The theoretical analysis code named REACT, devised for covering pulsed operation of the HTR, is based on a two-point kinetic model, one point representing the average of the core, while the other is optional (e. g. hottest spot). The behavior of reactor power and other parameters are calculated by the average point, and the temperature of the hottest spot by the other point at same time.
Improvements were brought to the code based on experimental data, and good agreement was found between calculation and experimental results obtained for pulsed operation. The feedback reactivity due to the Doppler effect was comprehensively evaluated by recalculation of experimental data; the void behavior was also investigated.

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The finite element method is applied to the axisymmetric buckling of shallow spherical shells under uniform external pressure. In order to analyze the nonlinear behavior of shells with sufficient accuracy and efficiency, the simple load incremental method is modified by introducing the mid-increment stiffness matrix and residual load correction. The analysis is performed on a fairly wide range of the geometric parameter α to obtain pressure-deflection curves in both pre-and post-buckling states. The results appear generally satisfactory, at least up to the early stage of post-buckling. In particular, the upper and lower buckling pressures are obtained in excellent agreement with known analytical solutions.

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Analytical forms of the spatially distributed noise sources used in a companion study on their propagation (this journal, 10_??_5_??_, 259 (1973)) have been examined and determined.
Nuclear noise sources can be regarded as white both in frequency and space. The thermal noise source due to the fact that heat energy is transferred with a definite magnitude at the transfer surface is also white. On the other hand, noise sources that are associated with fluctuations in the relaxation time constants of temperature are usually not white whether in frequency or in space. Their properties can be completely determined from the measured power spectral densities (PSD) of the fluctuations in coolant velocity and density, as well as from their cross PSD. The hydrodynamical noise is due to the random variations in the heat removal rate of the coolant and turns out to be non-white. The mechanical noise source is determined from the PSD of the statistical reactivity modulations caused by fluctuations in the fuel and control-rod positions.

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When a D-T fusion reactor is operated in steady state, the core plasma deviates from equilibrium below a critical temperature. This instability has already been analyzed when the plasma is distributed uniformly. In nonuniform plasmas, however, the stability is not yet known. In this article the stability of a radially nonuniform plasma is discussed analytically, assuming cylindrical symmetry. As a result, it is made clear that the stability of a nonuniform plasma is essentially the same as that of a uniform plasma, and that the critical temperature is determined by the dependence of the diffusion rate and the thermal conduction on the plasma density and temperature. When the diffusion rate is independent of the density and temperature, the critical temperature is higher than that in Bohm diffusion, and lower than in classical diffusion.
When a reactor is operated in an unstable region, feedback control is indispensable. The possibility of stabilization is discussed when the feedback is applied to the plasma density or temperature with different periods of delay after monitoring the peak temperature deviation from equilibrium.
The instability and its control are also analyzed numerically. The analytical results are in a good agreement with computer calculations.

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A mixture of SiC and SrO was heated in air, in Ar and in vacuum, and the reaction products were determined by powder X-ray diffraction. The reaction began above 1, 000°C and the products varied, depending on the oxygen content in the heating atmosphere. For example, in the case of a mixture of 1:1 in atomic ratio, SrO•SiO₂ formed in air, while in oxygen-free atomsphere, SrC₂ and Sr were produced besides 2SrO•SiO₂. It was also found that the powder X-ray diffraction pattern of 2SrO•SiO₂ obtained in the present experiment differed in part from O'Daniel's data. It is concluded that study of the microstructural changes in the SiC coating layer at high temperatures should be most important for assuring the retainment of the fission products Sr and Ba in coated particle fuels.

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When nitric acid solution containing carrier-free ¹³¹I was heated, a fraction of the ¹³¹I was converted to organic iodides, among which methyl iodide was a major component. The yield of organic iodides showed a strong dependence on the concentration of nitric acid. Furthermore, the yield curve referred to reaction time revealed the particular characteristic of a hump appearing in the case of diluted nitric acid solutions. The distribution of ¹³¹I among the various oxidation states in the acid solution has also been studied in its relation to reaction time. The results obtained indicate that organic iodide formation is related with the I°-IO₃^- oxidation step. The yield curve of the organic iodides is discussed qualitatively, by assuming an intermediate state I^* existing between two extreme states of oxidation.

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An experimental study has been made on the carbothermic reaction of ThN with graphite in the form of loose powder mixture in vacuo. The effects of reaction temperature and particle size were examined. The graphite and ThN powders were mixed in proportions representing molecular ratios of about unity. Three kinds of samples of different particle sizes were utilized.
Gravimetric measurements on the reaction were made isothermally in the temperature range of 1, 420°∼1, 630°C. For these samples of different particle sizes, the reaction kinetics were represented in common by first-order plots for the early part of the reaction. The apparent activation energies were about 60kcal/mol for the three samples.
X-ray examination revealed the reaction products to be mainly ThC_xN_1-x with some ThC₂ and/or ThC_x'N_1-x' in the case of the fine and the medium particle size samples. As for the coarse powder, the products were identified to be ThC_xN_1-x and ThC_x'N_1-x' without any ThC₂. As the reaction proceeded, each carbonitride took up more carbon and at the same time the carbon-rich carbonitride increased in quantity and the other decreased.

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A low temperature in-pile loop for the irradiation of chemical reactants, especially by fission fragments, has been designed, constructed and operated successfully. The main features of the loop are briefly described: the capsule for sample irradiation, the design and performance of the helium cooling system and safety considerations. Gaseous chemical reactants charged in an instrumented double capsule at pressures up to 20kg/cm², can be irradiated in a nuclear reactor at any temperature between +20° and -190°C, for periods up to 5hr. The thermal performance of the helium cooling system was in good agreement with the design calculations. Irradiations of ethylene and other gases could be conducted at an absorbed fission fragment dose rate of about 20Mrad/hr, which is several times larger than the background reactor radiation. Thus, chemical reaction studies by fission fragments can be safely conducted at ambient to low temperatures in this loop.

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