The reactivity equivalent of control rods was measured by three-variable measurements. The reactivity of a core was measured upon simultaneous displacement of two control rods from a critical point. The worth of one of the control rods, when the other was not displaced from the critical point, could be obtained by data processing. The results are compared with those from two-variable measurement, in which the reactivity change is determined by the summation of a series of small changes. The results agreed very well between those of the three-variable and the two-variable measurements. The differences were within ±5%. The present experimental procedure is much simpler than required for the currently practiced two-variable measurement.

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The effect of group collapsing applied to the perturbation theory for sample worth analysis in fast reactor systems is theoretically and numerically examined assuming the validity of the thin sample approximation.
As a result, the calculated worths of scattering predominant materials placed at the center of core are found to be strongly influenced by the group collapsing. The effect on the sample worth when the sample is placed in positions off the core center decreases with increasing distance from the center. It is noted that the reactivity perturbation due to inelastic scattering is also affected significantly by group collapsing especially near the core-blanket interface.
Based on the above observations, it is concluded that the perturbation theory with about 70 energy groups appropriately arranged is necessary to reproduce the experimental values of Na, O and Fe sample reactivity worths with accuracy efficient for ordinary purposes.

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An experimental and theoretical study has been conducted on two interference effects of neutron waves in heterogeneous multiplying systems-the interference between elementary waves having greatly different characteristics of propagation and the interference due to lattice effect, taking as example a plate-type system consisting of natural uranium and graphite. The experiments on neutron wave propagation were performed with external modulated sources. The effects examined on the neutron waves covered the multiplicity of system, the direction of propagation and the source spectrum. Analyses by fission expansion method have proved the existence of the first-mentioned form of interference in the source transient region. The second form of interference effect on the dispersion law with account taken of heterogeneity, is discussed numerically in comparison with the homogeneous multiplying system.

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Fractional electro-deposition of ²¹²Pb and ²⁰⁸Tl from RdTh source using a capsule equipped with three electrodes was carried out under normal pressure. The experiment was conducted with variations brought to the applied voltage, the interval between the electrodes, time allowed for deposition and grid conditions. The active deposit thus obtained was examined by radiochemical and autoradiographical methods.
Measurements of the active deposits on the electrodes showed that ²¹²Pb had deposited on the grid net, and ²⁰⁸Tl on the target. The mechanism of the electro-deposition process under electric field with and without a grid is discussed on the basis of the decay pattern shown by each nuclide.

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Fast cross section sets are prepared for the analysis of fast critical assemblies to test the agreement of calculated and measured integral parameters. Modifications are brought to fissile element cross sections making use of recently measured cross section data, and these updated cross section data are utilized to compare the calculated integral parameters of these assemblies, and to determine the effect brought to the calculated results by the differences between the data. The results are presented for the eigenvalue, central fission ratios and central sample reactivity in the ²³⁹Pu fueled assemblies ZPR-3-48, 49, 50 and ZEBRA-6A, whose spectra simulate those of large fast reactors. In addition, for ZPR-6-7 and ZPPR-2, which are large fast critical assemblies intended for testing the calculational procedures of practical demonstration reactor design, the results of analysis obtained with the updated cross section set are compared with experiments. It is found that the cross section for ²⁴¹Pu recommended here satisfactorily agrees with the experimental reactivity worth.

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Oxide film was made to form on the outside surface of zircaloy-2 tube (inside diameter=12.672mm, wall thickness=0.826mm) under the following three different conditions, (a) 300°C saturated LiOH solution, (b) 450°C and 15 ata steam and (c) 450°C air. The resulting oxide presented colors ranging from white to black, depending on the oxidation temperature and duration of exposure.
Each of these oxidized specimens, together with a control specimen (unoxidized) was heated directly by alternating current in high temperature coolant water sealed in an autoclave.
The apparent thermal conductivity of the oxide film was obtained by comparison of the temperatures inside tube measured on the oxidized and clean specimens under conditions as similar as possible. The experiment was conducted under 110°, 150°, 200°, 250° and 300°C of coolant temperature.
At near 110°C of coolant temperature, the apparent thermal conductivity was not affected significantly by difference in the method adopted for oxidization, and the values obtained were distinctly lower than that of sintered ZrO₂ stabilized with CaO. At higher temperatures, the oxide films formed under different conditions each showed individual heat transfer characteristics. The apparent thermal conductivity of white oxide film was smaller than that of black oxide film at near 300°C of coolant temperature.

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An experimental study of transient boiling of sodium by direct heating is presented. Liquid sodium circulating by forced convection in a vertical tube of 14.9mm inner diameter is heated by direct current (∼4V, ∼5, 000A) applied through the sodium.
The velocity of ejection of the vapor bubble and the variations in the thickness of the film remaining on the channel wall were measured during a single bubble ejection. Furthermore, a simplified model formulated around the inertia of the liquid column was used to derive estimates for comparison with measured data. The experiments were performed within the following ranges:
Input power: 6∼12kW
Pressure at the condenser: 0.6∼1.1kg/cm²
Flow rate: 1.0∼3.5l/min
Inlet temperature: 750°∼850°C
The ejection velocity measured was between 2.0 and 16m/sec. The velocity increased with the degree of superheat, i. e., the velocity was governed by the degree of superheat in the region where the vapor phase expanded.
The film thickness ranged from near zero to 1.0mm. The film thickness increased with ejection velocity.

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A few concrete conditions for the existence of the time eigenvalue of fast multiplying systems are derived from the integral transport equation. In order that the real time eigenvalue may exist, there are limitations to the composition and the size of the systems. It may be considered, with very good approximation, that a necessary condition for the existence of the real time eigenvalue is that the infinite multiplication constant is not less then unity. With respect to the size of the system, if half the minimum chord length is not less than three times the inverse of {Σ_s(υ)+νΣ_f(υ)}_min, the system is not too small for the existence of the real time eigenvalue. These conditions can be applied to the planning and the analysis of pulsed neutron experiments on a fast multiplying system.

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