The diffusion of C in UC has been measured using radioactive tracer and sectioning technique. Self-diffusion coefficients of C in UC are described well by the equation
D=4.7×10^-2exp(-54.7±2.8/RTkcal/mole)cm²/sec over the temperature range of 1, 400°∼2, 000°C.

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The thermal neutron spectra in light water of slab geometry poisoned with Cd and/or In were measured by the time of flight method, using a 20-MeV electron linear accelerator. The thermal spectra were simulated to those in the Pu built-up core of a commercial light water reactor corresponding to a fuel burnup of about 15, 000MWD/T. The results of measurements were compared with calculations based on the S₄ method using the Haywood scattering law. Fairly good agreement was obtained between the calculated and measured results except in a limited range of energy above the 0.176eV resonance of Cd. It is concluded that the P₁ components of the source neutrons as well as the neutron scattering kernel play a significant role in the calculation of the thermal neutron spectra with large flux gradients, and that the scattering kernel of light water based on the Haywood model will be accurate enough to evaluate the infinite multiplication constant k_∞ of light water reactor cores with high fuel burnup within an error of about -0.17%, as estimated from the uncertainty in the spectrum calculation in the region above the Cd resonance. It is also emphasised from the two-dimensional S₄ calculations that the effect of reentrant hole perturbation should be evaluated quantitatively in the interpretation of the measured angular neutron spectra produced within finite media.

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A strain gage was used for the measurement of fuel cladding strain generated during pulse operation tests on the Hitachi Training Reactor. In the analysis of the measured strain, two kinds of correction were called for: (1) the radiation effect on the strain gage and lead wires, and (2) the temperature effect due to the lag of the gage filament temperature behind the true fuel cladding temperature. The experimental axial strain after the two corrections were applied was 781×10^-6cm/cm for the hottest fuel rod in the pulse operation test with an inserted reactivity of 1.20%Δk/k. This maximum strain corresponded to 2, 169kg/cm² of thermal stress and 111cal/cm²•sec of heat flux. These results were obtained under the condition of maximum temperature in the fuel center of 1, 200°C and a fuel cladding temperature of 140°C. When the axial strain was calculated with consideration given to the gap or contact conductance between the fuel and its cladding, a reasonable agreement was obtained between the calculation and the experimental results.

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A novel method was developed for preparing ³⁵S-labeled Cu³⁵SO₄•5H₂O directly from neutron irradiated KCl. One gram of neutron irradiated KCl was dissolved in water and passed through a cation exchange resin column in Cu²⁺-form for the purpose of converting KCl to CuCl₂. In the presence of carrier sulfate, the precipitates of ³⁵S-labeled sulfate were separated from the effluent by the addition of ethanol. Factors affecting the recovery and quality of the final products were studied and a standard procedure was devised at optimum conditions.

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The Snoek peak and the strain amplitude dependence of internal friction were measured in order to gain further information on the behavior of interstitial solute atoms in Fe-3.48%Ni alloy which had been neutron-irradiated and annealed. This alloy contained 37ppm N and 7ppm C.
Interstitial solute N and C atoms were trapped by irradiation-induced defects and released by annealing at above 250°C. The Snoek peak and the slope of Granato-Lücke plots which latter is proportional to 1/L_c (L_c being the dislocation loop length between impurity atoms) increased with annealing temperature.
The binding energy of interstitial solute N atoms to dislocations in the irradiated and annealed specimen was obtained from the strain-amplitude dependent internal friction. The value was about 16, 500cal/mol.

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The results of an experimental study on incipient sodium boiling with upward forced convection loop are reported. Forced convection sodium in a vertical tube of 14.9mm I. D. was directly heated by direct current (∼4V, ∼5, 000A). The experiments were carried out under the following conditions:
Input power: 6∼12kW, Pressure at the condenser: 0.9∼1.1atm
Flow rate: 1.0∼3.0l/min, Inlet temperature: 7, 500°∼830°C
The incipient boiling phenomena observed were either violent single bubble ejection or else slow vapor formation depending on the conditions.
It was indicated from observations of the position of boiling inception that the boiling did not always occur at the position of maximum temperature.

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The paper describes the principle of measuring digitally the flow velocity of local voids, and discusses the accuracy of this method. It is shown that the spectrum of local void velocities can be determined reliably with digital values.
The measurement errors of void velocity u_g (m/sec) are within about ±9% in cases when the probes have detector head lengths of 0.8∼1.0mm, and head diameters of 0.1mmφ, when the voids have a flow velocity in the range of 0.2m/sec_??_u_g_??_1.2m/sec and a radius of curvature of void of 2mm_??_R_??_16mm.

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