reflector was effective for the moderator with a large emission-surface and the beryllium reflector for the moderator with a small one. This result indicates that the moderator size is important for choosing the best reflector material to produce the highest beam intensity.

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