The solution of the Vlasov equation for the longitudinal electron plasma wave is obtained by using Neumann series method. The electron relaxation process is analytically described by the superposition of expressions for electron streamings and Coulomb collisions.
The currently established formula for the longitudinal plasma wave is also explained by the closed-form solution of the Vlasov equation.
The expression effectively describes the detailed physical processes of the electron plasma wave relaxation both in space and time.

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This paper deals with an experimental study on incipient boiling of sodium flowing in an annular channel electrically heated by a pin forming the channel core. In the present experi-ment, the inlet temperature and flow rate were held constant, and the heat flux was gradually increased up to inception of boiling. Measurements were made of the incipient-boiling superheat, and the boiling patterns were observed. The experimental results were compared with analy-tical calculations.
The incipient-boiling wall superheat decreased exponentially with increase in flow rate. The boiling patterns observed in the present experiment were not the single-bubble slug ex-pulsion model, as assumed in calculational code, but more similar to multiple-bubble slug ex-pulsion or homogeneous two-phase flow models.

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Control algorithms are proposed for on-line computer control of the repetitive-accelerator fast-pulsed reactor.
Firstly, simplification of the reactor core dynamics is obtained by regression analysis technique in order to obtain a compact model of the pulsed reactor.
Secondly, a start-up control algorithm to avoid severe thermal shock in the fuel rod is proposed. This is essentially a programmed control, whereby the external neutron source from the accelerator and the external reactivity are controlled in such manner that the fuel temperature would follow a predetermined time trajectory. Applicability of the method to the reactor start-up is proved by a computer simulation experiment.
Thirdly, a control concept for constant power operation is proposed. This is composed of two control loops: (1) a fine control loop, which responds quickly to small variations of reactor power, and (2) a diagnosis and adaptation loop, which judges whether a large deviation from set-up value observed on the reactor power output is due to an abnormality in the external neutron source, or to a change of the set-up value itself, and then adaptively returns the reactor power to the set-up value. A computer simulation experiment demonstrated the validity of the control system characteristics.
A discussion is also presented on the problems foreseen in applying the proposed methods to the actual design of a digital computer control system.

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Applying stepwise compressive disturbances of only 1.0 bar from 1.5 to 2.5 bar to a steam-water two-phase medium containing an initial void fraction up to 17%, very strong secondary pressure pulses with peaks that could approach 100 bar lasting a few milliseconds were observed to evolve by the impulse of hammer action, in a shock tube type vertical experimental device constituted of a stainless steel pipe of rigid structure, 40mm inner diameter and about 2.5 m long. The intensity of these secondary shock pulses agreed fairly well with values based on Karplus' theoretical treatment slightly modified. The measured propagation velocity of the primary disturbances showed good agreement with that based on the completely condensed shock wave model proposed by Karplus.
Comparing the present results with those obtained in a preliminary experiment by the present authors, it is concluded that the propagation of the primary shock pulse waves should be distinguished from that of usually considered pressure waves with small amplitude. It is suggested that, from considerations of safety, careful attention should be paid in the design of reactors with a low system pressure-e.g. sodium cooled FBR-to the possibility of such strong secondary pressure pulses being generated by hammer action. Appreciable damage was actually sustained in the experiment by detectors of semiconductor piezo type with 5mm outer diameter and 10kg/cm² rating.

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The initial-stage sintering mechanism of hyperstoichiometric urania prepared by sol-gel process was determined in relation to temperature during constant rate heating (CRH). The urania powder used in this experiment was prepared by crushing in Ar atmosphere the micro-spheric gel of UO₂ obtained by sol-gel process, and reducing the resulting powder by heating in H₂ for 1 hr at 500°C. The results obtained from densification measurements indicated that the initial-stage sintering proceeded in two phases governed by different shrinkage mechanisms, as follows.
(1) The sintering up to 675°C would be due to a mechanism such as rearrangement of grains and/or plastic flow.
(2) Sintering from 750°C to 800°C was interpreted as being controlled by uranium volume diffusion.
The estimated diffusion coefficient D=1.42×10^-6 exp(-52, 500/RT) cm²/sec. This value agreed in order of magnitude with the uranium diffusion coefficients measured by other workers for hyperstoichiometric urania.

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The phase equilibrium of non-stoichiometric U₃O_8-x has been studied by thermogravi-metry in the range 765° ?? T ?? 995°C and 10^-4 ?? Po2 ?? 1 atm. The results suggest the presence of six phases within the U₃O_8-x phase, separated by second (or higher) order transitions. The relative partial molar free energies, enthalpies and entropies within an each of the six phases, as well as the standard free energies, enthalpies and entropies for the phase changes are calculated and compared with previous works.

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