Phenomena engendered by the fast neutron pulse are analyzed by Neumann series solution of the reduced three-dimensional neutron transport equation.
The spectral functions of collision neutron fluxes are derived by Fourier-Laplace transform, and the time-, space-, energy-dependent collision neutron fluxes possessing definite wave fronts are obtained with use made of convolution theorems in Fourier-Laplace analysis. Thus, the behavior of fast neutron pulses is described in terms of a superposition solely of wave modes.
The present study has deepened our understanding of the detailed physical process of fast neutron pulses, and the process of their formation on the asymptotic mode is clearly explained by wave processes. This treatment has provided a clear explanation of the short time phenomena shown by pulsed neutrons, and should prove useful for analyzing situations where the continuous mode prevails over the asymptotic mode.
The experiment of the nano-second order fast-neutron pulse is also analyzed.

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A promising new approach to dynamics problems relevant to the repetitive pulsed reactor is proposed, in which the matrix iterative method is utilized to describe dynamic behavior during one pulse cycle, as well as between adjacent pulse cycles. The conditions requisite for realizing pulsed criticality and the state of stationary repetitive pulsed operation are derived in simple expressions for the repetitive Accelerator Fast-Pulsed Reactor (AFPR). The accurate neutron pulse shape is easily obtained by this method, whatever the shape of the reactivity pulse may be. A core dynamics model of the AFPR is also developed by combining thermo-hydraulic considerations with the neutron dynamics.
Quantitative examination of the design parameters of the Japan Linac Booster (JLB) based on dynamics considerations provides proof of the validity of the JLB design concept. It is noted, however, that the high sensitivity of the mean reactor power and the peak neutron density to changes in reactivity, as well as the severe thermal shock to the fuel elements that would be induced by accidents due to an excessively large reactivity would make it essential to ensure high precision, promptness and reliability in the system of reactivity control. The possibility is pointed out of improving the S/N ratio by varying the speed of the rotary reflector.

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Nitrous oxide, carbon dioxide and ethylene were utilized as gaseous chemical dosimeters, and measurements were made of the doses absorbed by these substances in a nuclear reactor. The neutron and photon fluxes as well as their contributions to the absorbed doses were deter-mined by analyzing the data obtained from the chemical dosimetry, with particular consider-ation given to the fast neutron energy spectrum. The values of the photon, thermal neutron and fast neutron fluxes thus determined were 2.6×10¹³γ/cm²•sec, 3.5×10¹³ n/cm²•sec and 7.0×10¹² exp(-E)sinh1/2E n/cm²•sec. These values were compared with the results from calorimetric and activation methods, and were found to be in fair agreement with each other.
From the above results, it was confirmed that the above combination of chemical dosimeters are serviceable for the measurement of both absorbed dose and fluxes in a nuclear reactor.

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Grain growth in sol-gel urania compacts was determined as a function of temperature by observing with scanning electron microscope the fractured surface of compacts heated up to predetermined temperatures at a constant rate of heating. Rapid grain growth, unexplainable by currently accepted square or cubic growth kinetics, was observed to occur in the sintered compact when the density reached 70 to 75%T.D., which was attained at about 1, 000°Cin the case of heating rate of 5°C/min. It was considered that this rapid grain growth proceeded by plastic flow of which driving force was the surface tension of the grains, when a large number of primary grains were closely packed against each other, i.e., arranged in quite high coordination number. A rough estimation showed that the rate of coalescence of grains was dependent on the temperature and the grain size or the surface tension of the material, and that the multi-grain-coalescence model, which was previously reported for the case of sintering of BeO, could also be applied to sol-gel urania.

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RR-4, which is a swimming pool type reactor (2.5 MWt), is particularly suitable as source for neutron radiography. The first phases of neutron radiography experiments at JRR-4 are reported in this paper.
A simple Al tube (I.D.=60 mm) with a graphite block (100 x 100 x 700 mm) attached at the end was inserted in the pool (10m deep) and the graphite block was positioned close to the reactor core. The upper part of the tube was curved to avoid direct radiation streaming from the core.
Using both direct exposure method (DEM) and transfer (indirect) exposure method (TEM), the device thus arranged was tested to determine its performance in terms of sensitivity, resolution and contrast. Two examples of application are described : namely, determination of the enrichment of UO₂ pellets, and detection of small quantities of hydrogeneous material.

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