Release of Xe from lightly sintered UO² was measured by post-irradiation heating in vacuum at 400°-800°C. The release rate after initial burst followed the diffusion model. An Arrhenius plot of the diffusion coefficients revealed a discontinuity at 600°C. Activation energies were calculated to be 3.4kcal/mol at temperatures lower than the discontinuity, and over 36kcal/mol in higher temperature range, which corresponded respectively to non-lattice and lattice diffusion. The discontinuity is explained by a changeover of the predominance of one process over another depending on temperature range. The initial burst could not be attributed to surface oxidation. Based on analyses of such factors as dependence of burst on temperature and on specific surface area, as well as percentage of burst release, a kind of activated extrication from superficial layer, approximately 5 ?? thick, was proposed as a concept for interpreting the burst phenomenon.

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Enrichment of ³²P was attempted by Szillard-Chalmers effect on a neutron irradiated tripolyphosphate hexahydrate in the first eluate from anion-exchange resin column by chromatographic separation with gradient elution. Forty to fifty % of radioactive ³²P was found as orthophosphate, having a specific activity of around 100mCi/g of phosphorus at the end of about 20 min irradiation with a neutron flux of 5×10¹²n/cm²•sec in the KUR 1MW reactor. The increase of specific activity is straightforward for a short period of irradiation. The enrichment factor was several thousand when recrystallized tripolyphosphate hexahydrate was used as target compound containing 0.02% of orthophosphate-P as impurity. Radiation effect, radiolysis by γ-rays and fast neutron, thermal degradation and isotopic exchange effect were examined as factors that might affect the enrichment. Therelation between the specific activity of the isolated phosphates and the exposure time was estabished from a series of experiments in which the time of irradiation was varied.

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The interference effects of the resonance scattering of Na on the resonance absorption of ²⁸⁸U were investigated. The results for self-shielding factor obtained from exact treatment were compared with those from the usual conventional method, as well as with results based on the assumption that φ(u)σ_t=const. (φ(u): flux, σ_t: total cross section).
In the exact treatment, the neutron spectrum used is obtained with accurate treatment of the slowing down by the elastic scattering of light and medium elements. In the φ(u)σ_t=const. assumption, the energy dependence of the Na resonance scattering cross section is taken into account as in the exact treatment, whereas only the averaged value is used in the conventional method.
Self-shielding factors and their temperature dependence were calculated for several compositions and energy regions. It was found that the conventional method is not satisfactory, while the assumption that φ(u)σ_t=const. compares well with the exact treatment.

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Spectra of backscattered radiation from finite thick slabs of polyethylene, aluminum and lead were obtained by means of a scintillation spectrometer. A point ⁶⁰Co γ-source was placed in close contact with a scattering slab. Variations of number and energy albedo values with increasing slab thickness are given as experimental formulas and graphical representations.

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A highly reliable solid state reactor safety system operated in dynamic mode is described. Direct current signals from detectors for reactor control are converted to pulse signals, which are continuously generated by the circuit until some abnormal condition occurs.
The design and construction of several kinds of logic circuits used for reactor control are explained in detail. The fail-safe property of the circuits have been ascertained by tests, and effects due to temperature variation are negligible in a range 10°-50°C.
Results of reliability analysis show that the reactor safety system described, with redundant dynamic logic circuits, has the necessary qualities to assure highly reliable and safe operation of reactor plants.

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